A century of science in America

by Unknown

“Gibbs’ grand Collection of Minerals.

One of the most zealous cultivators of mineralogy in the United States is Col. G. Gibbs of Rhode Island and his taste and his fortune have concurred in making him the proprietor of the most extensive and valuable assortment of minerals that probably exists in America.

This rich collection consists of the cabinets possessed by the late Mons. Gigot D’Orcy of Paris and the Count Gregoire de Rozamonsky, a Russian nobleman, long resident in Switzerland. To which the present proprietor has added a number, either gathered by himself on the spot, or purchased in different parts of Europe.... The whole consists of about twenty thousand specimens. A small part of this collection was opened to amateurs at Rhode Island, the last summer, and the next, if circumstances permit, the remainder will be exposed.”

In 1802 Benjamin Silliman was appointed professor of chemistry and mineralogy in Yale College. After the Gibbs Collection was brought to America he spent much time with the owner in studying it and, as a result, Col. Gibbs offered to place the collection on exhibition in New Haven if suitable quarters would be furnished by the college. This was quickly accomplished and in 1810, 1811 and 1812 the collection was transferred to New Haven and arranged for exhibition by Col. Gibbs. Later, in 1825, it was purchased by Yale and served as the nucleus about which the present Museum collection of the University has been formed. There is no doubt but that the presence at this early date of this large and unusual mineral collection had a great influence upon the development of mineralogical science at Yale, and in the country at large.

In the year 1810 Dr. Archibald Bruce started the “American Mineralogical Journal,” the title page of which reads in part as follows: “The American Mineralogical Journal, being a Collection of Facts and Observations tending to elucidate the Mineralogy and Geology of the United States of America, together with other Information relating to Mineralogy, Geology and Chemistry, derived from Scientific Sources.” Unfortunately the health of Dr. Bruce failed, and the journal lasted only through its first volume. It had, however, “been most favorably received,” as Silliman remarks, and it was felt that another journal of a similar type should be instituted. Such a suggestion was made by Col. Gibbs to Professor Silliman in 1817 and this led directly to the founding of the American Journal of Science in 1818 under the latter’s editorship. Although the field of the Journal at the very beginning was made broad and inclusive it has always published many articles on mineralogical subjects. Three of its editors-in-chief have been eminent mineralogists, and without question it has been the most important single force in the development of this science in the country. More than 800 well-established mineral species have been described since the year 1800, of which approximately 150 have been from American sources. More than two-thirds of the articles describing these new American minerals have first appeared in the pages of the Journal. While the description of new species is not always the most important part of mineralogical investigation, still these figures serve to show the large part that the Journal has played in the growth of American mineralogy.

It is convenient to review the progress in Mineralogy according to the divisions formed by the different series, consisting of fifty volumes each, in which the Journal has been published. These divisions curiously enough will be found to correspond closely to four quite definite phases through which mineralogical investigation in America has passed. The first series covered the years from 1817 to 1845. In looking through these volumes one finds a large number of mineralogical articles, the work of many contributors. The great majority of these papers are purely descriptive in character, frequently giving only general accounts of the mineral occurrences of particular regions. However, a number of articles dealing with more detailed physical and chemical descriptions of rare or new species also belong in this period. Among the mineralogists engaged at this time in the description of individual species, none was more indefatigable than Charles U. Shepard. He was graduated from Amherst College in 1824, at the age of twenty. In 1827 he became assistant to Professor Silliman in New Haven, continuing in this position for four years. Later he was a lecturer in natural history at Yale, and was at various times connected with Amherst College and the South Carolina Medical College at Charleston. His articles on mineralogy were very numerous. He assigned a large number of new names to minerals, although with the exception of some half dozen cases, these have later been shown to be varieties of minerals already known and described, rather than new species. In spite, however, of his frequent hasty and inaccurate decision as to the character of a mineral, his influence on the progress of mineralogy was marked. His great enthusiasm and ceaseless industry throughout a long life could not help but make a definite contribution to the science. His “Treatise on Mineralogy” will be spoken of in a later paragraph. He died in May, 1886, having published his last paper in the Journal in the previous September.

The first book on mineralogy published in America was that by Parker Cleaveland, professor of mathematics, natural philosophy, chemistry and mineralogy in Bowdoin College. The first edition was printed in 1816 and an exhaustive notice is given in the first volume of the Journal (1, 35, 308, 1818); a second edition followed in 1822. In his preface Cleaveland gives an interesting discussion concerning the two opposing European methods of classifying minerals. The German school, led by Werner, classified minerals according to their external characters while the French school, following Haüy, put the emphasis on the “true composition.” Cleaveland remarks that “the German school seems to be most distinguished by a technical and minutely descriptive language; and the French, by the use of accurate and scientific principles in the classification or arrangement of minerals.” He, himself, tried to combine in a measure the two methods, basing the fundamental divisions upon the chemical composition and using the accurate description of the physical properties to distinguish similar species and varieties from each other.

Cleaveland’s mineralogy was followed nearly twenty years later by the Treatise on Mineralogy by Charles U. Shepard already mentioned. The first part of this book was published in 1832. This contained chiefly an account of the natural history classification of minerals according to the general plan adopted by Mohs, the Austrian mineralogist. The second part of the book, which appeared in 1835, gave the description of individual species, the arrangement here being an alphabetical one throughout. Subsequent editions appeared in 1844, 1852 and 1857.

James Dwight Dana was graduated from Yale College in 1833 at the age of twenty. Four years later (1837) he published “The System of Mineralogy,” a volume of 580 pages. The appearance of this book was an event of surpassing importance in the development of the science. The book, of course, depended largely upon the previous works of Haüy, Mohs, Naumann and other European mineralogists, but was in no sense merely a compilation from them. Dana, particularly in his discussion of mathematical crystallography, showed much original thought. He also proved his originality by proposing and using an elaborate system of classification patterned after those already in use in the sciences of botany and zoology. He later became convinced of the undesirability of this method of classification and abandoned it entirely in the fourth edition of the System, published in 1854, substituting for it the chemical classification which, in its essential features, is in general use to-day. The System of Mineralogy started in this way in 1837, has continued by means of successive editions to be the standard reference book in the subject. The various editions appeared as follows: I, 1837; II, 1844; III, 1850; IV, 1854; V, 1868; VI, 1892 (by Edward S. Dana).

J. D. Dana also contributed numerous mineralogical articles to the first series of volumes of the Journal. It is interesting to note that they are chiefly concerned with the more theoretical aspects of the subject, in fact they constitute practically the only articles of such a character that appeared during this period. Among the subjects treated were crystallographic symbols, formation of twin crystals; pseudomorphism, origin of minerals in metamorphosed limestones, origin of serpentine, classification of minerals, etc.

The volumes of the Second Series of the Journal covered the years from 1846 through 1870. This period was characterized by great activity in the study of the chemical composition of minerals. A number of skilled chemists, notably J. Lawrence Smith, George J. Brush and Frederick A. Genth, began about 1850 a long series of chemical investigations of American minerals. Very few articles during this time paid much attention to the physical properties of the minerals under discussion, practically no description of optical characters was attempted, and only occasionally were the crystals of a mineral mentioned. J. D. Dana was almost the only writer who constantly endeavored to discover the fundamental characters and relationships in minerals. He published many articles in these years which were concerned chiefly with the classification and grouping of minerals, with similarities in the crystal forms of different species, with relations between chemical composition and crystal form, chemical formulas, mineral nomenclature, etc. The following titles give an idea of the character of the more important series of articles by him which belong to this category: On the isomorphism and atomic volume of some minerals (9, 220, 1850); various notes and articles on homœomorphism of minerals (17, 85, 86, 210, 430; 18, 35, 131, 1854); on a connection between crystalline form and chemical constitution, with some inferences therefrom (44, 89, 252, 398, 1867).

A great many new mineral names were proposed between 1850 and 1870, a large number of which have continued to be well-recognized species. But there was also a tendency, which has not wholly disappeared even now, to base a mineral determination upon insufficient evidence, and to propose a new species with but little justification for it. In this connection a quotation from the introduction by J. D. Dana to the 3rd Supplement to the System of Mineralogy (4th edition) published in the Journal (22, page 246, 1856), will be of interest. He says:

“It is a matter of regret, that mineral species are so often brought out, especially in this country, without sufficient investigation and full description. It is not meeting the just demands of the science of mineralogy to say that a mineral has probably certain constituents, or to state the composition in a general way without a complete and detailed analysis, especially when there are no crystallographic characters to afford the species a good foundation. We have a right to demand that those who name species, should use all the means the science of the age admits of, to prove that the species is one that nature will own, for only such belong to science, and if enough of the material has not been found for a good description there is not enough to authorize the introduction of a new name in the science. The publication of factitious species, in whatever department of science, is progress not towards truth, but into regions of error; and often much and long labor is required before the science recovers from these backward steps.”

J. Lawrence Smith was born in 1818 and died in 1883. He was a graduate of the University of Virginia and of the Medical College of Charleston and later spent three years studying in Paris. Shortly after the completion of his studies he went to Turkey as an advisor to the government of that country in connection with the growing of cotton there. During this time he investigated the emery mines of Asia Minor, and wrote a memoir upon them which was later published by the French Academy. He served as professor of chemistry in the University of Virginia and later held the same chair in the University of Illinois. He published a long series of papers on the chemical composition of minerals and meteorites, as well as on pure chemical subjects. Among the more notable of his contributions are the “Memoir on Emery” (1850), a series of papers on the “Reëxamination of American Minerals” (1853) written with the collaboration of George J. Brush, and his “Memoir on Meteorites” (1855).

George J. Brush entered on his scientific career at the moment when science and scientific methods of research were just beginning to be appreciated in this country, and he soon became one of the leading pioneers in the movement. While his half century of active service was largely occupied by administrative duties in connection with the Sheffield Scientific School, his interest in mineralogy never flagged. His papers on mineralogical subjects number about thirty, all of which were published in the Journal. These began in 1849, even before his graduation from college, and continued until his last paper (in collaboration with S. L. Penfield) appeared in 1883. Three of the early papers were written with J. Lawrence Smith as noted above. These papers first set in this country the standard for thorough and accurate scientific mineral investigation. Later in life he was active in the development of the remarkable mineral locality at Branchville, Conn., and, with the collaboration of E. S. Dana, published in the Journal (1878–90) five important articles on its minerals. This locality, with the exception of the zinc deposits at Franklin Furnace, N. J., was the most remarkable yet discovered in this country. Nearly forty different mineral species were found there, of which nine (mostly phosphates) were new to science. There has certainly been no other series of descriptive papers on a mineralogical locality of equal importance published in this country.

In addition to publishing original papers, Brush did considerable editorial work in connection with the fourth (1854) and fifth (1868) editions of the System of Mineralogy and the Appendices to them. His Manual of Determinative Mineralogy, with a series of determinative tables adapted from similar ones by von Kobell, was first published in 1874. It was revised in 1878 and later rewritten by S. L. Penfield. This book did much to make possible the rapid and accurate determination of mineral species. Throughout his life, Brush was an enthusiastic collector of minerals, building up the notable collection that now bears his name. Perhaps, however, his most important contribution to the development of mineralogy in America lay rather in his influence upon his many students. With his enthusiasm for accurate and painstaking investigation he was an inspiration to all who came in contact with him and his own field and science in general owes much to that influence.

Among the early mineralogists in this country, who were concerned in the chemical analyses of minerals, none accomplished more or better work than Frederick A. Genth. He was born in Germany in 1820 and lived in that country until 1848, when he came to the United States and settled in Philadelphia. He had studied in various German universities and worked under some of the most famous chemists of that time. His papers in mineralogy number more than seventy-five, in the great majority of which chemical analyses are given. He published fifty-four successive articles, the greater part of which appeared in the Journal, which were entitled Contributions to Mineralogy. In these he gave descriptions of more than two hundred different minerals, most of which were accompanied by analyses. He described more than a dozen new and well-established mineral species. He was especially interested in the rarer elements and many of his analyses were of minerals containing them. Especially interesting was his work with the tellurides, the species coloradoite, melonite and calaverite being first described by him. A long and important investigation was recorded on Corundum, “Its Alterations and Associate Minerals,” published in the Proceedings of the American Philosophical Society in 1873 (13, 361). Dr. Genth died in 1893.

The period from 1860 until 1875 was not very productive in mineralogical investigations. The first ten volumes of the Third Series of the Journal, covering the years 1871–1876, contained mineralogical articles by only some fifteen different authors. But from that time on, the amount of work done and the number of investigators grew rapidly. With this increase in activity came also a decided change in the character of the work. The period between 1871 and 1895 can be characterized as one in which all the various aspects of mineral investigation received more nearly equal prominence. While the chemical composition of minerals still held rightly its prominent place, the investigation of the crystallographic and optical characters and the relationships existing between all three were of much more frequent occurrence. Edward S. Dana commenced his scientific work by publishing in 1872 an article on the crystals of datolite which was probably the first American article concerned wholly with the description of the crystallography of a mineral. Samuel L. Penfield began his important investigations in 1877 and the first articles by Frank W. Clarke appeared during this period. The first edition of the Text Book of Mineralogy by Edward S. Dana with its important chapters on Crystallography and Optical Mineralogy was published in 1877 and his revision of the System of Mineralogy (sixth edition) appeared in 1892.

Unquestionably the foremost figure in American mineralogy during this period was that of Samuel L. Penfield. He embodied in an unusual degree the characters making for success in this science, for few investigators in mineralogy have shown, as he did, equal facility in all branches of descriptive mineralogy. He was a skilled chemist and possessed in a high degree that ingenuity in manipulation so necessary to a great analyst. He was also an accurate and resourceful crystallographer and optical mineralogist. His contributions to the science of mineralogy can be partially judged by the following brief summary of his work. He published over eighty mineralogical papers, practically all of which were printed in the Journal. These included the descriptions of fourteen new mineral species, the establishment of the chemical composition of more than twenty others, and the crystallization of about a dozen more. By a series of brilliant investigations he established the isomorphism between fluorine and the hydroxyl radical. He first enunciated the theory that the crystalline form of a mineral was due to the mass effect of the acid present rather than that of the bases. He contributed also a number of articles on the stereographic projection and its use in crystallographic investigations, devising a series of protractors and scales to make possible the rapid and accurate use of this projection in solving problems in crystallography.

Penfield was born in 1856, was graduated from the Sheffield Scientific School in 1877 and immediately became an assistant in the chemical laboratory of that institution. At this time he, together with his colleague Horace L. Wells, made the analyses of the minerals from the newly discovered Branchville locality. He spent the years 1880 and 1881 in studying chemistry in Germany, returning to Yale as an instructor in mineralogy in the fall of 1881. Except for another semester in Europe at Heidelberg he continued as instructor and professor of mineralogy in the Sheffield Scientific School until his early death in 1906.

It is difficult to choose for mention the names of other investigators in Mineralogy during this period. Toward its end a great many writers contributed to the pages of the Journal, more than fifty different names being counted for the volumes 41 to 50 of the Third Series. Many of these are still living and still active in scientific research. Mention should be made of Frank W. Clarke, who contributed many important articles concerning the chemical constitution of the silicates. His work on the mica and zeolite groups is especially noteworthy. The work of W. H. Hillebrand, particularly in regard to his analytical investigations of the minerals containing the rarer elements, was of great importance. The name of W. E. Hidden should be remembered, because, with his keen and discriminating eye and active search for new mineral localities, he was able to make many additions to the science.

In glancing over the indices to the Journal the close interrelation of mineralogy to the other sciences is strikingly shown by the fact that so many scientists whose particular fields are along other lines have published occasional mineralogical papers. Frequently a young man has commenced with mineralogical investigations and then later been drawn definitely into one of these allied subjects. Men, who have won their reputation in chemistry, physics, and all the various divisions of geology, even that of palaeontology, have all contributed articles distinctly mineralogical in character. For this reason the number of American writers who have published what may be called casual papers on mineralogy is very great in comparison to the number of those who continue such publications over a series of years.

That the subject of meteorites is one which has been constantly studied by American mineralogists and petrographers is shown by the long list of papers concerning it that have been published in the Journal; it should, therefore, be considered briefly here. Many of these papers are short and of a general descriptive nature but others which give more fully the chemical, mineralogical and physical details are numerous. Among the earlier writers on this subject Benjamin Silliman, Jr., and C. U. Shepard should be mentioned. The latter was the first to recognize a new mineral in the Bishopville meteorite which he called chladnite. The same substance was afterwards found in a terrestial occurrence and was more accurately described by Kenngott under the name of enstatite. J. Lawrence Smith later showed that these two substances were identical. Smith did a large amount of important chemical work on meteorites. He was the first to note the presence of ferrous chloride in meteoric iron, the mineral being afterwards named lawrencite in his honor. The iron-chronium sulphide, daubreelite, was also first described by him. Other names that should be mentioned in this connection are those of A. W. Wright who studied the gaseous constituents of meteorites, G. F. Kunz, W. E. Hidden, A. E. Foote and H. A. Ward, all of whom published numerous descriptions of these bodies. Among the more recent workers in this field the names of G. P. Merrill and O. C. Farrington deserve especial mention.

The publication of the Fourth Series of the Journal began in 1896. Although the years since then have seen a great amount of very important work accomplished, the history of the period is fresh in the minds of all and as the majority of the active workers are still living and productive it seems hardly necessary to go into great detail concerning it. Twenty years ago it seemed to some mineralogists that the science could almost be considered complete. All the commoner minerals had certainly been discovered and exhaustively studied. Little apparently was left that could be added to our knowledge of them. New occurrences would still be recorded, new crystal habits would be observed, and an occasional new and small crystal face might be listed, but few facts of great importance seemed undiscovered. This view was not wholly justified because new facts of interest and importance have continuously been brought forward, and the finding of new minerals does not appear to diminish in amount with the years. The work of the investigators on the United States Geological Survey along these lines is especially noteworthy.

This last of our periods, however, is chiefly signalized by a practically new development along the lines that might be characterized as experimental mineralogy. New ways have been discovered in which to study minerals. The important but hitherto baffling problems of their genesis, together with their relations to their surroundings, and to associated minerals, have been attacked by novel methods.

In this pioneer work that of the Geophysical Laboratory of the Carnegie Institution of Washington has been of the greatest importance. This laboratory was established in 1905 and, under the directorship of Arthur L. Day, a notable corps of investigators has been assembled and remarkable work already accomplished. While the field of investigation of the laboratory is broader than that of mineralogy, including much that belongs to petrography, vulcanology, etc., still the greater part of the work done can be properly classed as mineralogical in character and should be considered here. Because of its great value, however, it was felt that an authoritative, although necessarily, under existing conditions, a brief, account of it should be given. A concise summary of the objects, methods and results of the investigations of the laboratory has been kindly prepared by a member of its staff, Dr. R. B. Sosman, and is given later.

During the last few years another line of investigation has been opened by the discovery of the effect of crystalline structure upon X-rays. Through the refraction or reflection of the X-ray by means of the ordered arrangement of the particles forming the crystalline network, we are apparently going to be able to discover much concerning the internal structure of crystals. And, partly through these discoveries, is likely to come in turn the solution of the hitherto insolvable mystery of the constitution of matter. Without doubt the multitudinous facts of mineralogy assembled during the past century by the painstaking investigation of a large number of scientists are destined to play a large part in the solution of this problem. Further, it does not seem too bold a prophecy to suggest, that the time will come when it will be possible to assemble all these unorganized facts that we know about minerals into a harmonious whole and that we shall be then able to formulate the underlying and fundamental principles upon which they all depend. These are the great problems for the future of mineralogical investigation.

IX
THE WORK OF THE GEOPHYSICAL LABORATORY OF THE CARNEGIE INSTITUTION OF WASHINGTON

By R. B. SOSMAN

There are three methods of approach to the great problem of rock formation. The first undertakes to reproduce by suitable laboratory experiments some of the observed changes in natural rocks. The second seeks to apply the principles of physical chemistry to a great body of carefully gathered statistics. The third method of attack is like the first in being a laboratory method, and like the second in seeking to apply existing knowledge to the association of minerals as found in rocks, but in its procedure differs widely from both. It consists of bringing together pure materials under measurable conditions, and thus in establishing by strictly quantitative methods the relations in which minerals can exist together under the conditions of temperature and pressure that have the power to affect such relations.

It is to this third method of investigation of the problems of the rocks that the Geophysical Laboratory has been devoted since its establishment in 1905. It has proved entirely practicable to make quantitative studies of the relations among the principal earth-forming oxides (silica, alumina, magnesia, lime, soda, potash, and the oxides of iron) over a very wide range of temperatures. The resources of physics have proved adequate to establish temperature with a high degree of precision and to measure the quantity of energy involved in the various reactions. The chemist has been able to obtain materials in a high degree of purity, and to follow out in detail the chemical relationships that exist among the earth-forming oxides. The petrographic laboratory has been available for the comparison of synthetic laboratory products with the corresponding natural minerals.

It has also proved entirely practicable to extend the same methods of research to some of the principal ore minerals such as the sulphides of copper. Other information which is certain to be of ultimate economic value has also come out of the thorough study of the silicates, which are basic materials for the vast variety of industries which are classed under the name of ceramic industries. The best example of this is the facility with which the experience and the personnel of the laboratory has been adapted to the very important problem of manufacturing an adequate supply of optical glass for the needs of the United States in the present war.

It has further been possible to show within the last two years that rock formation in which volatile ingredients play a necessary and determining part can be completely studied in the laboratory with as much precision as though all the components were solids or liquids.

Along with the laboratory work on the formation of minerals and rocks has gone an increasing amount of field work on the activities of accessible volcanoes, such as Kilauea and Vesuvius, where the fusion and recrystallization of rocks on a large scale can be observed and studied.

There was once a time when the confidence of the laboratory in the capacity of physics and chemistry to solve geological problems was not shared by all geologists. There were some who were inclined to view with considerable apprehension the vast ramifications and complications of natural rock formation as a problem impossible of adequate solution in the laboratory. It is, therefore, a matter of satisfaction to all those who have participated in these efforts to see the evidences of this apprehension disappearing gradually as the work has progressed. A careful appraisement of the situation to-day, after ten years of activity, reveals the fact that the tangible grounds for anxiety about the accessibility of the problems which were confronted at first are now for the most part dissipated.

It will not be possible to review in detail the lines of work sketched above. An outline of the synthetic work on systems of the mineral oxides and a paragraph on the volcano researches will perhaps suffice to indicate the general plan and purpose of the laboratory’s work. It should be added that the results of many of the researches of the laboratory, detailed below, have been published in the pages of the Journal (see 21, 89, 1906, and later volumes).

Mineral Researches.—The mineral studies include:

I. One-component systems: silica, with its numerous polymorphic forms and their relations to temperature and the conditions of rock formation; alumina; magnesia; and lime.

II. Two-component systems: silica-alumina, including sillimanite and related minerals; silica-magnesia, including the tetramorphic metasilicate MgSiO₃; silica-lime, including wollastonite; the alkali silicates, particularly with reference to their equilibria with carbon dioxide and with water; ferric oxide-lime; alumina-lime; alumina-magnesia, including spinel; and hematite-magnetite, a solid-solution series of an unusual type.

III. Three-component systems: silica-alumina-magnesia, completed but not yet published; silica-alumina-lime, complete, including the compounds that enter into the composition of portland cement; silica-magnesia-lime, completed but not yet published, including, however, published work on the diopside-forsterite-silica system, and on the CaSiO₃-MgSiO₃ series; and alumina-magnesia-lime.

IV. Four components: SiO₂-Al₂O₃-MgO-CaO: the incomplete system anorthite-forsterite-silica; SiO₂-Al₂O₃-CaO-Na₂O: the series of lime-soda feldspars (albite-anorthite), and the series nephelite (carnegieite)-anorthite; SiO₂-Al₂O₃-Na₂O-K₂O: the sodium-potassium nephelites.

V. Five components: SiO₂-Al₂O₃-MgO-CaO-Na₂O: the ternary system diopside-anorthite-albite (haplo-basaltic and haplo-dioritic magmas).

Fairly complete studies have also been made of the mineral sulphides of iron, copper, zinc, cadmium, and mercury, and the conditions controlling the secondary enrichment of copper sulphide ores are now being investigated. In connection with the sulphide investigations, the hydrated oxides of iron have been studied chemically and microscopically and the results will soon be ready for publication.

Throughout the work the mere accumulation of bodies of facts has been held to be secondary in importance to the development of new methods of attack and the evaluation of new general principles, and the specific problems studied have been selected from this point of view.

Volcano Researches.—A branch of the laboratory’s work that is of general as well as petrological interest is the study of active volcanoes. Observations and collections have been made at Kilauea, Vesuvius, Etna, Stromboli, Vulcano, and (through the courtesy of the directors of the National Geographic Society) Katmai in Alaska. The great importance of gases in volcanicity is emphasized by all the studies. The active gases include hydrogen and water vapor, carbon monoxide and carbon dioxide, and sulphur and its oxides, as well as a variety of other compounds of lesser importance. The crater of Kilauea proves to be an active natural gas-furnace, in which reactions are continuously occurring among the gases, often resulting in making the lava basin hotter at the surface than it is at some depth. These reactions are being studied in the laboratory on mixtures of the pure constituent gases in known proportions, in order to lay the foundation for accurate interpretation and prediction concerning the gases as actually collected from the volcanoes themselves.

X
THE PROGRESS OF CHEMISTRY DURING THE PAST ONE HUNDRED YEARS

By HORACE L. WELLS and HARRY W. FOOTE

Introduction.

As we look back to the time of the founding of the Journal in 1818, we see that the science of chemistry had recently made and was then making great advances. That the scientific men of those days were much impressed with what was being accomplished is well shown by the following statement made in an early number of the Journal (3, 330, 1821) by its founder in reviewing Gorham’s Elements of Chemical Science. He says: “The present period is distinguished by wonderful mental activity; it might indeed be denominated as the intellectual age of the world. At no former period has the mind of man been directed at one time to so many and so useful researches.”

A very remarkable revolution in chemical ideas had recently taken place. Soon after the discovery of oxygen by Priestley in 1774, and the subsequent discovery by Cavendish that water was formed by the combustion of hydrogen and oxygen, Lavoisier had explained combustion in general as oxidation, thus overthrowing the curious old phlogiston theory which had prevailed as the basis of chemical philosophy for nearly a century.

The era of modern chemistry had thus begun, and the additional views that matter was indestructible and that chemical compounds were of constant composition had been generally accepted at the beginning of the nineteenth century.

Dalton had announced his atomic theory in 1802, having based it largely upon the law of multiple proportions which he had previously discovered, and he had begun to express the formulas for compounds in terms of atomic symbols.

In 1808 Gay-Lussac had discovered his law of gas combination in simple proportions,[^[153]] a law of supreme importance in connection with the atomic theory, but neither he nor Dalton had seen this theoretical connection. Avogadro had understood it, however, and in 1811 had reached the momentous conclusion that all gases and vapors have equal numbers of molecules in equal volumes at the same temperature and pressure.

Davy in 1807 had isolated the alkali-metals, sodium and potassium, by means of electrolysis, thus practically dispelling the view that certain earthy substances might be elementary; and about four years later he had demonstrated that chlorine was an element, not an oxide as had been supposed previously, thus overthrowing Lavoisier’s view that oxygen was the characteristic constituent of all acids.

At the time that our period of history begins, the atomic theory had been accepted generally, but in a somewhat indefinite form, since little attention had been paid to Avogadro’s principle, and since Dalton had used only the principle of greatest simplicity in writing the formulas of compounds, considering water as HO and ammonia NH, for example. At this time, however, Berzelius for ten or fifteen years had been devoting tremendous energy to the task of determining the atomic weights of nearly all of the elements then known by analyzing their compounds. He had confirmed the law of multiple proportions, accepted the atomic theory, and utilized Avogadro’s principle, and it is an interesting coincidence that his first table of atomic weights was published in the year 1818.

An interesting account of the views on chemistry held at about that time was published in the Journal by Denison Olmsted (11, 349, 1826; 12, 1, 1827), who had recently become professor of natural philosophy in Yale College.

The most illustrious European chemists of that time were Berzelius of Sweden, Davy of England, and Gay-Lussac of France, and the curious circumstance may be mentioned that all three of them and also Benjamin Silliman, the founder of the Journal, were born within a period of eight months in 1778–1779.

In this country Robert Hare of Philadelphia and Benjamin Silliman were undoubtedly the most prominent chemists of those days. Hare is best known for his invention of the compound blowpipe, but his contributions to the Journal were very numerous, beginning almost with the first volume and continuing for over thirty years. Among the first of these contributions was a most vigorous but well-merited attack upon a Doctor Clark of Cambridge, England, who had copied his invention without giving him proper credit. He begins (2, 281, 1820) by saying: “Dr. Clark has published a book on the gas blowpipe in which he professes a sincere desire to render everyone his due. That it would be difficult for the conduct of any author to be more discordant with these professions, I pledge myself to prove in the following pages.”

Hare also invented a galvanic battery which he called a “deflagrator,” consisting of a large number of single cells in series. With this, using carbon electrodes, he was able to obtain a higher temperature than with his oxy-hydrogen blowpipe. He was the first to apply galvanic ignition to blasting (21, 139, 1832), and he first carried out electrolyses with the use of mercury as the cathode (37, 267, 1839). In this way he prepared metallic calcium and other metals from solutions of their chlorides, while the principle employed by him has in recent times been used as the basis of a very important process for manufacturing caustic potash and soda.

Silliman, who had become an intimate friend of Hare during two periods of chemical study under Woodhouse in Philadelphia in 1802–1804, and who soon afterwards spent fourteen months as a student abroad, chiefly in England and Scotland, took a broad interest in science and gave much attention to geology as well as to chemistry. In spite of this divided interest and his work as a teacher, popular scientific lecturer, and editor, he found time for a surprising amount of original chemical work. For instance, using Hare’s deflagrator, he showed that carbon was volatilized in the electric arc (5, 108, 1822); he was the first in this country to prepare hydrofluoric acid (6, 354, 1823), and he first detected bromine in one of our natural brines (18, 142, 1830).

Atomic Weights.

As soon as the atomic theory was accepted, the relative weights of the atoms became a matter of vital importance in connection with formulas and chemical calculations. In advancing his theory, Dalton had made some very rough atomic weight determinations, and it has been mentioned already that Berzelius, at the time that our historical period begins, was engaged in the prodigious task of accurately determining these constants for nearly all the known elements. It is recorded that he analyzed quantitatively no less than two thousand compounds in connection with this work during his career. His table of 1818 has proved to be remarkably accurate for that pioneer period, and it indicates his remarkable skill as an analyst.

It is to be observed that Berzelius in this early table made use of Avogadro’s principle in connection with elements forming gaseous compounds, and thus obtained correct formulas and atomic weights in such cases, but that in many instances his atomic weights and those now accepted bear the relation of simple multiples to one another, because he had then no means of deciding upon the formulas of many compounds except the rule of assumed simplicity. For example, the two oxides of iron now considered to be FeO and Fe₂O₃ he regarded as FeO₂ and FeO₃, knowing as he did that the ratio of oxygen in them was 2 to 3, and believing that a single atom of iron in each was the simplest view of the case, so that as the consequence of these formulas the atomic weight of iron was then considered to be practically twice as great in its relation to oxygen as at present.

These old atomic weights of Berzelius, used with the corresponding formulas, were just as serviceable for calculating compositions and analytical factors as though the correct multiples had been selected. As time went on, the true multiples were gradually found from considerations of atomic heats, isomorphism, vapor densities, the periodic law, and so on, and suitable changes were made in the chemical formulas.

Berzelius used 100 parts of oxygen as the basis of his atomic weights, a practice which was generally followed for several decades. Dalton, however, had originally used hydrogen as unity as the basis, and this plan finally came into use everywhere, as it seemed to be more logical and convenient, because hydrogen has the smallest atomic weight, and also because the atomic weights of a number of common elements appeared to be exact multiples of that of hydrogen, thus giving simpler numbers for use in calculations.

Within a few years a slight change has been made by the adoption of oxygen as exactly 16 as the basis, which gives hydrogen the value of 1·008.

As early as 1815, Prout, an English physician, had advanced the view that hydrogen is the primordial substance of all the elements, and consequently that the atomic weights are all exact multiples of that of hydrogen. This hypothesis has been one of the incentives to investigations upon atomic weights, for it has been found that these constants in the cases of a considerable number of the elements are very close to whole numbers when based upon hydrogen as unity, or even still closer when based upon oxygen as 16.

With our present knowledge Prout’s hypothesis may be regarded as disproved for nearly all the elements whose atomic weights have been accurately determined, but the close or even exact agreement with it in a few cases is still worthy of consideration. There is an interesting letter from Berzelius to B. Silliman, Jr., in the Journal (48, 369, 1845) in which Berzelius considers the theory entirely disproved.

For a long time entire reliance was placed upon the atomic weights obtained by Berzelius, but it came to be observed that the calculation of carbon from carbon dioxide appeared to give high results in certain cases, so that doubt arose as to the accuracy of Berzelius’s work. Consequently in 1840 Dumas, assisted by his pupil Stas, made a new determination of the atomic weight of carbon, and found that the number obtained by Berzelius, 12·12, was slightly too large. Subsequently Dumas determined more than twenty other atomic weights, but this great amount of work did not bring about any considerable improvement, for it appears that Dumas did not greatly excel Berzelius in accuracy, and that the latter had made one of his most noticeable errors in connection with carbon.

Soon after assisting Dumas in the work upon carbon, Stas began his very extensive and accurate, independent determinations, leading to the publication of a book in 1867 describing his work. Stas made many improvements in methods by the use of great care in purifying the substances employed, and especially by using large quantities of material in his determinations, thus diminishing the proportional errors in weighing. His results, which dealt with most of the common elements, were accepted with much confidence by chemists everywhere.

Stas reached the conclusion that there could be no real foundation for Prout’s hypothesis, since so many of his atomic weights varied from whole numbers, and this opinion has been generally accepted.

The first accurate atomic weight determination published in the Journal was that by Mallett on lithium (22, 349, 1856; 28, 349, 1859), showing a result almost identical with that accepted at the present time. Johnson and Allen’s determination (35, 94, 1863) on the rare element cæsium was carried out with extraordinary accuracy. Lee, working with Wolcott Gibbs, made good determinations on nickel and cobalt (2, 44, 1871). The work of Cooke on antimony (15, 41, 107, 1878) was excellent.

Concerning the more recent work published elsewhere than in the Journal, attention should be called particularly to the investigations that have been carried on for the past twenty-five years by Richards and his associates at Harvard University. Richards has shown masterly ability in the selection of methods and in avoiding errors. His results have displayed such marvelous agreements among repeated determinations by the same and by different processes as to inspire the greatest confidence. His work has been very extensive, and it is a great credit to our country that this atomic weight work, so superior to all that has been previously done, is being carried out here.

It may be mentioned that for a number of years the decision in regard to the atomic weights to be accepted has been in the hands of an International Committee of which our fellow countryman F. W. Clarke has been chairman. In connection with this position and previously, Clarke has done valuable service in re-calculating and summarizing atomic weight determinations.

Analytical Chemistry.

Analysis is of such fundamental importance in nearly every other branch of chemical investigation that its development has been of the utmost importance in connection with the advancement of the science. It attained, therefore, a comparatively early development, and one hundred years ago it was in a flourishing condition, particularly as far as inorganic qualitative and gravimetric analysis were concerned. There is no doubt that Berzelius, whose atomic weight determinations have already been mentioned, surpassed all other analysts of that time in the amount, variety, and accuracy of his gravimetric work. He lived through three decades of our period, until 1848.

During the past century there has been constant progress in inorganic analysis, due to improved methods, better apparatus and accumulated experience. An excellent work on this subject was published by H. Rose, a pupil of Berzelius, and the methods of the latter, with many improvements and additions by the author and others, were thus made accessible. Fresenius, who was born in 1818, did much service in establishing a laboratory in which the teaching of analytical chemistry was made a specialty, in writing text-books on the subject and in establishing in 1862 the “Zeitschrift für analytische Chemie,” which has continued up to the present time.

Besides Berzelius, who was the first to show that minerals were definite chemical compounds, there have been many prominent mineral analysts in Europe, among whom Rammelsberg and Bunsen may be mentioned, but there came a time towards the end of the nineteenth century when the attention of chemists, particularly in Germany, was so much absorbed by organic chemistry that mineral analysis came near becoming a lost art there. It was during that period that an English mineralogist, visiting New Haven and praising the mineral analyses that were being carried out at Yale, expressed regret that there appeared to be no one in England, or in Germany either, who could analyze minerals.

The best analytical work done in this country in the early part of our period was chiefly in connection with mineral analysis, and a large share of it was published in the Journal. Henry Seybert, of Philadelphia, in particular, showed remarkable skill in this direction, and published numerous analyses of silicates and other minerals, beginning in 1822. It was he who first detected boric acid in tourmaline (6, 155, 1822), and beryllium in chrysoberyl (8, 105, 1824). His methods for silicate analyses were very similar to those used at the present time.

J. Lawrence Smith in 1853 described his method for determining alkalies in minerals (16, 53), a method which in its final form (1, 269, 1871) is the best ever devised for the purpose. He also described (15, 94, 1853) a very useful method, still largely used in analytical work, for destroying ammonium salts by means of aqua regia. Carey Lea (42, 109, 1866) described the well-known test for iodides by means of potassium dichromate. F. W. Clarke (49, 48, 1870) showed that antimony and arsenic could be quantitatively separated from tin by the precipitation of the sulphides in the presence of oxalic acid. In 1864 Wolcott Gibbs (37, 346) began an important series of analytical notes from the Lawrence Scientific School, and he worked out later many difficult analytical problems, particularly in connection with his extensive researches upon the complex inorganic acids.

From 1850 on, Brush and his students made many important investigations upon minerals, and from 1877 Penfield (13, 425), beginning with an analysis of a new mineral from Branchville, Connecticut, described by Brush and E. S. Dana, displayed remarkable skill and industry in this kind of work. Both of the writers of this article were fortunate in being associated with Penfield in some of his researches upon minerals and one of us began as he did with the Branchville work. It is probably fair to say that Penfield did the most accurate work in mineral analysis that has ever been accomplished, and that he was similarly successful in crystallography and other physical branches of mineralogy.

The American analytical investigations that have been mentioned were all published in the Journal, with the exception of a part of Gibbs’s work. Many other American workers at mineral analysis might be alluded to here, but only the excellent work of a number of chemists in the United States Geological Survey will be mentioned. Among these Hillebrand deserves particular praise for the extent of his investigations and for his careful researches in improving the methods of rock analysis.

To our own Professor Gooch especial praise must be accorded for the very large number of analytical methods that have been devised, or critically studied, by him and his students, and for the excellent quality of this work. The publications in the Journal from his laboratory began in 1890 (39, 188), and the extraordinary extent of this work is shown by the fact that the three hundredth paper from the Kent Laboratory appeared in May, 1918. These very numerous and important investigations have been of great scientific and practical value, and they have formed a striking feature of the Journal for nearly 30 years. In 1912 Gooch published his “Methods in Chemical Analysis,” a book of over 500 pages, in which the work in the Kent Chemical Laboratory up to that time was concisely presented. Among the many workers who have assisted in these investigations, P. E. Browning, W. A. Drushel, F. S. Havens, D. A. Kreider, C. A. Peters, I. K. Phelps and R. G. Van Name are particularly prominent. Besides many other useful pieces of apparatus, the perforated filtering crucible was devised by Gooch, and this has brought his name into everyday use in all chemical laboratories.

Volumetric analysis was originated by Gay-Lussac, who described a method for chlorimetry in 1824, for alkalimetry in 1828, and for the determination of silver and chlorides in 1832. Margueritte devised titrations with potassium permanganate in 1846, while Bunsen, not far from the same time, introduced the use of iodine and sulphur dioxide solutions for the purpose of determining many oxidations and reductions. We owe to Mohr some improvements in apparatus and a German text-book on the subject, while Sutton wrote an excellent English work on volumetric analysis, of which many editions have appeared.

While volumetric analysis began to be used less than one hundred years ago, its applications have been gradually extended to a very great degree, and it is not only exceedingly important in investigations in pure chemistry, but its use is especially extensive in technical laboratories where large numbers of rapid analyses are required.

Not a few volumetric methods have been devised or improved in the United States, but mention will be made here only of Cooke’s important method for the determination of ferrous iron in insoluble silicates, published in the Journal (44, 347, 1867); to Penfield’s method for the determination of fluorine in 1878; and to the more recent general method of titration with an iodate in strong hydrochloric acid solutions, due to L. W. Andrews, a number of applications of which have been worked out in the Sheffield Laboratory.

A considerable amount of work with gases had been done by Priestley, Scheele, Cavendish, Lavoisier, Dalton, Gay-Lussac, and others before our hundred-year period began. Cavendish, about 1780, had analyzed atmospheric air with remarkable accuracy, and had even separated the argon from it and wondered what it was, and later Gay-Lussac had shown great skill in the study of gas reactions. During our period gas analysis has been further developed by many chemists. Bunsen, in particular, brought the art to a high degree of perfection in the course of a long period beginning about 1838, the last edition of his “Methods of Gas Analysis” having been published in 1877.

Important devices for the simplification of gas analysis in order that it might be used more conveniently for technical purposes have been introduced by Orsat in France and by Winkler, Hempel and Bunte in Germany.

It appears that our countryman Morley has surpassed all others in accurate work with gases in connection with his determinations of the combining weights and volumes of hydrogen and oxygen about the year 1891. Some of his publications have appeared in the Journal (30, 140, 1885; 41, 220, 1891; and others).

Electrolytic analysis, involving the deposition of metals, or sometimes of oxides, usually upon a platinum electrode, was brought into use in 1865 by Wolcott Gibbs through an article published in the Journal (39, 58, 1865). He there described the electrolytic precipitation of copper and of nickel by the methods still in use. The application of the process has been extended to a number of other metals, and it has been largely employed, particularly in technical analyses. Important investigations and excellent books on this subject have been the contributions of Edgar F. Smith of the University of Pennsylvania, and the useful improvement, the rotating cathode, was devised by Gooch and described in the Journal (15, 320, 1903).

General Inorganic Chemistry.

The Chemical Symbols.—It is to Berzelius that we owe our symbols for the atoms, derived usually from their Latin names, such as C for carbon, Na for sodium, Cl for chlorine, Fe for iron, Ag for silver, and Au for gold. We owe to him also the use of small figures to show the number of atoms in a formula, as in N₂O₅. This was a marked improvement over the hieroglyphic symbols proposed by Dalton, which were set down as many times as the atoms were supposed to occur in formulas, forming groups of curious appearance, but in some respects not unlike some of our modern developed formulas. The advantages of Berzelius’s symbols were their simplicity, legibility, and the fact that they could be printed without the need of special type. It is true that at a later period Berzelius used certain symbols with horizontal lines crossing them to represent double atoms, and that these made some difficulty in printing. It should be mentioned also that Berzelius at one time made an effort to simplify formulas by placing dots over other symbols to represent oxygen, and commas to represent sulphur atoms. Examples of these are:

ĊaS⃛, calcium sulphate; F̋e, iron disulphide

This form of notation was quite extensively employed for a time, especially by mineralogists, but it was entirely abandoned later.

It is interesting to notice that Dalton, who lived until 1844, to reach the age of 78, differed from other chemists in refusing to accept the letter-symbols of Berzelius. In a letter written to Graham in 1837 he said: “Berzelius’s symbols are horrifying. A young student in chemistry might as soon learn Hebrew as to make himself acquainted with them. They appear like a chaos of atoms ... and to equally perplex the adepts of science, to discourage the learner, as well as to cloud the beauty and simplicity of the atomic theory.”

This forcibly expressed opinion was apparently tinged with self-esteem, but there is no doubt that Dalton was sincere in believing that the atoms were best represented by his circular symbols, because, as is well known, he thought that all the atoms were spherical in form, and it is evident that circles give the proper picture of spherical objects. At the present time some insight as to the structure of atoms is being gained, and it appears possible that the time may come when pictures of their external appearance that are not wholly imaginary may be made.

Changes in Formulas.—Even before the year 1826, Berzelius displayed great skill in arriving at many formulas that agree with our present ones, for example, H₂O for water, ZnCl₂ for zinc chloride, N₂O₅ for nitric acid (anhydride), CaO for calcium oxide, CO and CO₂ for the oxides of carbon, and many others. But at the same period other authorities, especially Gay-Lussac in France and Gmelin in Germany, on account of a lack of appreciation for Avogadro’s principle and for other reasons, such as the use of symbols to represent combining weights rather than atoms, were using different formulas for some of these compounds, such as HO, ZnCl and NO₅, so that their formulas for many of the compounds of hydrogen, chlorine, nitrogen and several other elements differed from those of Berzelius. The employment of different formulas involved the use of different atomic or combining weights. For example, with the formula H₂O for water the composition by weight requires the ratio 1 to 16 for the weights of the hydrogen and oxygen atoms, while with HO the ratio is 1 to 8.

Berzelius attempted to bring about greater uniformity in formulas and atomic weights by making changes in his table of atomic weights published in 1826. He practically doubled the relative atomic weights of hydrogen, chlorine, nitrogen, and of the other elements that gave twice as many atoms in his formulas as in those of others, and at the same time he wrote the symbols of these elements with a bar across them to indicate that they represented double atoms. For example, he wrote:

H̶O ZnC̶l N̶O₅,

instead of

H₂O, ZnCl₂ N₂O₅

This appears to have been an unfortunate concession to the views of others on the part of Berzelius, for the barred symbols were not generally adopted, partly on account of difficulties in printing, and the great achievement in theory made by him was lost sight of for a long period of time.

The Law of Atomic Heats.—In 1819, Dulong and Petit of France, from experiments upon the specific heats of a number of solid elementary substances, came to the conclusion that the atoms of simple substances have equal capacities for heat, or in other words, that the specific heats of elements multiplied by their atomic weights give a constant called the atomic heat. For instance, the specific heats of sulphur, iron, and gold have been given as 0·2026, 0·110, and 0·0324, while their atomic weights are about 32, 56, and 197, respectively; hence the atomic heats obtained by multiplication are 6·483, 6·116, and 6·383.

Further investigations showed that the atomic heats display a considerable variation. Those of carbon, boron, beryllium, and silicon are very low at ordinary temperatures, although they increase and approach the usual values at higher temperatures. More recent work has shown, however, that the specific heats of other elements vary greatly with the temperature, almost disappearing at the temperature of liquid hydrogen, and hence possibly disappearing entirely at the absolute zero, where the electrical resistance of the metals appears to vanish likewise.

It has been found that most of the solid elements near ordinary temperatures give atomic heats that are approximately 6·4. Berzelius applied the law in fixing a number of atomic weights, and its importance for this purpose is still recognized.

It may be mentioned here that two well-known Yale men, W. O. Mixter and E. S. Dana, while students in Bunsen’s laboratory at Heidelberg in 1873, made determinations of the specific heats of boron, silicon, and zirconium. This was the first determination of this constant for zirconium, and it was consequently important in establishing the atomic weight of that element.

Isomorphism and Polymorphism.—Mitscherlich observed in 1818 that certain phosphates and arsenates have the same crystalline form, and afterwards he reached the conclusion that identity in form indicates similarity in composition in connection with the number of atoms and their arrangement. This law of isomorphism was of much assistance in the establishment of correct formulas and consequently of atomic weights. For instance, since the carbonates of barium, strontium, and lead crystallize in the same form, the oxides of these metals must have analogous formulas. From such considerations Berzelius was able to make several improvements in his atomic weight table of 1826.

Mitscherlich was the first to observe two forms of sulphur crystals, and from this and other cases of dimorphism or of polymorphism it became evident that analogous compounds were not necessarily always isomorphous, a circumstance which has restricted the application of the law to some extent.

Besides its application in fixing analogous formulas, the law of isomorphism has come to be of much practical use in the understanding and simplification of the formulas for minerals, for these natural crystals very often contain several isomorphous compounds in varying proportions, and an understanding of this “isomorphous replacement,” as it is called, makes it possible to deduce simple general formulas for them.

In some cases isomorphism takes place to a greater or less extent between substances which are not chemically similar, and this brings about a variation in composition which at times has caused confusion. For instance, the mineral pyrrhotite has a composition which usually varies between Fe₇S₈ and Fe₁₁S₁₂, and both these formulas have been assigned to it. It was recently shown by Allen, Crenshaw and Johnston in the Journal (33, 169, 1912) that this is a case where the compound FeS is capable of taking up various amounts of sulphur isomorphously.

The idea of solid solution was advanced by van’t Hoff to explain the crystallization of mixtures, including cases of evident isomorphism. This view has been widely accepted, and it has been particularly useful in cases where isomorphism is not evident. Solid solution between metals has been found to be exceedingly common, many alloys being of this character. A case of this kind was observed by Cooke and described in the Journal (20, 222, 1855). He prepared two well-crystallized compounds of zinc and antimony to which he gave the formulas Zn₃Sb and Zn₂Sb, but he observed that excellent crystals of each could be obtained which varied largely in composition from these formulas. As the two compounds were dissimilar in their formulas and crystalline forms, Cooke assumed that isomorphism was impossible and concluded “that it is due to an actual perturbation of the law of definite proportions, produced by the influence of mass.” We should now regard this as a case of solid solution.

A Lack of Confidence in Avogadro’s Principle.—One reason why chemists were so slow in arriving at the correct atomic weights and formulas was a partial loss of confidence in Avogadro’s principle. About 1826 the young French chemist Dumas devised an excellent method for the determination of vapor densities at high temperatures, and his results and those of others showed some discrepancies in the expected densities. For example, the vapor density of sulphur was found to be about three times too great, that of phosphorus twice too great, that of mercury vapor and that of ammonium chloride only about half large enough to correspond to the values expected from analogy and other considerations. Thus, one volume of oxygen with two volumes of hydrogen make two volumes of steam, but only one third of a volume of sulphur vapor was found to unite with two volumes of hydrogen to make two volumes of hydrogen sulphide. Berzelius saw clearly that the results pointed to the existence of such molecules as S₆, P₄, and Hg₁, but it was not generally realized in those days that Avogadro’s rule is fundamentally reliable, and Berzelius himself appears to have lost confidence in it on account of these complications, for he did not apply Avogadro’s principle to decisions about atomic weights, except in the cases of substances gaseous at ordinary temperatures.

Electro-chemical Theories.—The observation was made by Nicholson and Carlisle in 1800 that water was decomposed into its constituent gases by the electric current. Then in 1803 Berzelius and Hisinger found that salts were decomposed into their bases and acids by the same agency, and in 1807 Davy isolated potassium, sodium, and other metals afterwards, by a similar decomposition. Since those early times a vast amount of attention has been paid to the relation of electricity to chemical changes, a relation that is evidently of great importance from the fact that while electric currents decompose chemical compounds, these currents, on the other hand, are produced by chemical reactions.

Berzelius was particularly prominent in this direction, and in 1819 he published an elaborate electro-chemical theory. He believed that atoms were electrically polarized, and that this was the cause of their combination with one another. He extended this idea to groups of atoms, particularly to oxides, and regarded these groups as positive or negative, according to the excess of positive or negative electricity derived from their constituent atoms and remaining free. He thus arrived at his dualistic theory of chemical compounds, which attained great prominence and prevailed for a long time in chemical theory. According to this idea, each compound was supposed to be made up of a positive and a negative atom or group of atoms. For example, the formulas for potassium nitrate, calcium carbonate, and sulphuric acid corresponded to K₂O.N₂O₅, CaO.CO₂ and H₂O.SO₃ where we now write KNO₃, CaCO₃ and H₂SO₄, and the theory was extended to embrace organic compounds also.

The eminent English chemist and physicist Faraday announced the important law of electro-chemical equivalents in 1834. This law shows that the quantities of elements set free by the passage of a given quantity of electricity through their solutions correspond to the chemical equivalents of those elements. Faraday made a table of the equivalents of a number of elements, regarding them important in connection with atomic weights, but at that time no sharp distinction was usually made between equivalents and atomic weights, and it was not fully realized that one atom of a given element may be the electrical equivalent of several atoms of another.

Faraday’s law, which is still regarded as fundamentally exact, has been of much practical use in the measurement of electric currents and in calculations connected with electro-chemical processes. In discussing his experiments, Faraday made use of several new terms, such as “electrolyte” for a substance which conducts electricity when in solution, and is thus “electrolyzed,” “electrode,” “anode,” and “cathode,” terms that have come into general use, and finally “ions” for the particles that were supposed to “wander” towards the electrodes to be set free there.

This term “ion” remained in comparative obscurity for more than half a century, when it was brought into great prominence among chemists by Arrhenius in connection with the ionic theory.

Cannizzaro’s Ideas.—Up to about 1869 chaos reigned among the formulas used by different chemists. Various compound radicals and numerous type-formulas were employed, dualistic and unitary formulas of several kinds were in use, but the worst feature of the situation was the fact that more than one system of atomic weights was in vogue, so that water might be written

HO, H̶O, or H₂0

and similar discrepancies might appear in nearly all formulas containing elements of different valencies. In 1858, however, an article by the Italian chemist Cannizzaro appeared in which the outlines of a course in chemical philosophy were presented. This acquired wide circulation in the form of a pamphlet at a chemical convention somewhat later, and it dealt so clearly and ably with Avogadro’s principle, Dulong and Petit’s law, and other points in connection with formulas that it led to a rapid and almost universal reform among those who were using unsatisfactory formulas.

At about this time also the dualistic formulas of Berzelius were generally abandoned, and hydrogen came to be regarded as the characteristic element of all acids. For instance, CaO.SO₃, called “sulphate of lime,” came to be written CaSO₄ and was called “calcium sulphate,” and while it had been shown as early as 1815 by Davy that “iodic acid,” I₂O₅, showed no acid reaction until it was combined with water, the accumulation of similar facts led to the formulation of sulphuric acid as H₂SO₄ instead of SO₃ or H₂O.SO₃, and that of other “oxygen acids” in a similar way. As a necessary consequence of this view of acids, the bases came to be regarded as compounds of the “hydroxyl” group, OH. Therefore the formula for caustic soda came to be written NaOH instead of Na₂O.H₂O, and so on.

The Periodic System of the Elements.—The periodicity of the elements in connection with their atomic weights was roughly grasped by Newlands in England, who announced his “law of octaves” in 1863. This was at the time when the atomic weights were being modified and their numerical relations properly shown. The subject was worked out more fully by L. Meyer in Germany a little later, but it was most clearly and elaborately presented by the Russian chemist Mendeléeff in 1869.

In order that this subject may be explained to some extent Mendeléeff’s table is given here, with the addition of the recently discovered elements and some other modifications.

Note.—Distinctions in printing: Gaseous elements. Other non-metallic elements, metallic elements. The heavy line encloses approximately the acid-forming elements.

In this table the elements arranged in the order of their atomic weights fall into eight groups where the known oxides progress regularly, with the exception of two or three elements, from R₂O in Group I to R₂O₇ in Group VII, while in Group VIII two oxides (of ruthenium and osmium) are known which carry the progression to RO₄.

It was pointed out by Mendeléeff that, with the exception of series 1 and 2 at the top of the table, the alternate members of the groups show particularly close relationships. These subordinate groups, marked A and B, in most cases show remarkable analogies and gradations in their properties, for example, in the alkali-metals from lithium to cæsium, and in the halogens from fluorine to iodine. The two divisions of a group do not usually show very close relations to each other, except in their valency, and they even display, in several instances, opposite gradations in chemical activity in the order of their atomic weights. For instance, cæsium stands at the electro-positive end, while gold stands at the electro-negative end of its subordinate group. The difference between the two divisions is very great in Groups VI and VII, but it is extreme in Group VIII, where heavy metals are on one side and inactive gases on the other. Many authorities separate these gases into a “Group O” by themselves at the left-hand side of the table, but this does not change their relative positions, and the plan may be objected to on the ground that many vacant places are thus left in the groups VIII and O.

The periodic law has been useful in rectifying certain atomic weights. At the outset Mendeléeff was obliged to change beryllium from 14·5 (assuming Be₂O₃) to 9 (assuming BeO), and later the atomic weights of indium and uranium were changed to make them fit the system. All of these changes have been confirmed by physical means.

Mendeléeff found a number of vacant places in his table, and was thus able to render further service to chemical science by predicting the properties of undiscovered elements, and his predictions were very closely confirmed by the later discovery of scandium, gallium, and germanium. The table indicates that there are still two undiscovered elements below manganese and probably two more among the rare-earth metals. The interesting observation has just recently been made by Soddy that the products of radioactive disintegration appear to pass in a symmetrical way through positions in the periodic system, giving off a helium molecule at alternate transformations until the place of lead is reached. It appears, therefore, that the five vacant places in the table above bismuth are probably occupied by these evanescent elements, and it is to be noticed that all of the elements that have been placed in this region of high atomic weights are radioactive.

There are some inconsistencies in the periodic system. The increments in the atomic weights are irregular, and there are three cases, argon and potassium, cobalt and nickel, and tellurium and iodine, where a higher atomic weight is placed before a lower one in order to bring these elements into their undoubtedly proper places. There is a peculiarity also in the heavy-metal division of Group VIII, where three similar elements occur in each of three places, and where the usual periodicity appears to be suspended, or nearly so, in comparison with most of the other elements. However, there seems to be a still more remarkable case of this kind in Group III, where fourteen metals of the rare earths have been placed. They are astonishingly similar in their chemical properties, hence it seems necessary to assume that periodicity is suspended here throughout the wide range of atomic weights from 139 to 174, where no elements save these have been found.

Several other interesting features of the table may be pointed out. The chlorides and hydrides, as indicated by the “typical compounds,” show a regular progression in both directions towards Group IV. (Where the type-formulas do not apply, as far as is known, to more than one or two elements, they have been placed in parentheses in the table given here.) It is a striking fact that the acid-forming elements occur together in a definite part of the table, and that the gases and other non-metallic elements, except the inactive gases of Group VIII, occur in the same region.

Atomic Numbers.—As the result of a spectroscopic study of the wave lengths or frequencies of the X-rays produced when cathode rays strike upon anticathodes composed of different elements, Moseley in 1914 discovered that whole numbers in a simple series can be attributed to the atoms. These atomic numbers are: 1 for hydrogen, 2 for helium, 3 for lithium, 4 for beryllium, and so on, in the order in which the elements occur in Mendeléeff’s periodic table, and in the cases of argon and potassium, cobalt and nickel, and tellurium and iodine, they follow the correct chemical order, while the atomic weights do not. They appear to indicate, therefore, an even more fundamental relation between the atoms than that shown by the atomic weights.

These numbers are now available for every element up to lead, and they are particularly interesting in indicating, on account of missing numbers, the existence of two undiscovered elements in the manganese group, and two more among the rare-earth metals, in confirmation of the vacant places below lead in Mendeléeff’s table.

The Isolation of Elements.—In the year 1818 about 53 elements were recognized, and since that time about 30 more have been discovered, but the elements already known comprised the more common ones, and nearly all of those which have been commercially important. A few of them, including beryllium, aluminium, silicon, magnesium, and fluorine, were then known only in their compounds, as they had not yet been isolated in the free condition.

Berzelius in 1823 prepared silicon, a non-metallic element resembling carbon in many respects. This element has recently been prepared on a rather large scale in electric furnaces at Niagara Falls, and has been used for certain purposes in the form of castings.

Wöhler created much sensation in 1827 by isolating aluminium and finding it to be a very light, strong and malleable metal, stable in the air, and of a silver-white color. For a long time this metal was a comparative rarity, being prepared by the reduction of aluminium chloride with metallic sodium; but about 25 years ago Hall, an American, devised a method of preparing it by electrolyzing aluminium oxide dissolved in fused cryolite. This process reduced the cost of aluminium to such an extent that it has now come into common use.

Wöhler and Bussy prepared beryllium in 1828, and Liebig and Bussy did the same service for magnesium in 1830. The latter metal has come to be of much practical importance, both as a very powerful reducing agent in chemical operations, and as an ingredient of flash-light powders and of mixtures used for fireworks. It is also used in making certain light alloys.

After almost innumerable attempts to isolate fluorine, during a period of nearly a century, this was finally accomplished in 1886 by Moissan in France by the electrolysis of anhydrous hydrogen fluoride. The free fluorine proved to be a gas of extraordinary chemical activity, decomposing water at once with the formation of hydrogen fluoride and ozonized oxygen. This fact explains the failure of many previous attempts to prepare it in the presence of water.

Early Discoveries of New Elements.—The remarkable activity of chemical research at the beginning of our period is illustrated by the fact that three new elements were discovered in 1817. In that year Berzelius had discovered selenium, Arfvedson, working in Berzelius’s laboratory had discovered the important alkali-metal lithium, and Stromeyer had discovered cadmium.

In 1826 Ballard in France discovered bromine in the mother-liquor from the crystallization of common salt from sea water. Bromine proved to be an unusually interesting element, being the only non-metallic one that is liquid at ordinary temperatures, and being strikingly intermediate in its properties between chlorine and iodine. It has been obtained in large quantities from brines, and is produced extensively in the United States. The elementary substance and its compounds have found important applications in chemical operations, while the bromides have been found valuable in medicine and silver bromide is very extensively used in photography.

In 1828 Berzelius discovered thorium. The oxide of this metal has recently been employed extensively as the principal constituent of incandescent gas-mantles, and the element has acquired particular importance from the fact that, like uranium, it is radioactive, decomposing spontaneously into other elements.

Vanadium had been encountered as early as 1801 by Del Rio, who named it “erythronium,” but a little later it was thought to be identical with chromium and was lost sight of for a while. In 1830, however, it was re-discovered by, and received its present name from Sefström in Sweden. Berzelius immediately made an extensive study of vanadium compounds, but he gave them incorrect formulas and derived an incorrect atomic weight for the element, because he mistook a lower oxide for the element itself. Roscoe in England in 1867 isolated vanadium for the first time, found the right atomic weight, and gave correct formulas to its compounds. Vanadium is particularly interesting from the fact that it displays several valencies in its compounds, many of which are highly colored. It has found important use as an ingredient in very small proportions in certain “special steels” to which it imparts a high degree of resistance to rupture by repeated shocks.

Columbium was discovered early in the nineteenth century in the mineral columbite from Connecticut by Hatchett, an Englishman, who did not, however, obtain the pure oxide. It was afterwards obtained by Rose who named it niobium. Both names for the element are in use, but the former has priority. Attention was called to this fact by an article in the Journal by Connell, an Englishman (18, 392, 1854).

The Platinum Group of Metals.—In 1854 a new member of the platinum group of metals, ruthenium, was discovered by Claus. Platinum had been discovered about the middle of the eighteenth century, while its other rarer associates, iridium, osmium, palladium, and rhodium, had been recognized in the very early years of the nineteenth century. It was during the latter period that platinum ware began to be employed to a considerable extent in chemical operations, and this use was greatly extended as time went on. The discovery was made by Phillips in 1831 that finely divided platinum by contact would bring about the combination of sulphur dioxide with atmospheric oxygen, and this application during the past 20 years has become enormously important in the sulphuric acid industry, while other important applications of platinum as a “catalytic agent” have also been made. Wolcott Gibbs and Carey Lea have contributed perhaps more than any other recent chemists to a knowledge of the platinum metals. Carey Lea (38, 81, 248, 1864) dealt chiefly with the separation of the metals from each other, while Gibbs’s work (31, 63, 1861; 34, 341, 1862) included investigations of many of the compounds.

It may be mentioned that while platinum and its associates were formerly known only in the uncombined condition in nature, the arsenide sperrylite, PtAs₂, was described by the late S. L. Penfield, and the senior writer of this chapter, in articles published in the Journal (37, 67, 71, 1889).

Applications of the Spectroscope.—The discovery in certain mineral waters of the rare alkali-metals rubidium and cæsium by Bunsen and Kirchoff in 1861 was in consequence of the application of spectroscopy by these same scientists a short time previously to the identification of elements imparting colors to the flame. Since that time the employment of the spectroscope for chemical purposes has been much extended, as it has been used in the examination of light from electric sparks and arcs, as well as from Geissler tube discharges and from colored solutions.

The metals rubidium and cæsium are interesting in being closely analogous to potassium and in standing at the extreme electro-positive end of the series of known metals. It should be noticed here that Johnson and Allen of our Sheffield Laboratory, having obtained a good supply of rubidium and cæsium material from the lepidolite of Hebron, Maine, made some important researches upon these elements, accounts of which were published in the Journal (34, 367, 1862; 35, 94, 1863). They established the atomic weight of cæsium, thus correcting Bunsen’s determination which was unsatisfactory on account of the small quantity and impurity of his material. Pollucite, a mineral rich in cæsium, which had been found in very small amount on the Island of Elba, has more recently been obtained in large quantities—hundreds of pounds—at Paris, Maine, and its vicinity. This American pollucite was first analyzed and identified by the senior writer of this article (41, 213, 1891), and later (43, 17, 1892 et seq.) the results of many investigations on cæsium and rubidium compounds, in which the junior writer played an important part, carried out in Sheffield Laboratory, were published in the Journal.

The application of the spectroscope led to the discovery of thallium in 1861 by Crookes of England, and to that of indium in 1863 by Reich and Richter in Germany. Both of these metals are extremely rare, but they are of considerable theoretical interest. Thallium is particularly remarkable in showing resemblances in its different compounds to several groups of metals.

The spectroscope was employed again in connection with the discovery of gallium in 1875 by Boisbaudran. It is in the same periodic group as thallium and indium, and it has a remarkably low melting point, just above ordinary room-temperature. It has been among the rarest of the rare elements, but within two or three years a source of it has been found in the United States in certain residues from the refining of commercial zinc. The recent issues of the Journal (41, 351, 1916; 42, 389, 1916) show that Browning and Uhler of Yale have availed themselves of this new material in order to make important chemical and physical researches upon this metal.

Germanium.—The discovery of germanium in the mineral argyrodite in 1886 by Winkler revealed a curious metal which gives a white sulphide that may be easily mistaken for sulphur and which is volatilized completely when its hydrochloric acid solution is evaporated, so that it is evasive in analytical operations. This element had been predicted with much accuracy by Mendeléeff, and it is rather closely related to tin.

A few years after the discovery of germanium, Penfield published in the Journal (46, 107, 1893; 47, 451, 1894) some analyses of argyrodite, correcting the formula given by Winkler to the mineral; also he described canfieldite, an analogous mineral from Bolivia, in which a large part of the germanium was replaced by tin.

The Rare Earths.—Before the year 1818 two rare earths, the oxides of yttrium and cerium, were known in an impure condition. Since that time about fourteen others have been discovered as associates of the first two. The rare earths are peculiar from the fact that many of them are always found mixed together in the minerals containing them, and also from the circumstance that most of them are remarkably similar in their chemical reactions and consequently exceedingly difficult to separate from each other. In many cases multitudes of fractional precipitations or crystallizations are needed to obtain pure salts of a number of these metals. The solutions of the salts of several of these elements give characteristic absorption bands when examined spectroscopically by the use of transmitted light.

No important practical application has been found for any of these earthy oxides, except that about one per cent of cerium oxide is mixed with thorium oxide in incandescent gas-mantles in order to obtain greatly increased luminosity.

The Inactive Gases.—As long ago as 1785, Cavendish, that remarkable Englishman who first weighed the world and first discovered the composition of water, actually obtained a little argon in a pure condition by sparking atmospheric nitrogen with oxygen converting it into nitric acid (another discovery of his) and absorbing the excess of oxygen. The volume of this residual gas as estimated by him corresponds very closely to the volume of argon in the atmosphere, as now known.

It was more than a century later, in 1894, that Rayleigh and Ramsay discovered argon in the air. Lord Rayleigh had found that atmospheric nitrogen was about one-half per cent heavier than chemical nitrogen, a fact which led to the investigation. It was only necessary to repeat Cavendish’s experiment on a large scale, or to absorb oxygen with hot copper and nitrogen with hot magnesium, in order to obtain argon. The gas attracted much attention, both on account of having but a single atom in its molecule, and particularly because it failed to enter into chemical combination of any kind. This gas has been used of late for filling the bulbs of incandescent electric lamps in cases where a gas pressure without chemical action is desired.

In 1890 and 1891, Hillebrand published in the Journal (40, 384, 1890: 42, 390, 1891) a series of analyses of the mineral uraninite and reported in some samples of the mineral as much as 2·5 per cent of an inactive gas. Hillebrand examined the gas spectroscopically but, just missing an important discovery, he detected only the spectrum lines of nitrogen. Ramsay, in searching for argon in some sort of natural combination, and doubtless remembering Hillebrand’s work, heated some cleveite, a variety of uraninite, and obtained, not argon, but a new gas. This gave a yellow spectrum-line corresponding to a line previously observed in the light of the sun’s corona and attributed to an element in the sun called helium. Helium, therefore, in 1895 had been found on the earth. This gas is a constant constituent of uranium minerals, as it is produced by the breaking down of radioactive elements. It has been found in very small quantity in the atmosphere, and is the most difficult of all known gases to liquefy, as its boiling point, as shown by Onnes in 1908, is only 4° above the absolute zero. It has not yet been solidified.

In 1898 Ramsay and Travers, by the use of ingenious methods of fractional distillation and absorption by charcoal, obtained three other much rarer inactive gases from the atmosphere which they called neon, krypton and xenon.

The inactive gases are all colorless, and as they form no chemical compounds they are characterized by their densities, which give their atomic weights, by their boiling points, and by their characteristic Geissler-tube spectra.

The gaseous radium emanation, or niton, belongs also to the inactive group, and it was also collected and studied by Ramsay who was compelled to work with only 0·0001 cc. of it, as the volume obtained by heating radium salts is very small. It is an evanescent element, disappearing within a few days on account of radioactive disintegration. Meanwhile it glows brilliantly when liquefied and cooled to the temperature of liquid air. It has an atomic weight of 222, four units below that of radium, and the difference is considered as due to the loss by radium of an atom of helium in passing into the emanation.

The Radioactive Elements.—The discovery of radium in 1898 by Madame Curie, and the study of that and other radioactive elements has produced a profound effect upon chemical theory. It was found that the two elements of the highest atomic weights, uranium and thorium, are always spontaneously decomposing into other elements at a fixed rate of speed which can be controlled by no artificial means, and that the elements resulting from these decompositions likewise undergo spontaneous changes into still other elements at greatly varying rates of speed, forming in each case a remarkable series of temporary elements. These transformations are accompanied by the emission at enormous velocities of three kinds of rays, one variety of which has been shown to consist of helium atoms. The greater number of the elements formed in these transformations have not as yet been obtained in a pure condition, and they are known only in connection with their radioactivity, volatility, etc.; but radium and niton, two of these products, have been obtained in a pure condition, so that their atomic weights and their places in the periodic system have been fixed.

We owe much of our knowledge of the radioactive transformations to the researches of Rutherford and of Soddy, and of their co-workers, but one of the important products of the transformation of uranium, an element which he called ionium, was characterized by Boltwood of Yale (25, 365, 1908).

Radium and niton, apart from their radioactive properties, resemble barium and the inert gases of the atmosphere, respectively. The rates at which their progenitors produce them, and the rates at which they themselves decompose, bring about a state of equilibrium after a time. Therefore a given amount of uranium, which decomposes exceedingly slowly, can yield even after thousands of years only a very small proportional quantity of undecomposed radium, one-half of which disappears in about 2500 years, because the amount decomposed must eventually be equal to the amount produced. The first conclusive evidence that radium is a product of the decomposition of uranium was given by Boltwood in the Journal (18, 97, 1904). He found that all uranium minerals contain radium; and the amount of radium present is always proportional to the amount of uranium, which shows the genetic relation between the two.

In the case of niton, which is produced by radium, and is called also the radium emanation, the rate of decay is rapid, so that if the gas is expelled from radium by heating, equilibrium is reached after a few days, with the accumulation of the largest possible amount of niton.

The conclusion has been reached by Rutherford and others that the final product besides helium, in the radioactive transformations, is lead, or at least an element or elements resembling lead to such a degree that no separation of them by chemical means is possible. Atomic weight determinations by Richards and others have shown that specimens of lead found in radioactive minerals give distinctly different atomic weights from that of ordinary lead. This fact has led to the view that possibly the atoms of the elements are not all of the same weight, but vary within certain limits—a view that is contrary to previous conclusions derived from the uniformity in atomic weights obtained with material from many different sources.

The results of the investigations upon radioactivity have led to modified views in regard to the stability of the elements in general. There has been little or no proof obtained that any artificial transmutation of the elements is possible, but the spontaneous transformation of the radioactive elements brings forward the possibility that other elements are changing imperceptibly, and that a state of evolution exists among them. All of the radioactive changes that we know proceed from higher to lower atomic weights, and we are entirely ignorant of the process by which uranium and thorium must have been produced originally.

Since radioactive changes have been found to be accompanied by the release of vast amounts of energy, compared with which the energy of chemical reactions is trivial, a new aspect in regard to the structure of atoms has arisen,—they must be complex in structure, the seats of enormous energy.

The determination of the amount of radium in the earth’s crust has indicated that the heat produced by it is amply sufficient to supply the loss of heat due to radiation, and this source of heat is regarded by many as the cause of volcanic action. The sun’s radiant heat also has been supposed to be supplied by radioactive action, so that the older views regarding the limitation of the age of the earth and the solar system on account of loss of heat have been considerably modified by our knowledge of radioactivity.

Physical Chemistry.

The application of physical methods as aids to chemical science began in early times, and some of these, such as the determinations of gas and vapor densities, specific heats, and crystalline forms have been mentioned already in this article. Within recent times physical chemistry has greatly developed and a few of its important achievements will now be described.

Molecular Weight Determinations.—Gas and vapor densities in connection with Avogadro’s principle, formed the only basis for molecular weight determinations until comparatively recent times. The early methods of Gay-Lussac and Dumas for vapor density were supplemented in 1868 by the method of Hofmann, whereby vapors were measured under diminished pressure over mercury. In 1878 Victor Meyer introduced a simpler method depending upon the displacement of air or other gas by the vapor in a heated tube. As refractory tubes, such as those of porcelain or even iridium, could be used in this method, molecular weights at extremely high temperatures were determined with interesting results. For instance, it was found that iodine vapor, which shows the molecule I₂ at lower temperatures, gradually becomes monatomic with rise in temperature, that sulphur vapor dissociates from S₈ to S₂ under similar conditions, and that most of the metals, including silver, have monatomic vapors.

In 1883 and later it was pointed out by Raoult that the molecular weights of substances could be found from the freezing points of their solutions, but this method was complicated from the fact that salts, strong acids and strong bases behaved quite differently from other substances in this respect, and allowances had to be made for the types of substances used. The complication was afterwards explained by the ionization theory of Arrhenius. Better apparatus for this method was soon devised by Beckmann, who introduced also a method depending upon the boiling points of solutions, and these two methods are still the standard ones for determining molecular weights in solution. They are very extensively employed by organic chemists.

It has been found that the majority of substances when dissolved have the same molecular weight as in the gaseous condition, provided that they can be volatilized at comparable temperatures. For instance, sulphur in solution has the formula S₈, iodine is I₂ and the metals are monatomic.

Van’t Hoff’s Law and Arrhenius’s Theory of Ions.—Modern views on solutions date largely from 1886, when van’t Hoff called attention to the relations existing between the osmotic pressure exerted by dissolved substances and gas pressure.

Pfeffer, a botanist, was the first to measure osmotic pressure (1877). Basing his conclusions chiefly upon Pfeffer’s determinations, van’t Hoff formulated a new and highly important law, which may be stated as follows: The osmotic pressure exerted by a substance in solution is equal to the gas pressure that the substance would exert if it were a gas at the same temperature and the same volume. Further investigations have fully established the fact that molecules in dilute solution obey the simple laws of gases.

It was pointed out by van’t Hoff that salts, strong acids and strong bases showed marked exceptions to his law in exerting much greater osmotic pressures than those calculated for them.

The next year in 1887, Arrhenius explained this abnormal behavior of salts, strong acids and strong bases by assuming that they dissociate spontaneously into ions when they dissolve, and that these more numerous particles act like molecules in producing osmotic pressure. He showed that these exceptional substances all conduct electricity in solution, while those conforming with van’t Hoff’s law do not, and according to his theory the ions become positively or negatively charged when they are formed, and these charged ions conduct the current. For example a molecule of sodium chloride was supposed to give the two ions Na⁺ and Cl^-, thus exerting twice as much osmotic pressure as a single molecule.

Determinations of osmotic pressure or related values, such as depression of the freezing point and of electric conductivity, indicated that ionization could not be regarded as complete in any case except in exceedingly dilute solutions, and that the extent of ionization varied with different substances. The fact that osmotic pressures and electric conductivities gave closely agreeing results in regard to the extent of ionization in various cases, is the strongest evidence in support of the theory.

It was difficult at first for many chemists to believe that atoms, such as those of sodium and chlorine, and groups such as NH₄ and SO₄ could exist independently in solution, even though electrically charged. However, the theory rapidly gained ground and is now accepted by nearly every chemist as a satisfactory explanation of many facts.

During recent years, many investigations relating to osmotic pressure and ionization have been carried out in the United States, but only the work of Morse, A. A. Noyes, and the late H. C. Jones can be merely alluded to here. It should be mentioned that the eminent author of the ionic hypothesis gave the Silliman Memorial course of lectures at Yale in 1911 on Theories of Solution.

Colloidal Solutions.—Graham, an English chemist, in 1861 was the first to make a distinction between substances forming true solutions, which he called crystalloids, and those of a gummy nature resembling glue, which in solution do not diffuse readily through parchment membranes, as crystalloids do, and which he called colloids. The separation of colloids by means of parchment was called dialysis, and this process has come into extensive use in preparing pure colloidal solutions. Slow diffusion is now regarded as characteristic of colloids rather than their gummy condition.

Colloidal solutions occupy an intermediate position between true solutions and suspensions, resembling one or the other according to the kind of colloid and the fineness of division. By preparing filters with pores of varying degrees of fineness, Bechold has been able to separate colloids from each other in accordance with the size of their particles. It has also been possible to prepare different solutions of a colloid varying gradually from one in which the particles were undoubtedly in suspension to one which had many of the properties of a true solution.

Beginning in 1889, Carey Lea described in the Journal (37, 476, 1889 et seq.) a variety of methods for preparing colloidal solutions of the metals, consisting in general of treating solutions of metallic salts with mild reducing agents. His work on colloidal silver was particularly extensive and interesting. Solutions of this kind have recently yielded some extremely interesting results by means of the ultra-microscope, an apparatus devised by Zsigmondy and Siedentopf. A very intense beam of light is passed through the solution and observed at right angles with a powerful microscope. Under these conditions, particles much too small to be seen by other means, reveal their presence by reflected light. It has been possible in a very dilute solution of known strength to count the particles and thus to calculate their size. The smallest colloidal particles measured in this way were of gold and were shown to have approximately ten times the diameter, or 1000 times the volume, attributed to ordinary molecules. It is of interest that the particles appear in rapid motion corresponding to the well-known Brownian movement.

The chemistry of colloids has now assumed such importance that it may be considered as a separate branch of the science. It has its own technical journal and deals largely with the chemistry of organic products. All living matter is built up of colloids, and hæmoglobin, starch, proteins, rubber and milk are examples of colloidal substances or solutions. Among inorganic substances, many sulphides, silicic acid, and the amorphous hydroxides, like ferric hydroxide, frequently act as colloids.

Law of Mass Action.—Berthollet about the beginning of the last century was the first chemist to study the effect of mass, or more correctly, the concentration of substances on chemical action. His views summarized by himself are as follows: “The chemical activity of a substance depends upon the force of its affinity and upon the mass which is present in a given volume.” The development of this idea, which is fundamentally correct, was greatly hindered by the fact that Berthollet drew the incorrect conclusion that the composition of chemical compounds depended upon the masses of the substances combining to produce them, a conclusion in direct contradiction to the law of definite proportions, and since this view was soon disproved by Proust and others, Berthollet’s law in its other applications received no immediate attention. Mitchell, however, pointed out in the Journal (16, 234, 1829) the importance of Berthollet’s work, and Heinrich Rose in 1842 again called attention to the effect of mass, mentioning as one illustration the effect of water and carbonic acid in decomposing the very stable natural silicates. Somewhat later several other chemists made important contributions to the question of the influence of concentration upon chemical action, but it was the Norwegians, Guldberg and Waage, who first formulated the law of mass action in 1867.

This law has been of enormous importance in chemical theory, since it explains a great many facts upon a mathematical basis. It applies particularly to equilibrium in reversible reactions, where it states that the product of the concentrations on the one side of a simple reversible equation bears a constant relation to the products of the concentrations on the other side, provided that the temperature remains constant. In cases of this kind where two gases or vapors react with two solids, the latter if always in excess may be regarded as constant in concentration, and the law takes on a simpler aspect in applying only to the concentrations of the gaseous substances. For example, in the reversible reaction

3Fe + 4H₂O ⇄ Fe₃O₄ + 4H₂,

which takes place at rather high temperatures, a definite mixture of steam and hydrogen at a definite temperature will cause the reaction to proceed with equal rapidity in both directions, thus maintaining a state of equilibrium, provided that both iron and the oxide are present in excess. If, however, the relative concentrations of the hydrogen and steam are changed, or even if the temperature is changed, the reaction will proceed faster in one direction than in the other until equilibrium is again attained.

The principle of mass action also explains why it is sometimes possible for a reversible reaction to become complete in either direction. For instance, in connection with the reaction that has just been considered, if steam is passed over heated iron and if hydrogen is passed over the heated oxide, the gaseous product in each case is gradually carried away, and the reaction continually proceeds faster in one direction than in the other until it is complete, according to the equations

3Fe + 4H₂O → 3Fe₃O₄ + 4H₂, and

Fe₃O₄ + 4H₂ → 3Fe + 4H₂O.

Many other well-known and important facts, both chemical and physical, depend upon this law. It explains the circumstance that a vapor-pressure is not dependent upon the amount of the liquid that is present; it also explains the constant dissociation pressure of calcium carbonate at a given temperature, irrespective of the amounts of carbonate and oxide present; in connection with the ionic theory, it furnishes the reason for the variable solubility of salts due to the presence of electrolytes containing ions in common; and it elucidates Henry’s law which states that the solubilities of gases are proportional to their pressures.

Ostwald, more than any other chemist, has been instrumental in making general applications of this law, and he made particularly extensive use of it in connection with analytical chemistry in a book upon this subject which he published.

The Phase Rule.—In 1876 Willard Gibbs of Yale published a paper in the Proceedings of the Connecticut Academy of Science on the “Equilibrium of Heterogeneous Substances,” and two years later he published an abstract of the article in the Journal (16, 441, 1878). He had discovered a new law of nature of momentous importance and wide application which is called the “Phase-Rule” and is expressed by a very simple formula.

The application of this great discovery to chemical theory was delayed for ten years, partly, perhaps, because it was not sufficiently brought to the attention of chemists, but largely it appears because it was not at first understood, since its presentation was entirely mathematical.

It was Rooseboom, a Dutch chemist, who first applied the phase-rule. It soon attracted profound attention, and the name of Willard Gibbs attained world-wide fame among chemists. When Nernst, who is perhaps the most eminent physical chemist of the present time, was delivering the Silliman Memorial Lectures at Yale a few years ago, he took occasion to place a wreath on the grave of Willard Gibbs in recognition of his achievements.

To understand the rule, it is necessary to define the three terms, introduced by Gibbs, phase, degrees of freedom and component.

By the first term, is meant the parts of any system of substances which are mechanically separable. For instance, water in contact with its vapor has two phases, while a solution of salt and water is composed of but one. The degrees of freedom are the number of physical conditions, including pressure, temperature and concentration, which can be varied independently in a system without destroying a phase. The exact definition of a component is not so simple, but in general, the components of a system are the integral parts of which it is composed. Any system made up of the compound H₂O, for instance, whether as ice, water or vapor, contains but one component, while a solution of salt and water contains two. Letting P, F, and C stand for the three terms, the phase-rule is simply

F = C + 2 − P

that is, the number of degrees of freedom in a system in equilibrium equals the number of components, plus two, minus the number of phases. The rule can be easily understood by means of a simple illustration. In a system composed of ice, water and water vapor, there are three phases and one component and therefore

F = 1 + 2 − 3 = 0

Such a system has no degrees of freedom. This means that no physical condition, pressure or temperature can be varied without destroying a phase, so that such a system can only exist in equilibrium at one fixed temperature, with a fixed value for its vapor-pressure.

For instance, if the system is heated above the fixed temperature, ice disappears and if the pressure is raised, vapor is condensed. If this same system of water alone contains but two phases, for instance, liquid and vapor, F = 1 + 2 − 2 = 1, or there is one degree of freedom. In such a system, one physical condition such as temperature can be varied independently, but only one, without destroying a phase. For instance, the temperature may be raised or lowered, but for every value of temperature there is a corresponding value for the vapor-pressure. One is a function of the other. If both values are varied independently, one phase will disappear, either vapor condensing entirely to water or the reverse. Finally if the system consists of one phase only, as water vapor, F = 2, or the system is divariant, which means that at any given temperature it is possible for vapor to exist at varying pressures.

The illustration which has been given relates to physical equilibrium, but the rule is applicable to cases involving chemical changes as well. In comparing the phase-rule with the law of mass action, it will be noticed that both have to do with equilibrium. The great advantage of the former is that it is entirely independent of the molecular condition of the substances in the different phases. For instance, it makes no difference so far as the application of the rule is concerned, whether a substance in solution is dissociated, undissociated or combined with the solvent. In any case, the solution constitutes one phase. On the other hand, the rule is purely qualitative, giving information only as to whether a given change in conditions is possible. The law of mass action is a quantitative expression so that when the value of the constant is once known, the change can be calculated which takes place in the entire system if the concentration of one substance is varied. The law, however, requires a knowledge of the molecular condition of the reacting substances, which may be uncertain or unknown, and chiefly on this account it has, like the phase-rule, often only a qualitative significance.

The phase rule has served as a most valuable means of classifying systems in equilibrium and as a guide in determining the possible conditions under which such systems can exist. As illustrations of its practical application, van’t Hoff used it as an underlying principle in his investigations on the conditions under which salt deposits have been formed in nature, and Rooseboom was able by its means to explain the very complicated relations existing in the alloys of iron and carbon which form the various grades of wrought iron, steel and cast iron.

Thermochemistry.—This branch of chemistry has to do with heat evolved or absorbed in chemical reactions. It is important chiefly because in many cases it furnishes the only measure we have of the energy changes involved in reactions. To a great extent, it dates from the discovery by Hess in 1840 of a fundamental law which states that the heat evolved in a reaction is the same whether it takes place in one or in several stages. This law has made it possible to calculate the heat values of a large number of reactions which cannot be determined by direct experiment.

Thermochemistry has been developed by a comparatively few men who have contributed a surprisingly large number of results. Favre and Silbermann, beginning shortly after 1850, improved the apparatus for calorimetric determinations, which is called the calorimeter, and published many results. At about the same time Julius Thomsen, and in 1873 Berthelot, began their remarkable series of publications which continued until recently. Thomsen’s investigations were published in 1882 in 4 volumes. It is probably safe to say that the greater part of the data of thermochemistry was obtained by these two investigators. The bomb calorimeter, an apparatus for determining heat values by direct combustion, was developed by Berthelot. The recent work of Mixter at Yale, published in the Journal, and of Richards at Harvard should be mentioned particularly. Mixter’s work in this field began in 1901 (12, 347). Using an improved bomb calorimeter, he has developed a method of determining the heats of formation of oxides by combustion with sodium peroxide. By this same method as well as by direct combustion in oxygen, he has obtained results which appear to equal or excel in accuracy any which have ever been obtained in his field of work. Richards’s work has consisted largely of improvements in apparatus. He developed the so-called adiabatic calorimeter which practically eliminates one of the chief errors in thermal work caused by the heating or cooling effect of the surroundings. This modification is being generally adopted where extremely accurate work is required.

Organic Chemistry.

One hundred years ago qualitative tests for a few organic compounds were known, the elements usually occurring in them were recognized, and some of them had been analyzed quantitatively, but organic chemistry was far less advanced than inorganic, and almost the whole of its enormous development has taken place during our period.

Berzelius made a great advance in the subject by establishing the fact, which had been doubted previously, that the elements in organic compounds are combined in constant, definite proportions. In 1823 Liebig brought to light the exceedingly important fact of isomerism by showing that silver fulminate had the same percentage composition as silver cyanate, a compound of very different properties. Isomeric compounds with identical molecular weight as well as the same composition have since been found in very many cases, and they have played a most important part in determining the arrangements of atoms in molecules. They have been found to be very numerous in many cases. For instance, three pentanes with the formula C₅H₁₂are known, all that are possible according to theory, and in each case the structure of the molecule has been established. On theoretical grounds it has been calculated that 802 isomeric compounds with the formula C₁₃H₂₈ are possible, while with more complex formulas the numbers of isomers may be very much greater.

A particularly interesting case of isomerism was observed by Wöhler in 1828, when he found that ammonium cyanate changes spontaneously into urea

(NH₄CNO → N₂H₄CO).

This was the first synthesis of an organic compound from inorganic material, and it overthrew the prevailing view that vital forces were essential in the formation of organic substances. A great many natural organic compounds have been made artificially since that time, and some of them, such as artificial alizarin, indigo, oil of wintergreen, and vanillin, have more or less fully replaced the natural products. The preparation of a vast number of compounds not known in nature, many of which are of practical importance as medicines, dyes, explosives, etc., has been another great achievement of organic chemistry.

The development of our present formulas for organic compounds, by means of which in many cases the relative positions of the atoms can be shown with the greatest confidence, has been gradual. Formulas based on the dualistic idea of Berzelius were used for some time, type-formulas, with the employment of compound radicals, came later, the substitution of atoms or groups of atoms for others in chemical reactions came to be recognized, but one of the most important steps was the recognition of the quadrivalence of carbon and the general application of valency to atoms by Kekulé about 1858. This led directly to the use of modern structural formulas which have been of the greatest value in the theoretical interpretation of organic reactions. It was Kekulé also who proposed the hexagonal ring-formula for benzene, C₆H₆, which led to exceedingly important theoretical and practical developments. The details of the formulas for many other rings and complex structures have been established since that time, and there is no doubt that the remarkable achievements in organic chemistry during the past sixty years have been much facilitated by the use of these formulas.

Many important researches in organic chemistry have been carried out in the United States, and the activity in this direction has greatly increased in recent years. In this connection the large amount of work of this kind accomplished in the Sheffield Laboratory, at present under the guidance of Professor T. B. Johnson, should be mentioned.

It has happened that comparatively few publications on organic chemistry have appeared in the Journal, but it may be stated that the preparation of chloroform and its physiological effects were described by Guthrie (21, 64, 1832). Unknown to him, it had been prepared by Souberain, a French chemist, the previous year, but the former was the first to describe its physiological action. Silliman gave a sample to Doctor Eli Ives of the Yale Medical School, who used it to relieve a case of asthma. This was the first use of chloroform in medical practice (21, 405, 1832). Guthrie also described in the Journal (21, 284, 1832) his new process for converting potato starch into glucose, a method which is essentially the same as that used to-day in converting cornstarch into glucose. Lawrence Smith (43, 301, 1842 et seq.), Horsford (3, 369, 1847 et seq.), Sterry Hunt (7, 399, 1849), Carey Lea (26, 379, 1858 et seq.), Remsen (5, 179, 1873 et seq.), and others have contributed articles on organic chemistry.

Agricultural Chemistry.

Until near the middle of the nineteenth century, it was believed that plants, like animals, used organic matter for food, and depended chiefly upon the humus of the soil for their growth. This view was held even long after it was known that plant leaves absorb carbon dioxide and give off oxygen, and after the ashes of plants had been accurately analyzed.

This incorrect view was overthrown by the celebrated German chemist, Liebig, who made many investigations upon the subject, and, properly interpreting previous knowledge, published a book in 1840 upon the application of chemistry to agriculture and physiology in which he maintained that the nutritive materials of all green plants are inorganic substances, namely, carbon dioxide, water, ammonia (nitrates), sulphates, phosphates, silica, lime, magnesia, potash, iron, and sometimes common salt. He drew the vastly important conclusion that the effective fertilization of soils depends upon replenishing the inorganic substances that have been exhausted by the crops.

The fundamental principles set forth by Liebig have been confirmed, and it has been found that the fertilizing constituents most commonly lacking in soils are nitrogen compounds, phosphates, and potassium salts, so that these have formed the important constituents of artificial fertilizers. Liebig himself found that humus is valuable in soils, because it absorbs and retains the soluble salts.

The foundation established by Liebig in regard to artificial fertilizers has led to an enormous application of these materials, much to the advantage of the world’s food supply.

It was Liebig’s belief, in accordance with the prevailing views, that decay and putrefaction as well as alcoholic and other fermentations were spontaneous processes, and when the eminent French chemist, Pasteur, in 1857, explained fermentation as directly caused by yeast, an epoch-making discovery which led to the explanation of decay and putrefaction by bacterial action and to the germ-theory of disease, the explanation was violently opposed by Liebig and other German chemists. Pasteur’s view prevailed, however, and since that time it has been found that various kinds of bacteria are responsible for the formation of ammonia from nitrogenous organic matter and also for the change of ammonia into the nitrates that are available as plant-food.

The long-debated question as to the availability of atmospheric nitrogen for plant-food was settled in 1886 by the discovery of Hellriegel that bacteria contained in nodules on the roots, especially of leguminous plants, are capable of bringing nitrogen into combination and furnishing it to the plants.

No more than an allusion can be made to agricultural experiment stations where soils, fertilizers, foods and other products are examined, and where other problems connected with agriculture are studied.

The late S. W. Johnson of Yale studied with Liebig and subsequently did much service for agricultural chemistry in this country, by his investigations, his teaching, and his writings. His book, “How Crops Grow,” published in 1868, gave an excellent account of the principles of agricultural chemistry. He did much to bring about the establishment of agricultural experiment stations in this country, and for a long time he was the director of the Connecticut Station.

In the Journal, as early as 1827, Amos Eaton (12, 370) published a simple method for the mechanical analysis of soils to determine their suitability for wheat-culture, and Hilgard, between 1872 and 1874, described an elaborate study of soil-analysis. J. P. Norton, a Yale professor, in 1847 (3, 322) published an investigation on the analysis of the oat, which was awarded a prize of fifty sovereigns by a Scotch agricultural society, while Johnson, Atwater, and others have contributed articles on the analysis of various farm products.

Industrial Acids and Alkalies.

One hundred years ago sulphuric acid was manufactured on a comparatively very small scale in lead chambers. In 1818, an English manufacturer of the acid introduced the modern feature of using pyrites in the place of brimstone, while the Gay-Lussac tower in 1827 and the Glover tower in 1859 began to be applied as great improvements in the chamber process. Within about twenty years the contact process, employing platinized asbestos, has replaced the old chamber process to a large extent. It has the advantage of producing the concentrated acid, or the fuming acid, directly.

During our period the manufacture of sulphuric acid has increased enormously. Very large quantities of it have been used in connection with the Leblanc soda process in its rapid development. It came to be employed extensively for absorbing ammonia in the illuminating-gas industry, which was in its infancy one hundred years ago. New industries such as the manufacture of “superphosphates” as artificial fertilizers, the refining of petroleum, the manufacture of artificial dyestuffs and many other modern chemical products have greatly increased the demand for it, while its employment in the production of nitric and other acids, and for many other purposes not already mentioned, has been very great.

The manufacture of nitric acid has been greatly extended during our period on account of its employment for producing explosives, artificial dyestuffs, and for many other purposes. Chile saltpeter became available for making it about 1852. This acid has been manufactured recently from atmospheric nitrogen and oxygen by combining them by the aid of powerful electric discharges. This process has been used chiefly in Norway where water-power is abundant, as it requires a large expenditure of energy. A still more recent method for the production of nitric acid depends upon the oxidation of ammonia by air with the aid of a contact substance, such as platinized asbestos.

The production of ammonia, which was very small a hundred years ago, has been vastly increased in connection with the development of the illuminating-gas industry and the employment of by-product coke ovens. This substance is very extensively used in refrigerating machines and also in a great many chemical operations, including the Solvay soda process. Ammonium salts are of great importance also as fertilizers in agriculture. The conversion of atmospheric nitrogen into ammonia on a commercial scale is a recent achievement. It has been accomplished by heating calcium carbide, an electric-furnace product made from lime and coke, with nitrogen gas, thus producing calcium cyanamide, and then treating this cyanamide with water under proper conditions. Another method devised by Haber consists in directly combining nitrogen and hydrogen gases under high pressure with the aid of a contact substance.

Leblanc’s method for obtaining sodium carbonate from sodium chloride by first converting the latter into the sulphate by means of sulphuric acid and then heating the sulphate with lime and coal in a furnace was invented as early as 1791, but it was not rapidly developed and did not gain a foothold in England until 1826 on account of a high duty on salt up to that time. Afterwards the process flourished greatly in connection with the sulphuric acid industry upon which it depended, and with the bleaching-powder industry which utilized the hydrochloric acid incidentally produced by it, and, of course, in connection with soap manufacture and many other industries in which the soda itself was employed.

About 1866 the Solvay process appeared as a rival to the Leblanc process. This depends upon the precipitation of sodium bicarbonate from salt solutions by means of carbon dioxide and ammonia, with the subsequent recovery of the ammonia. It has displaced the older process to a large extent, and it is carried on extensively in this country, for instance, at Syracuse, New York.

Other processes for soda depend upon the electrolysis of sodium chloride solutions. In this case caustic soda and chlorine are the direct products, and the chlorine thus produced and liquefied by pressure in steel cylinders, has become an important commercial article.

In earlier times wood-ashes were the source of potash and potassium salts. Wurtz in the Journal (10, 326, 1850) suggested the availability of New Jersey greensand as a source of potash and showed how this mineral could be decomposed, but it does not appear that this mineral has ever been utilized for the purpose. About 1861 the German potash-salt deposits began to be developed, and these have since become the chief source of this material. At present many efforts are being made to obtain potassium compounds from other sources, such as brines, cement-kiln dust, and feldspar and other minerals but thus far the results have not satisfied the demand.

Conclusion.

This account of chemical progress has given only a limited view of small portions of the subject, because the amount of available material is so vast in comparison with the space allowed for its presentation. Since the Journal has published comparatively little organic chemistry, it was decided to make room for a better presentation of other things by giving only a brief discussion of this exceedingly active and important branch of the science. For similar reasons industrial and metallurgical chemistry, and other branches besides, in spite of their great growth and importance, have been neglected, except for some incidental references to them, and some account of a few of the more important industrial chemicals.

It appears that we have much reason to be proud of the advances in chemistry that have been made during the Journal’s period, and of the part that the Journal has taken in connection with them, and there seems to be no doubt that this progress has not diminished during more recent times.

The present tendency of chemical research is evidently towards a still greater development of organic chemistry, and an increased application of physics and mathematics to chemical theory and practice.

The very great improvements that have been made in chemical education, both in the number of students and the quality of instruction, during the period under discussion, and particularly in rather recent times, gives promise for excellent future progress.

Note.

[153]. It appears that the most accurate experimental demonstration ever made of this law was that of E. W. Morley, published in the Journal (41, 220, 276, 1891). He showed that 2·0002 volumes of hydrogen combine with one volume of oxygen.

XI
A CENTURY’S PROGRESS IN PHYSICS

By LEIGH PAGE

Dynamics.—At the beginning of the nineteenth century mechanics was the only major branch of physical science which had attained any considerable degree of development. Two centuries earlier, Galileo’s experiments on the rate of fall of iron balls dropped from the top of the Leaning Tower of Pisa, had marked the origin of dynamics. He had easily disproved the prevalent idea that even under conditions where air resistance is negligible heavy bodies would fall more rapidly than light ones, and further experiments had led him to conclude that the increase in velocity is proportional to the time elapsed, and not to the distance traversed, as he had at first supposed. Less than a century later Newton had formulated the laws of motion in the same words in which they are given to-day. These laws of motion, coupled with his discovery of the law of universal gravitation, had enabled him to correlate at once the planetary notions which had proved so puzzling to his predecessors. His success gave a tremendous stimulus to the development and extension of the fundamental dynamical principles that he had brought to light, which culminated in the work of the great French mathematicians, Lagrange and Laplace, a little over a hundred years ago.

Newton’s laws of motion, it must be remembered, apply only to a particle, or to those bodies which can be treated as particles in the problem under consideration. In his “Mécanique Analytique” Lagrange extended these principles so as to make it possible to treat the motion of a connected system by a method almost as simple as that contained in the second law of motion. Instead of three scalar equations for each of the innumerably large number of particles involved, he showed how to reduce the ordinary dynamical equations to a number equal to that of the degrees of freedom of the system. This is made possible by a combination of d’Alembert’s principle, which eliminates the forces due to the connections between the particles, and the principle of virtual work, which confines the number of equations to the number of possible independent displacements. The aim of Lagrange was to make dynamics into a branch of analysis, and his success may be inferred from the fact that not a single diagram or geometrical figure is to be found in his great work.

Celestial Mechanics.—Almost simultaneously with the publication of the “Mécanique Analytique” appeared Laplace’s “Mécanique Celeste.” Laplace’s avowed aim was to offer a complete solution of the great dynamical problem involved in the solar system, taking into account, in addition to the effect of the sun’s gravitational field, those perturbations in the motion of each planet caused by the approach and recession of its neighbors. So successful was his analysis of planetary motions that his contemporaries believed that they were not far from a complete explanation of the world on mechanical principles. Laplace himself was undoubtedly convinced that nothing was needed beyond a knowledge of the masses, positions, and initial velocities of every material particle in the universe in order to completely predetermine all subsequent motion.

The greatest triumph of these dynamical methods was to come half a century later. The planet Uranus, discovered in 1781 by the elder Herschel, was at that time the farthest known planet from the sun. But the orbit of Uranus was subject to some puzzling variations. After sifting all the known causes of these disturbances, Leverrier in France and Adams in England independently reached the conclusion that another planet still more remote from the sun must be responsible, and computed its orbit. Leverrier communicated to Galle of Berlin the results of his calculations, and during the next few days the German astronomer discovered Neptune within one degree of its predicted position!

We shall mention but one other achievement of the methods of celestial mechanics. Those visitors of the skies, the comets, which become so prominent only to fade away and vanish perhaps forever, had interested astronomers from the earliest times. Soon after the discovery of the law of gravitation, Newton had worked out a method by which the elements of a comet’s orbit can be computed from observations of its position. It was found that the great majority of these bodies move in nearly parabolic paths and only a few in ellipses. Of the latter the most prominent is the brilliant comet first observed by Halley in 1681. It has reappeared regularly at intervals of seventy-six years; the last appearance in the spring of 1910 is no doubt well remembered by the reader. Kant had considered comets to be formed by condensing solar nebulæ, whereas Laplace had maintained that they originate in matter which is scattered throughout stellar space and has no connection with the solar system. A study of the distribution of inclinations of comet orbits by H. A. Newton (16, 165, 1878) of New Haven substantiated Laplace’s hypothesis, and led to the conclusion that the periodic comets have been captured by the attraction of those planets near to which they have passed. Of these comets a number have comparatively short periods, and are found to have orbits which are in general only slightly inclined to those of the planets, and are traversed in the same direction. Moreover, the fact that the orbit of each of these comets comes very close to that of Jupiter made it seem probable that they have been attached to the solar system by the attraction of this planet. Further confirmation of this hypothesis was furnished by H. A. Newton’s (42, 183 and 482, 1891) explanation of the small inclination of their orbits and the scarcity of retrograde motions among them.

In 1833 occurred one of the greatest meteoric showers of history. Olmstead (26, 132, 1834) and Twining (26, 320, 1834) of New Haven noticed that these shooting stars traverse parallel paths, and were the first to suggest that they must be moving in swarms in a permanent orbit. From an examination of all accessible records, H. A. Newton (37, 377, 1864; 38, 53, 1864) was able to show that meteoric showers are common in November, and of particular intensity at intervals of 33 or 34 years. He confidently predicted a great shower for Nov. 13th, 1866, which not only actually occurred but was followed by another a year later, showing that the meteoric swarm extended so far as to require two years to cross the earth’s orbit. H. A. Newton (36, 1, 1888) in America and Adams in England took up the study of meteoric orbits with great interest, and the former concluded that these orbits are in every sense similar to those of the periodic comets, implying that a swarm of meteors originates in the disintegration of a comet. In fact Schiaparelli actually identified the orbit of the Perseids, or August meteors, with Tuttle’s comet of 1862, and shortly after the orbit of the Leonids, or November meteors, was found to be the same as that of Tempel’s comet.

Electromagnetism.—During the eighteenth century much interest had been manifested in the study of electrostatics and magnetism. Du Fay, Cavendish, Michell and Coulomb abroad and Franklin in America had subjected to experimental investigation many of the phenomena of one or both of these sciences, and in the early years of the nineteenth century Poisson developed to a remarkable extent the analytical consequences of the law of force which experiment had revealed. Both Laplace and he made much use of the function to which Green gave the name “potential” in 1828, and which is such a powerful aid in solving problems involving magnetism or electricity at rest.

Meantime electric currents had been brought under the hand of the experimenter by the discoveries of Galvani and Volta. Large numbers of cells were connected in series, and interest seemed to lie largely in producing brilliant sparks or fusing metals by means of a heavy current. Hare (3, 105, 1821) of the University of Pennsylvania constructed a battery consisting of two troughs of forty cells each, so arranged that the coppers and zincs can be lowered simultaneously into the acid and large currents obtained before polarization has a chance to interfere. This “deflagrator” was used to ignite charcoal in the circuit, or melt fine wires, and was for some time the most powerful arrangement of its kind. That “galvanism” is something quite different from static electricity was the opinion of many investigators; Hare considered the heat developed to be the distinguishing mark of the electric current. He says: “It is admitted that the action of the galvanic fluid is upon or between atoms; while mechanical electricity when uncoerced, acts only upon masses. This difference has not been explained unless by my hypothesis, in which caloric, of which the influence is only exerted between atoms, is supposed to be a principal agent in galvanism.”

Questioning minds were beginning to suspect that there must be some connection between electricity and magnetism. For lightning had been known to make magnets of steel knives and forks, and Franklin had magnetized a sewing needle by the discharge from a Leyden jar. Finally Oersted of Copenhagen undertook systematic investigation of the effect of electricity on the magnetic needle. His researches were without result until during the course of a series of lectures on “Electricity, Galvanism, and Magnetism” delivered during the winter of 1819–20 it occurred to him to investigate the action of an electric current on a magnetic needle. At first he placed the wire bearing the current at right angles to the needle, with, of course, no result; then it occurred to him to place it parallel. A deflection was observed, for to his surprise the needle insisted on turning until perpendicular to the wire.

Oersted’s discovery that an electric current exerts a couple on a magnetic needle was followed a few months later by Ampère’s demonstration before the French Academy that two currents flowing in the same direction attract each other, while two in opposite directions repel. The story goes that a critic attempted to belittle this discovery by remarking that as it was known that two currents act on one and the same magnet, it was obvious that they would act upon each other. Whereupon Arago arose to defend his friend. Drawing two keys out of his pocket he said, “Each of these keys attracts a magnet; do you believe that they therefore attract each other?”

A few years later Ampère showed how to express quantitatively the force between current elements, and indeed developed to a considerable degree the equivalence between a closed circuit carrying a current and a magnetic shell. So convincing was his analysis and so thorough his discussion of the subject, that Maxwell said of this memoir half a century later, “The whole, theory and experiment, seems as if it had leaped, full grown and full armed, from the brain of the ‘Newton of electricity.’ It is perfect in form and unassailable in accuracy; and it is summed up in a formula from which all the phenomena may be deduced, and which must always remain the cardinal formula of electrodynamics.”

Shortly afterwards the dependence of a current on the conductivity of the wire used and the grouping of cells employed, was made clear by the work of Ohm. Many of his results were obtained independently by Joseph Henry (19, 400, 1831) of the Albany Academy, who described in 1831 a powerful electromagnet in which a great many coils of wire insulated with silk were wound around an iron core and connected in parallel with a single cell. He remarks in this paper that with long wires, as in the telegraph, many cells arranged in series should be used, whereas for several short wires connected in parallel a single cell with large plates is more efficient.

Current Induction.—Impressed by the fact that electric charges have the power of inducing other charges on neighboring conductors without coming into contact with them, Faraday was engaged in investigating the possibility of an analogous phenomenon in the case of electric currents. His idea at first seems to have been that a current should induce another current in any closed conducting circuit which happens to be in its vicinity. Experiment readily showed the falsity of this conception, but a brief deflection of the galvanometer in the secondary circuit was noticed at the instant of making and breaking the current in the primary. Further experiments showed that thrusting a permanent steel magnet into a coil connected to a galvanometer caused the needle to deflect. In fact Faraday’s report to the Royal Society on November 24th, 1831, contains a complete account of all experimental methods available for inducing a current in a closed circuit.

While Faraday is entitled to credit for the discovery of current induction by virtue of the priority of his publication, it must not pass unnoticed that Henry obtained many of the same experimental results independently and some even earlier. Henry was at this time instructor in mathematics at the Albany Academy, and seven hours of teaching a day made it well nigh impossible to carry on original research except during the vacation month of August. As early as the summer of 1830 he had wound 30 feet of copper wire around the armature of a horseshoe electromagnet and connected it to a galvanometer. When the magnet was excited, a momentary deflection was observed. “I was, however, much surprised,” he says, “to see the needle suddenly deflected from a state of rest to about 20° to the east, or in a contrary direction, when the battery was withdrawn from the acid, and again deflected to the west when it was re-immersed.” In addition a deflection was obtained by detaching the armature from the magnet, or by bringing it again into contact. Had the results of these experiments been published promptly, America would have been entitled to credit for the most important discovery of the greatest of England’s many great experimenters. But Henry desired first to repeat his experiments on a larger scale, and while new magnets were being constructed, the news of Faraday’s discovery arrived. This occasioned hasty publication of the work already done in an appendix to volume 22, 1832, of the Journal.

At almost the same time Henry made another important discovery and this time he was anticipated by no other investigator in making public his results. In the paper already referred to be describes the phenomenon known to-day as self-induction. “When a small battery is moderately excited by diluted acid and its poles, which must be terminated by cups of mercury, are connected by a copper wire not more than a foot in length, no spark is perceived when the connection is either formed or broken; but if a wire thirty or forty feet long be used, instead of the short wire, though no spark will be perceptible when the connection is made, yet when it is broken by drawing one end of the wire from its cup of mercury a vivid spark is produced.... The effect appears somewhat increased by coiling the wire into a helix; it seems to depend in some measure on the length and thickness of the wire; I can account for these phenomena only by supposing the long wire to become charged with electricity which by its reaction on itself projects a spark when the connection is broken.”

Soon after, Henry went to Princeton and there continued his experiments in electromagnetism. No difficulty was experienced in inducing currents of the third, fourth and fifth orders by using the first secondary as primary for yet another secondary circuit, and so on (38, 209, 1840). The directions of these currents of higher orders when the primary is made or broken proved puzzling at first, but were satisfactorily explained a year later (41, 117, 1841). In addition induced currents were obtained from a Leyden jar discharge. Faraday failed to find any screening effect of a conducting cylinder placed around the primary and inside the secondary. Henry examined the matter, and found that the screening effect exists only when the induced current is due to a make or break of the primary circuit, and not when it is caused by motion of the primary.

Henry’s work was mainly descriptive; it remained for Faraday to develop a theory to account for the phenomena discovered and to prepare the way for quantitative formulation of the laws of current induction. This he did in his representation of a magnetic field by means of lines of force; a conception which he found afterwards to be equally valuable when applied to electrostatic problems. Every magnet and every current gives rise to these closed curves; in the case of a magnet they thread it from south pole to north, while a straight wire bearing a current is surrounded by concentric rings. The connection between lines of force and the induction of currents is contained in the rule that a current is induced in a closed circuit only when a change takes place in the number of lines of force passing through it. Furthermore the dependence of the current strength on the conductivity of the wire employed has led to recognition of the fact that it is the electromotive force and not the current itself which is conditioned by the change in magnetic flux.

Great interest was attached to the utilization of the newly discovered forces of electromagnetism. In 1831 Henry (20, 340, 1831) described a reciprocating engine depending on magnetic attraction and repulsion, and C. G. Page (33, 118, 1838; 49, 131, 1845) devised many others. The latter’s most important work, however, was the invention of the Ruhmkorff coil. In 1836 (31, 137, 1837) he found the strongest shocks to be obtained, from a secondary coil of many windings forming a continuation of a primary of half the number of turns. His perfection of the self-acting circuit breaker (35, 252, 1839) widened the usefulness of the induction coil, and his substitution of a bundle of iron wires for a solid iron core (34, 163, 1838) greatly increased its efficiency.

Conservation of Energy.—Perhaps the most important advance of the nineteenth century has been the establishment of the principle of conservation of energy. Despite the fact that the “principe de la conservation des force vives” had been recognized by the French mathematicians of the early part of the century, the application of this principle even to purely mechanical problems was contested by some scientists. Through the early numbers of the Journal runs a lively controversy as to whether there is not a loss of power involved in imparting momentum to the reciprocating parts of a steam engine only to check the motion later on in the stroke. Finally Isaac Doolittle (14, 60, 1828), of the Bennington Iron Works, ends the discussion by the pertinent remark: “If there be, as is contended by one of your correspondents, a loss of more than one third of the power, in transforming an alternating rectilinear movement into a continuous circular one by means of a crank, I should like to be informed what would be the effect if the proposition were reversed, as in the case of the common saw mill, and in many other instances in practical mechanics.”

A realization of the equivalence of heat and mechanical work did not come until the middle of the century, in spite of the conclusive experiments of the American Count Rumford and the English Davy before the year 1800. So firmly enthroned was the caloric theory, according to which heat is an indestructible fluid, that evidence against it was given scant consideration. In fact the success of the analytical method introduced by Fourier in 1822 for the solution of problems in conduction of heat only added to the difficulties of the adherents of the kinetic theory. But recognition of heat as a form of energy was on the way, and when it came it made its appearance almost simultaneously in half a dozen different places. Perhaps Robert Mayer of Heilbronn was the first to state explicitly the new principle. His paper “On the Forces of Inorganic Nature” was refused publication in Poggendorff’s Annalen, but fared better at the hands of another editor. During the next few years Joule determined the mechanical equivalent of heat experimentally by a number of different methods, some of which had already been devised by Carnot. Of those he used, the most familiar consists in churning up a measured mass of water by means of paddles actuated by falling weights and calculating the heat developed from the rise in temperature. However, the work of the young Manchester brewer received little attention from the members of the British Association before whom it was reported until Kelvin showed them its significance and attracted their interest to it. Meanwhile Helmholtz had completed a very thorough disquisition on the conservation of energy not only in dynamics and heat but in other departments of physics as well. His paper on “Die Erhaltung der Kraft” was frowned upon by the members of the Physical Society of Berlin before whom he read it, and received the same treatment as Mayer’s from the editor of Poggendorff’s Annalen. Helmholtz’s “Kraft,” like the “vis viva” of other writers, is the quantity which Young had already christened energy. Not many years elapsed, however, until the convictions of Mayer, Joule, Kelvin and Helmholtz became the most clearly recognized of all physical principles. As early as 1850 Jeremiah Day (10, 174, 1850), late president of Yale College, admitted the improbability of constructing a machine capable of perpetual motion, even though the “imponderable agents” of electricity, galvanism and magnetism be utilized.

Thermodynamics.—The importance of the principle of conservation of energy lies in the fact that it unites under one rule such diverse phenomena as gravitation, electromagnetism, heat and chemical action. Another principle as universal in its scope, although depending upon the coarseness of human observations for its validity rather than upon the immutable laws of nature, was foreshadowed even before the first law of thermodynamics, or principle of conservation of energy, was clearly recognized. This second law was the consequence of efforts to improve the efficiency of heat engines. In 1824 Carnot introduced the conception of cyclic operations into the theory of such engines. Assuming the impossibility of perpetual motion, he showed that no engine can have an efficiency greater than that of a reversible engine. Finally Clausius expressed concisely the principle toward which Carnot’s work had been leading, when he asserted that “it is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature.” Kelvin’s formulation of the same law states that “it is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.”

The consequences of the second law were rapidly developed by Kelvin, Clausius, Rankine, Barnard (16, 218, 1853, et seq.) and others. Kelvin introduced the thermodynamic scale of temperature, which he showed to be independent of such properties of matter as condition the size of the degree indicated by the mercury thermometer. This scale, which is equivalent to that of the ideal gas thermometer, was used subsequently by Rowland in his exhaustive determination of the mechanical equivalent of heat by an improved form of Joule’s method. He found different values for different ranges in temperature, showing that the specific heat of water is by no means constant. Since then electrical methods of measuring this important quantity have been used to confirm the results of purely mechanical determinations.

The definition of a new quantity, entropy, was found necessary for a mathematical formulation of the second law of thermodynamics. This quantity, which acts as a measure of the unavailability of heat energy, was given a new significance when Boltzmann showed its connection with the probability of the thermodynamic state of the substance under consideration. If two bodies have widely different temperatures, a large amount of the heat energy of the system is available for conversion into mechanical work. From the macroscopic point of view this is expressed by saying that the entropy is small, or if the motions of the individual molecules are taken into account, the probability of the state is low. The interpretation of entropy as the logarithm of the thermodynamic probability has thrown much light on the meaning of this rather abstruse quantity. Gibbs’s “Elementary Principles in Statistical Mechanics” treats in detail the fundamental assumptions involved in this point of view, its limitations and its consequences. In his “Equilibrium of Heterogeneous Substances”[^[154]] he had already extended the principle of thermal equilibrium to include substances which are no longer homogeneous. The value of the chemical potential he introduced determines whether one phase is to gain at the expense of another or lose to it. It is unfortunate that the analytical rigor and austerity of his reasoning combined with lack of mathematical training on the part of the average chemist, delayed true appreciation of his work and full utilization of the new field which he opened up.

Liquefaction of Gases.—Meanwhile the problem of liquefying gases was attracting much attention on the part of experimental physicists. Faraday had succeeded in making liquid a number of substances which had hitherto been known only in the gaseous state. His method consists in evolving the gas from chemicals placed in one end of a bent tube, the other end of which is immersed in a freezing mixture. The high pressure caused by the production of the gas combined with the low temperature is sufficient to bring about liquefaction in many cases. Failure with other more permanent gases was unexplained until the researches of Andrews in 1863 showed that no amount of pressure will produce liquefaction unless the temperature is below a certain critical value. The method of reducing the temperature in use to-day depends on a fact discovered by Kelvin and Joule in connection with the free expansion of a gas. These investigators allowed the gas to escape through a porous plug from a chamber in which the pressure was relatively high. With the single exception of hydrogen, the effect of the sudden expansion is to cool the gas, and even with it cooling is found to take place after the temperature has been made sufficiently low. By this method all known gases have been liquefied. Helium, with a boiling point of –269°C, or only 4°C. above the absolute zero, was the last to be made a liquid, finally yielding to the efforts of Kammerlingh Onnes in 1907. This investigator[^[155]] finds that at temperatures near the absolute zero the electrical conductivity of certain substances undergoes a profound modification. For example, a coil of lead shows a superconductivity so great that a current once started in it persists for days after the electromotive force has ceased to act.

Electrodynamics.—Faraday’s representation of electric and magnetic fields by lines of force had been of great value in predicting the results of experiments in electromagnetism. But a more mathematical formulation of the laws governing these phenomena was needed in order to make possible quantitative development of the theory. This was supplied by Maxwell in his epoch-making treatise on “Electricity and Magnetism.” Starting with electrostatics and magnetism, he gives a complete account of the mathematical methods which had been devised for the solution of problems in these branches of the subject, and then turning to Ampère’s work he shows how the Lagrangian equations of motion lead to Faraday’s law if the single assumption is made that the magnetic energy of the field is kinetic. In the treatment of open circuits Maxwell’s intuition led to a great advance, the introduction of the displacement current. Consider a charged condenser, the plates of which are suddenly connected by a wire. A current will flow through the wire from the positively charged plate to the negative, but in the gap between the two plates the conduction current is missing. So convinced was Maxwell that currents must always flow in closed circuits, that he postulated an electrical displacement in the medium between the plates of a charged condenser, which disappears when the condenser is short-circuited. Thus even in the so-called open circuit the current flows along a closed path.

Maxwell’s theory of the electromagnetic field is based essentially on Faraday’s representation by lines of force of the strains and stresses of a universal medium. So it is not surprising that he was led to a consideration of the propagation of waves through this medium. The introduction of the displacement current made the form of the electrodynamic equations such as to yield a typical wave equation for space free from electrical charges and currents. Moreover, the disturbance was found to be transverse, and its velocity turned out to be identical with that of light. The conclusion was irresistible. That light could consist of anything but electromagnetic waves of extremely short length was inconceivable. In fact so certain was Maxwell of this deduction from theory that he felt it altogether unnecessary to resort to the test of experiment. For the electromagnetic theory explained so many of the details which had been revealed by experiments in light, that no doubt of its validity could be entertained. Even dispersion received ready elucidation on the assumption that the dispersing medium is made up of vibrators having a natural period comparable with that of the light passing through it.

Maxwell’s book was published in 1873. Fifteen years later, Hertz,[^[156]] at the instigation of Helmholtz, succeeded in detecting experimentally the electromagnetic waves predicted by Maxwell’s theory. His oscillator consisted of two sheets of metal in the same plane, to each of which was attached a short wire terminating in a knob. The knobs were placed within a short distance of each other, and connected to the terminals of an induction coil. By reflection standing waves were formed, and the positions of nodes and loops determined by a detector composed of a movable loop of wire containing an air gap. Thus the wave length was measured. Hertz calculated the frequency of his radiator from its dimensions, and then computed the velocity of the disturbance. In spite of an error in his calculations, later pointed out by Poincaré, he obtained very nearly the velocity of light for waves traveling through air, but a velocity considerably smaller for those propagated along wires. Subsequent work by Lecher, Sarasin and de la Rive, and Trowbridge and Duane (49, 297, 1895; 50, 104, 1895) cleared up this discrepancy, and showed the velocity to be in both cases identical with that of light. The last-named investigators increased the size of the oscillator until it was possible to measure the frequency by photographing the spark in the secondary with a rotating mirror. The positions of nodes and loops were obtained by means of a bolometer after the secondary had been tuned to resonance with the vibrator. The velocity thus found for electromagnetic waves along wires is within one-tenth of one percent of the accepted value of the velocity of light. Hertz’s later experiments showed that waves in air suffer refraction and diffraction, and he succeeded in polarizing the radiation by passing it through a grating constructed of parallel metallic wires.

In order to satisfy the law of action and reaction, it is found necessary to attribute a quasi-momentum to electromagnetic waves. When a train of such waves is absorbed, their momentum is transferred to the absorbing body, while if they are reflected an impulse twice as great is imparted. This consequence of theory, foreseen by Maxwell and developed in detail by Poynting, Abraham and Larmor, has been verified by the experiments of Lebedew, and Nichols and Hull.[^[157]] The latter used a delicate torsion balance from which was suspended a couple of silvered glass vanes. In order to eliminate the effect of impulses imparted by the molecules of the residual gas, such as Crookes had observed in his radiometer, readings were made at many different pressures and the ballistic rather than the static deflection recorded. After the pressure produced by light from a carbon arc had been measured, the intensity of the radiation was determined with a bolometer. Preliminary experiments indicated the existence of a pressure of the order expected, and later more careful measurements showed good quantitative agreement with theory. This pressure had already found an important application in Lebedew’s explanation of the solar repulsion of comet’s tails. These tails are made up of enormous swarms of very minute particles, and as the comet swings around the sun they suffer a repulsion due to the pressure of the intense solar radiation which counteracts the sun’s gravitational attraction. Hence the tail, instead of following after the comet in its orbit, points in a direction away from the sun.

Some uncertainty existed as to whether a convection current produces a magnetic field. A compass needle is deflected by a current from a Daniell cell; is the same effect obtained when a conductor is charged electrostatically and then whirled around the needle by means of an insulating handle? The experimental difficulties involved in settling this question are realized when the enormous difference between the electrostatic and electromagnetic units of current is taken into consideration. For a sphere one centimeter in radius, charged to a potential of 20,000 volts, and revolving in a circle sixty times a second, constitutes a current of little over a millionth of an ampere.

This problem was undertaken by Rowland (15, 30, 1878) in Helmholtz’s laboratory at Berlin in 1876. A hard rubber disk coated on both sides with gold was charged and rotated about a vertical axis at a rate of sixty revolutions a second. On reversing the sign of the electrification on the disk, the astatic needle hung above its center showed a deflection of over five millimeters. The current was calculated in electrostatic units from the charge on the disk and its rate of motion, and in electromagnetic units from the magnetic deflection. The ratio of these two quantities gave fair agreement with its theoretical value, the velocity of light.

Although the result of this experiment was confirmed by Rowland and Hutchinson in 1889, Crémieu was convinced by an investigation carried out at Paris in 1900 that the Rowland effect did not exist. Consequently further repetition of the experiment was desirable. So the following year Adams (12, 155, 1901) arranged two rings of eight spheres each so that they could be rotated about their common axis from fifty to sixty times a second. One set of spheres was connected by brushes to the positive pole of a battery of 20,000 volts, the other to the negative pole. The deflection of a nearby magnetometer needle was observed when the electrification of the two rings was reversed, and from the reading so obtained the ratio of the electromagnetic to the electrostatic unit of current computed. This quantity was found to differ from the velocity of light by only a few percent. This experiment and the even more exhaustive investigations carried out by Pender, both independently and in collaboration with Crémieu, finally convinced the scientific world that a convection current produces the same magnetic field as a conduction current of the same magnitude.

In discussing the ponderomotive force experienced in a magnetic field by a conductor through which a current is passing, Maxwell had said, “It must be carefully remembered, that the mechanical force which urges a conductor carrying a current across the lines of magnetic force, acts, not on the electric current, but on the conductor which carries it.” Hall (19, 200, 1880), one of Rowland’s students, questioned this statement, and determined to put it to the test of experiment. Efforts to find an increase in the resistance of a wire placed at right angles to the lines of magnetic force were unsuccessful. So the current was passed through a moderately broad strip of gold leaf and the effect of the magnetic field on the equipotential lines investigated. The results obtained confirmed Hall’s belief that the force exerted by the field acts on the current itself, and is transmitted through it to the conductor. Further investigation (20, 161, 1880) revealed the same deflection of equipotential lines in thin strips of other metals, although the effect was found to be reversed in iron.

During the closing years of the nineteenth century occurred three events of far reaching importance. The electron was isolated, and its charge and mass measured by J. J. Thomson in England; X-rays were discovered by Röntgen in Germany; and the first indications of radioactivity were found by Becquerel in France. The first two are certainly to be attributed largely to the great advances which had been made in obtaining high vacua, and the last two might not have occurred so soon had it not been for the photographic plate.

The Electron.—The atomic theory of electricity dates from the time of Faraday. His experiments on electrolysis showed that each monovalent atom or radical, whatever its nature, carries the same charge, each bivalent ion a charge twice as great. Only a lack of knowledge of the number of atoms in a gram of the dissociated salt prevented him from calculating the value of the elementary charge. As the discharge of electricity through gases at low pressures became a subject for experimental investigation, another line of approach to the study of the atom of electricity was opened up. As early as the seventies Hittorf and Goldstein had observed that a shadow is cast by a screen placed in front of the cathode of a Crookes tube. Varley suggested that the cathode rays producing the shadow consist of “attenuated particles of matter, projected from the negative pole by electricity.” The discovery that these rays are deflected by a magnetic field led English physicists to the conclusion that they must be composed of charged particles, and the direction of the deflection was such as to require the charge to be negative. Hertz contested this view on the ground that his experiments showed the rays to be unaffected by an electrostatic field, and suggested that they consist of etherial disturbances. Finally Perrin succeeded in passing the rays into a metal cylinder which received from them a negative charge, and Lenard showed how excessively minute these negatively charged particles must be by actually passing them through a thin sheet of aluminium in the wall of a vacuum tube, and detecting their presence in the air outside. Conclusive information as to the nature of the electron, as it was named by Johnstone Stoney, was supplied by the classic experiments of J. J. Thomson.[^[158]] First he showed that Hertz’s failure to find a deflection when a stream of electrons passes between the plates of a charged condenser was due to the screening effect of the gaseous ions produced by the discharge. With a much more highly evacuated tube he found no difficulty in obtaining a deflection in an electrostatic field. By using crossed electric and magnetic fields the deflection produced by one was just balanced by that caused by the other, and from the field strengths employed both the velocity of the particles and the ratio e
m of charge to mass was calculated. The former was found to be about one-tenth the velocity of light, but the most startling result of the experiment was that the same value of e
m was obtained no matter what residual gas was contained in the tube or of what metal the cathode was made.

To calculate e and then m other methods are necessary. C. T. R. Wilson has shown that in supersaturated air, water drops form easily on charged molecules, and that negative ions are more effective in causing condensation than positive ones. By making use of the results of this research Thomson has been able to measure the elementary charge. For suppose a stream of negative ions to pass through supersaturated air. A little drop forms on each charged particle, and the cloud of condensed vapor settles to the bottom of the vessel. The charge carried and the mass of water deposited can be measured directly. Stokes’ law for the rate of fall of a minute particle through a gaseous medium enables the average size of the drops to be computed from the observed rate of descent of the cloud. Hence the number of drops formed and the charge carried by each follows at once. H. A. Wilson improved the method by noting the effect of an electric field upon the rate of fall of the charged drops, and subsequent experiments undertaken by Millikan[^[159]] have been of such a character as to enable him to follow the motion of a single drop. Instead of water, the latter uses oil drops less than one ten-thousandth of a centimeter in diameter. A drop, after one or more electrons have attached themselves to it, is actually weighed in terms of the charge on its surface by applying an upward electric force just sufficient to balance the force of gravity. Then its weight is independently obtained from the density of the oil and the radius of the drop as determined by the rate of fall when the electric field is absent. Comparison of these two expressions gives 4·774(10)^–10 electrostatic units for the elementary charge. Combining this result with the value of e
m found by Thomson, the mass of the electron comes out to be about one eighteen-hundredth that of an atom of the lightest known element, hydrogen.

That the electron is a fundamental constituent of all matter is attested by the fact that charge and mass are the same regardless of the source or manner of production. Whether emitted by a heated metal, under the action of ultra-violet light, from a radioactive substance, by a body exposed to X-rays, as a result of friction, it is the same negatively charged particle that constitutes the cathode ray of the discharge tube. Moreover, it makes its effect felt indirectly in many other phenomena, and from an investigation of some of these the ratio of charge to mass can be determined independently. Of such perhaps the most interesting is the Zeeman effect.

Spectroscopy.—Early in the nineteenth century Fraunhofer had observed that the solar spectrum is crossed by a large number of dark lines. Their presence was unexplained until in 1859 Kirchhoff and Bunsen showed “that a colored flame, the spectrum of which contains bright sharp lines, so weakens rays of the color of these lines when they pass through it, that dark lines appear in place of bright lines as soon as there is placed behind the flame a light of sufficient intensity, in which the lines are otherwise absent.” For intra-atomic oscillators must have the natural frequency of the radiation which they emit, and consequently resonance will take place when they are exposed to rays of this frequency coming from an outside source, and selective absorption ensue. By comparing the bright lines in the spectra of metallic vapors made luminous by a gas flame with the dark lines in the sun’s spectrum these investigators showed that many of the common terrestrial elements exist in the sun. The interest in spectroscopy grew rapidly. The excellent diffraction gratings made by Rutherfurd were succeeded by the superior concave gratings of Rowland. In 1877 Draper (14, 89, 1877) announced the discovery of the bright lines of oxygen in the solar spectrum, but his interpretation of his photographs has not been corroborated by the work of later investigators. Langley (11, 401, 1901), by the aid of his newly invented bolometer, succeeded in detecting the emission of energy from the sun in the infra-red in amounts far exceeding that contained in the visible spectrum. In 1842 Doppler drew attention to the fact that motion of the source should cause a displacement of the spectral lines, the shift being to the blue if the light is approaching and to the red if it is receding, and a few years later Fizeau suggested the application of Doppler’s principle to the measurement of the velocity of a star moving in the line of sight. Thus the spectroscope has been able to supply one of the deficiencies of the telescope, and the two together are sufficient to reveal all components of stellar motion. When spectra formed by light from the sun’s limb and from its center are compared, the same effect reveals the rotation of the sun about its axis. (C. S. Hastings, 5, 369, 1873; C. A. Young, 12, 321, 1876.)

Further Evidence of the Electron.—In 1845 Faraday discovered a rotation of the plane of polarization when light passes in the direction of the lines of force through a piece of glass placed between the poles of an electromagnet. Examination of the spectrum from a glowing vapor situated between the poles of a magnet, however, failed to reveal any effect of the field. The latter problem was attacked anew by Zeeman[^[160]] in 1896, and with the aid of the improved appliances of modern science he succeeded in detecting a broadening of the lines. Later experiments with more powerful apparatus resolved these broadening lines into several components.

Lorentz[^[161]] showed at once how the electron theory furnishes an explanation of the Zeeman effect. He found that when the source is viewed at right angles to the lines of magnetic force, a spectral line should be split into three components. Of these he predicted that the middle, or undisplaced component, would be found to be polarized at right angles to the direction of the field, and the other components parallel to the field. When the light proceeds from the source in a direction parallel to the magnetic lines of force, two components only should be formed, and these should be circularly polarized in opposite senses. Moreover, from the separation of the components can be calculated the ratio of charge to mass of the electronic vibrator which is responsible for the emission of radiant energy. Zeeman’s experiments confirmed Lorentz’s theory in every detail, and yielded a value of e
m in substantial agreement with that obtained for cathode rays. Subsequent research, however, has shown that in many cases more components are found than the elementary theory calls for. Hale has detected the Zeeman effect in light from sun spots, proving that these blemishes on the sun’s face are vortices caused by whirling swarms of electrified particles. Recently Stark and Lo Surdo have found a similar splitting up of lines in the spectrum formed by light from canal rays (rays of positively charged particles) passing through an intense electric field. This phenomenon has as yet received no adequate explanation.

On discovering that an electric current is capable of producing a magnetic field, Ampère had suggested that the magnetic properties of such substances as iron might be explained on the assumption of molecular currents. The electron theory considers these currents to be due to the revolution, inside the atom, of negatively charged particles about an attracting nucleus. It occurred to Richardson that this motion should give the atom the properties of a gyrostat. Hence if an iron bar be rotated about its axis, the atoms should orient themselves so as to make their axes more nearly parallel to the axis of rotation. Thus its rotation should cause the bar to become a magnet. Barnett[^[162]] has tested this hypothesis, and has found the effect Richardson had predicted. From the strength of the magnetization produced, the value of e
m can be computed. Barnett finds a value somewhat smaller than that for cathode rays, but of the right order of magnitude and sign. Einstein and De Haas have detected the inverse of this effect, i. e., the rotation of an iron rod when it is suddenly magnetized.

X-Rays.—In 1895, on developing a plate which had been lying near a vacuum tube, Röntgen[^[163]] was surprised to find distinct markings on it. As the plate had never been exposed to light, it was necessary to suppose the effect to be due to some new and unknown type of radiation. Further investigation showed that this radiation originates at the points where cathode rays impinge on the glass walls of the tube. Besides being able to pass with ease through all but the most dense material objects X-rays were found to have the power of ionizing gases through which they pass and ejecting electrons from metal surfaces against which they strike. The points at which these electrons are produced are in turn the sources of secondary X-rays whose properties are characteristic of the metal from which they come.

Röntgen’s discovery excited intense interest among laymen as well as in scientific circles. Of the many X-ray photographs taken, those of Wright (1, 235, 1896) of Yale were the first to be produced in this country. His experiments were made immediately on receipt of the news of Röntgen’s research, and resulted in the publication of a number of photographs showing the translucency for these rays of paper, wood, and even aluminium.

As X-rays are undeviated by electric or magnetic fields, Schuster, and later Wiechert and Stokes, suggested that they might be electromagnetic waves of the same nature as light, but much shorter and less regular. The great objection to this hypothesis was the failure either to refract or diffract these rays. In fact Bragg contended that they were not etherial disturbances at all, but consisted of neutral particles moving with very high velocities. Finally Laue[^[164]] demonstrated their undulatory nature by showing that diffraction took place under proper conditions. Just as the distance between adjacent lines of a grating must be comparable to the wave length of light for a spectrum to be formed, a periodic structure with a grating space of their very much shorter wave length is necessary to diffract X-rays. Such a structure is altogether too fine to be made by human tools. Nature, however, has already prepared it for man’s use. The distance between the atoms of a crystal is just right to make it an excellent X-ray grating, and Laue had no difficulty in obtaining diffraction patterns when Röntgen rays were passed through a block of zincblende. The distance between adjacent atoms of this cubic crystal can be computed at once from its density and molecular weight, and then the wave length of the radiation calculated from the deviation suffered. In this way X-rays are found to have a length less than one thousandth as great as visible light. Further study of this phenomenon, particularly by the two Braggs, father and son, has revealed many of the structural details of more complicated crystals.

The most significant investigation in the field opened up by Laue’s discovery is that undertaken by Moseley[^[165]] only a couple of years before he lost his life in the trenches at Gallipoli. Using many different metals as anticathodes in a vacuum tube, he measured the frequencies of the characteristic rays emitted. He found that if the elements are arranged in order of increasing atomic weight, the square roots of the characteristic frequencies form an arithmetical progression. If to each element is assigned an integer, beginning with one for hydrogen, two for helium, and so on, the square root of the frequency of the characteristic radiation is found to be proportional to this atomic number. Even though Uhler has shown recently that over wide ranges Moseley’s law does not hold within the limits of experimental error, there is undoubtedly much significance to be attached to this simple relation.

Radioactivity.—The year following the discovery of X-rays, Becquerel found that a photographic plate is similarly affected by radiations from uranium salts. Two years later the Curies separated from pitchblende the very active elements polonium and radium. Passage of the rays from these substances through electric and magnetic fields revealed the existence of three types. The alpha rays have been shown by Rutherford and his co-workers to be positively charged helium atoms; the beta rays are very rapidly moving electrons; and the gamma rays are electromagnetic pulses of the same nature as X-rays but somewhat shorter. In 1902 Rutherford and Soddy advanced the theory of atomic disintegration, according to which the emission of a ray is an indication of the breaking down of the atom to a simpler form. Thus in the radioactive substances there is going on before our eyes a continual transformation of one element into another, a change, by the way, which appears to be in no slightest degree either hastened or delayed by changes in temperature (H. L. Bronson, 20, 60, 1905) or external electrical condition of the radioactive element. Uranium is the progenitor of a long line of descendants, of which radium was supposed for some time to be the first member. Boltwood (25, 365, 1908) of Yale, however, showed that the slow growth of radium in uranium solutions is incompatible with this assumption, and soon isolated an intermediate product which he named ionium. Radium itself disintegrates into a gas known as radium emanation, which in turn gives rise to a succession of other products. Analyses by Boltwood (23, 77, 1907) of radioactive minerals from the same locality show such a constant ratio between the amounts of uranium and lead present that it is natural to conclude that lead is the end product of the series. This hypothesis is confirmed by the fact that the oldest rocks show relatively the greatest amounts of this element.

In addition to the Ionium-Radium series two others have been discovered. Of these Boltwood’s (25, 269, 1908) investigations seem to indicate that the one which starts with actinium is a collateral branch of the radium series and comes from the same parent uranium. The other begins with thorium and comprises ten members. As yet the end products of the actinium and thorium series have not been identified, although there is some reason for believing that an isotope of lead may be the final member of the latter.

As the amount of a radioactive element which disintegrates in a given time is proportional to the total mass present, an infinite time would be required for the substance to be completely transformed. Hence the life of such an element is measured by the half value period, or time taken for half the initial mass to disintegrate. This time varies widely for different radioactive substances, ranging from a small fraction of a second for actinium A to five billion years for uranium. Boltwood’s (25, 493, 1908) original determination of the life of radium from the rate of its growth in a solution containing ionium gave 2000 years as its result, although recent measurements by Miss Gleditsch (41, 112, 1916) agree more closely with the value 1760 years obtained by Rutherford and Geiger from the number of alpha particles emitted.

Under the action of X-rays or the radiations from radioactive substances, gases acquire a conductivity which has been attributed by Thomson and Rutherford to the formation of ions. Zeleny has found that ions of opposite sign have somewhat different mobilities in an electric field, and experiments of Wellisch (39, 583, 1915) show that at low pressures some of the negative ions are electrons. T. S. Taylor (26, 169, 1908 et seq.) and Duane (26, 464, 1908) have investigated the ionization produced by alpha particles, and Bumstead (32, 403, 1911 et seq.) has studied the emission of electrons from metals which are bombarded by these rays. The investigations of Franck and Hertz, and McLennan and Henderson, show a significant relation between the ionizing potential (energy which must be possessed by an electron in order to produce an ion on colliding with an atom) and a quantity, to be considered later in more detail, which has been introduced by Planck into the theory of radiation.

Methods of Science.—Scientific progress seems to follow a more or less clearly defined path. Experimentation brings to light the hidden processes of nature, and hypotheses are advanced to correlate the facts discovered. As more and more phenomena are found to fit into the same scheme, the hypotheses at first proposed tentatively, although often only after extensive alterations, become firmly established as theories. Finally there may appear a fundamental clash between two theories, each of which in its respective domain seems to represent the only possible manner in which a large group of phenomena can be correlated. The maze becomes more perplexing at every step. At last a genius appears on the scene, approaches the problem from a new and unsuspected point of view, and the paradox vanishes. Such changes in point of view are the milestones which mark the progress of science. That science is stagnant whose only function is to collect, classify and correlate vast stores of experimental data. The sign of vitality is the existence of clearly defined and fundamental problems any possible solution of which seems irreconcilable with the most basic truths of the science in question. The greater the paradox grows, the more certain the advent of a new point of view which will bring one step nearer the comprehensive picture of nature which is the goal of natural philosophy.

The Ether.—From the earliest times philosophers have been attracted by the possibility of explaining physical phenomena in terms of an all-pervading medium. So strong had this tendency become by the middle of the nineteenth century that the English school of physicists were attributing rigidity, density and nearly all the properties of material media to the ether. In fact most physicists seemed to have forgotten that no experiment had ever given direct evidence of the existence of such a medium. Not until the first decade of the twentieth century was it realized that the experimental evidence actually pointed in quite the opposite direction, and that a new point of view was needed in dealing with those phenomena of light and electromagnetism which had been previously described in terms of a universal medium. Some account of the development of the ether theory and of the origin and growth of the point of view which has its principal exemplification in the principle of relativity is essential for an understanding of present tendencies in formulating a philosophic basis for scientific thought.

In the time of Newton and for a century after there was much controversy between the adherents of two irreconcilable theories of light. Hooke had suggested that light is a wave motion traveling through a homogeneous medium which fills all space, and Huygens had shown that the law of refraction can be deduced at once from this hypothesis if it is assumed that the velocity of light in a transparent body is less than that in free ether. However, Newton, impressed by the fact that a ray obtained by double refraction in Iceland spar differs from a ray of ordinary light just as a rod of rectangular cross section differs from one of circular cross section, and seeing no way of explaining this dissymmetry in terms of a wave motion analogous to longitudinal sound waves, adhered to the view that light consists of infinitesimal particles shot out from the luminous body with enormous velocities. So great was his reputation on account of his discoveries in other fields that this theory of light held sway among his contemporaries and successors until the labors of Young and Fresnel at the beginning of the nineteenth century definitely established the undulatory theory. However, in spite of the fact that a corpuscular theory of light made the assumption of an ether unnecessary in so far as the simpler of the observed phenomena are concerned, even Newton postulated the existence of such a medium, partly in order to explain the more complicated results of experiments in light, and partly in order to provide a vehicle for the propagation of gravitational forces.

Now an ether, if it is to explain anything at all, must have at least some of the simpler properties of material media. The most fundamental of these, perhaps, is position in space. As a first approximation in explaining optical phenomena on the earth’s surface, the earth might be supposed to be at rest relative to the ether. But the establishment of the Copernican system made the sun the center of the solar system and gave the earth an orbital speed of eighteen miles a second. It may be remarked parenthetically that the speed of a point on the equator due to the earth’s diurnal rotation is quite insignificant compared to its orbital velocity. Hence as a second approximation the sun might be considered at rest relative to the ether and the earth as moving through this unresisting medium.

The first indication of this motion lay in the discovery of aberration by the British astronomer Bradley in 1728. Bradley noticed that stars near the pole of the ecliptic describe small circles during the course of a year, while those in the plane of the ecliptic vibrate back and forth in straight lines, stars in intermediate positions describing ellipses. The surprising thing, however, was that the time taken to complete one of these small orbits is in all cases exactly a year. Bradley concluded that the phenomenon is in some way dependent on the earth’s motion around the sun, and he was not long in reaching the correct explanation. For suppose the earth to be at rest. Then in observing a star at the pole of the ecliptic it would be necessary to keep the axis of the telescope exactly at right angles to the plane of the earth’s orbit. However, as the earth is in motion, the telescope must be pointed a little forward, just as in walking rapidly through the rain an umbrella must be inclined forward so as to intercept the raindrops which would otherwise fall on the spot to be occupied at the end of the next step. The angle through which the telescope has to be tilted is known as the angle of aberration, and the tangent of this angle may easily be shown to be equal to the ratio of the velocity of the earth to the velocity of light. Knowing the velocity of the earth, the velocity of light can then be calculated. This method was one of the first of obtaining the value of this important quantity.

More recently, terrestrial methods of great precision have been devised for measuring the velocity of light. The most accurate of these is that employed by the French physicist Foucault in 1862. A ray of light is reflected by a rotating mirror to a fixed mirror placed at some distance, which in turn reflects the ray back to the moving mirror. The latter, however, has turned through a small angle during the time elapsed since the first reflection, and consequently the direction of the ray on returning to the source is not quite opposite to that in which it had started out. This deviation in direction is determined from the displacement of the image formed by the returning light, and from it the velocity of light is calculated. In order to make the deflection appreciable the distance between the two mirrors should be very great. As originally arranged by Foucault, it was found impractical to make this distance greater than twenty meters, and consequently the displacement of the image was less than a millimeter. Such a small deflection limited the accuracy of the experiment to one percent. In 1879, however, Michelson (18, 390, 1879), then a master in the United States Navy, improved Foucault’s optical arrangements to such an extent that he was able to use a distance of nearly seven hundred meters between the two mirrors. With a rate of two hundred and fifty-seven revolutions a second for the rotating mirror, the displacement obtained was over thirteen centimeters. This experiment gave 299,910 kilometers a second for the velocity of light, with a probable error of one part in ten thousand. Later investigations by Newcomb and Michelson (31, 62, 1886) gave substantially the same result. So great has been the accuracy of these terrestrial determinations that recent practice has been to calculate from them and the angle of aberration the earth’s orbital velocity, and hence the distance of the earth from the sun. This indirect method of measuring the astronomical unit has a probable error no greater than the best parallax methods of the astronomer. (J. Lovering, 36, 161, 1863.)

Aberration is a first order effect, i. e., it depends upon the first power of the ratio of the velocity of the earth to the velocity of light, and at first sight it seemed to prove conclusively that the earth must be in motion relative to the luminiferous medium. Other questions had to be settled, however, and one of these was whether or not light coming from a star would be refracted differently when passing through optical instruments from light which had a terrestial origin. Arago subjected the matter to experiment, and concluded that in every respect the light from a star behaved as if the earth were at rest and the star actually occupied the position which it appears to occupy on account of aberration. Finally optical experiments with terrestrial sources seemed to be in no way affected by the motion of the earth through the ether.

In order to account for these facts Fresnel advanced the following theory. To explain the refraction that takes place when light enters a transparent body, it is necessary to assume that light waves travel more slowly through matter than in free ether. Now the velocity of sound is known to vary inversely with the square root of the density of the material medium through which it passes. Hence it is natural to assume that ether is condensed inside material objects to such an extent that this same relation connects its density with the velocity of light traveling through it. But when a lens or prism is set in motion, Fresnel supposed it to carry along only the excess ether which it contains, ether of the normal density remaining behind. This assumption suffices to explain Arago’s results, and yet fits in with the phenomenon of aberration. It gives for light traveling in the direction of motion through a moving material medium of index of refraction n an absolute velocity greater than that when the medium is at rest by an amount

(1 − ¹/_n²)v,

which is only a fraction of the velocity v which would have to be added if convected matter carried along all the ether which resides within it. This expression was tested directly, first by Fizeau in 1851, and later by Michelson and Morley (31, 377, 1886) in this country. The experiment consists in bifurcating a beam of light, passing one-half in one direction and the other in the opposite direction through a stream of running water. On reuniting the two rays the usual interference fringes are produced. Reversing the direction of motion of the water causes the fringes to shift, and from the amount of this shift the velocity imparted to the light by the motion of the stream is computed. The divergence between the experimental value of this quantity and that calculated from Fresnel’s coefficient of entrainment was found by Michelson and Morley to be less than one percent, which was about their experimental error. Thus Fresnel’s expression for the velocity of light in a moving medium is entirely confirmed by experiment. The derivation of it accepted to-day, however, is very different from his original deduction.

It has been noted that the phenomena of polarization led Newton to reject the wave theory of light. The only type of wave known to him was the longitudinal wave, in which the vibrations of the particles of the medium are in the same direction as that of propagation of the wave, and it was impossible to suppose that such a wave could have different properties in different directions at right angles to the line in which it is advancing. But in 1817 Young suggested that this inconsistency between the wave theory and the facts of polarization could be removed by supposing the vibrations constituting light to be executed at right angles to the direction of propagation. Thus in ordinary light the vibrations are to be conceived as taking place haphazard in all directions in the plane perpendicular to the ray, while in plane polarized light these vibrations are confined to a single direction. This supposition explained so many of the puzzling results of experiment, that it was accepted at once and led to the complete vindication of the undulatory theory.

Elastic Solid Theory.—Shortly afterwards Poisson succeeded in solving the differential equation which determines the motion of a wave through an elastic medium. His solution shows that such a medium is capable of transmitting two types of wave—one longitudinal, the other transverse. If κ denotes the volume elasticity, η the rigidity and ρ the density of the medium, the velocities of the two waves are respectively

√(^{(κ + (⁴⁄₃)η)}/_ρ) and √(^η/_ρ)

Now a solid has both compressibility and rigidity, and transmits in general both types of wave. A fluid, on the other hand, on account of its lack of rigidity, cannot support a transverse vibration. Hence it was natural that Green, in searching for a dynamical explanation of the ether, should have proposed in a paper read before the Cambridge Philosophical Society in 1837 that the ether has the elastic properties of a solid. One great difficulty presented itself; disturbances inside an elastic solid must give rise to compressional as well as to transverse waves. But no such thing as a compressional wave had been found in the experimental study of light. Green attempted to overcome this difficulty by attributing an infinite volume elasticity to the ether. The expression above shows that longitudinal waves originating in such an incompressible medium would be carried away with an infinite velocity, and it may be shown that the energy associated with them would be infinitesimal in amount. The next step was to calculate the coefficients of transmission and reflection for light passing from one material medium to another. Here the elastic solid theory is not altogether successful. If the ether is supposed to have different densities in the two media, as in Fresnel’s theory, but the same rigidity, certain of these coefficients fail to give the values demanded by experiment, while if the densities are assumed the same but the rigidities different, other of the coefficients have discordant values. In connection with the phenomena of double refraction even more serious difficulties are encountered.

Electromagnetic Theory.—It was beginning to be felt that an ether must explain more than the phenomena of light, for Faraday’s conception of electromagnetic action as carried on through the agency of a medium had added greatly to its functions. Finally Maxwell’s demonstration that electromagnetic waves are propagated with the velocity of light made the theory of light into a subdivision of electrodynamics. Maxwell himself did not apply electromagnetic theory to the explanation of reflection and refraction. This deficiency, however, was remedied by Lorentz in 1875. The results obtained, as well as those for double refraction (J. W. Gibbs, 23, 262, 1882 et seq.), and metallic reflection (L. P. Wheeler, 32, 85, 1911), provided a complete vindication of the electromagnetic theory of light. This is all the more significant when the extreme precision obtainable in optical experiments is taken into account. For instance, Hastings (35, 60, 1888) has tested Huygens’ construction for double refraction in Iceland spar and found that “the difference between a measured index of refraction ... at an angle of 30° with the crystalline axis, and the index calculated from Huygens’ law and the measured principal indices of refraction” is a matter of only 4–5 units in the sixth decimal place. Since Maxwell’s time the gamut of electromagnetic waves has been steadily extended. The shortest Hertzian waves merge almost imperceptibly into the longest heat waves of the infra-red, and from there the known spectrum runs continuously through the visible region to the short waves of the extreme ultra-violet recently disclosed by Lyman. Here there is a short gap until soft X-rays are reached, and finally the domain of radiation comes to an end with gamma rays a billionth of a centimeter in length.

Maxwell’s ether was not a dynamical ether in the sense of Green’s elastic solid medium. In spite of the fact that Maxwell was always active in devising mechanical analogues to illustrate the phenomena of electromagnetism, he was never enthusiastic over the speculations of the advocates of a dynamical ether. The electrodynamic equations provided an accurate representation of the electric and magnetic fields, and beyond that he felt it was needless to go. That Gibbs (23, 475, 1882) held the same view is made evident by the closing paragraphs of a paper in which he shows that the electromagnetic theory of light accounts in minutest detail for the intricate phenomena accompanying the passage of light through circularly polarizing media. He says:

“The laws of the propagation of light in plane waves, which have thus been derived from the single hypothesis that the disturbance by which light is transmitted consists of solenoidal electrical fluxes, ... are essentially those which are received as embodying the results of experiment. In no particular, so far as the writer is aware, do they conflict with the results of experiment, or require the aid of auxiliary and forced hypotheses to bring them into harmony therewith.

In this respect the electromagnetic theory of light stands in marked contrast with that theory in which the properties of an elastic solid are attributed to the ether,—a contrast which was very distinct in Maxwell’s derivation of Fresnel’s laws from electrical principles, but becomes more striking as we follow the subject farther into its details, and take account of the want of absolute homogeneity in the medium, so as to embrace the phenomena of the dispersion of colors and circular and elliptical polarization.”

Further Dynamical Theories.—Kelvin, however, was not satisfied with this type of ether. To him dynamics was the foundation of all physical phenomena, and nothing could be said to be explained until a mechanical model was provided. So he returned to the elastic solid theory, and developed the consequences of the assumption, already made use of by Cauchy, that the ether has a negative volume elasticity of such a value as to make the velocity of the compressional wave zero. In order to prevent such an ether from collapsing it is necessary to assume that it is rigidly attached at its boundaries and that cavities cannot be formed at any point in its interior. Now Gibbs (37, 129, 1889) has pointed out the remarkable fact that the equations describing the motion of Kelvin’s quasi-labile ether are of exactly the same form as the electromagnetic equations. Electric displacement is represented by an actual displacement of the ether, magnetic intensity by a rotation. Hence everything which can be explained by the electrodynamic equations finds an analogue in terms of Kelvin’s ether. Still another type of dynamic ether which fits the known facts was proposed by McCullagh and perfected by Larmor. In this ether a rotational elasticity is premised, such as would exist if each particle of the medium consisted of three rigidly connected gyrostats with mutually perpendicular axes. In this ether electrical displacements correspond to rotations, and magnetic strains to etherial displacement.

A New Point of View.—While the dynamical school was still dominant in England, another point of view was developing on the continent. Kirchhoff denied that it was the province of science to provide mechanical explanations of the ether and electrodynamic phenomena such as Kelvin conceived to be necessary in order to make these phenomena intelligible. Kirchhoff’s contention was that the object of science is purely descriptive,—phenomena must be observed, classified, and mutual connections described by the fewest number of differential equations possible. Mach expressed the same idea somewhat more concisely when he asserted that the aim of science is “economy of thought.” For instance, in the time of Newton, planetary motions could be described quite satisfactorily by means of the three laws of Kepler. The motion of falling bodies on the earth’s surface had been described with a fair degree of accuracy by Galileo. The value of Newton’s law of gravitation, however, lay in the fact that this great generalization made it possible to describe these and many other types of motion by a single simple formula, instead of leaving each to be governed by a number of separate and apparently unrelated laws. The importance of such a generalization is measured by the economy of thought which it introduces.

Fig. 1.      Fig. 2.      Fig. 3.

Electron Theory.—The electron theory was leading to a reversal of Kelvin’s idea that dynamical principles must underlie electrodynamics. Lorentz had shown that a rigorous solution of the electrodynamic equations did away entirely with Maxwell’s displacement current, but made the electromagnetic field at a point in space depend not upon the distribution of charges and currents at the same instant, but at a time earlier sufficient to allow the effect to travel with the velocity of light from the charges and currents producing the field to the point at which the electric and magnetic intensities are to be found. The position of a charge or current element at this earlier time he denoted its “effective position.” The effective distribution, then, is that actually seen by an observer stationed at the point under consideration at the instant for which the intensity of the electromagnetic field is to be determined. This solution of the electrodynamic equations led in turn to rigorous expressions for the electric and magnetic intensities produced by a very small charged particle, such as an electron. Fig. 1 shows the electrostatic field produced by a charged particle at rest. The lines of force spread out radially and uniformly in all directions. In fig. 2 the electron is supposed to have a velocity v horizontally to the right of an amount smaller than, though comparable with, the velocity of light c. It is seen that the lines of electric force still diverge radially from the charge, but are crowded in the equatorial plane and spread apart in the polar regions. The dissymmetry grows as the velocity increases until if the velocity of light should be reached the field would be entirely concentrated in a plane at right angles to the direction of motion. Now it may be shown that fig. 2 is obtainable from fig. 1 by reducing dimensions in the direction of motion in the ratio of

√(1 − β²) : 1, where β ≡ ^v/_c.

For a uniformly convected electric field differs from an electrostatic field only in that the dimensions in the direction of motion are contracted in this particular ratio. Fig. 3 represents the electric field of a charged particle which has a uniform acceleration to the right. Consider Faraday’s analogy between lines of force and stretched elastic bands. The symmetry of the first two figures shows that in neither of these cases would there be a resultant force on the charged particle. But in the third figure it is obvious that a force to the left is exerted on the charge by its own field. Calculation shows this force to be proportional in magnitude to the acceleration. Let it be postulated that the resultant force on a charged particle is always zero. Then if F is the applied force, the force on the particle due to the reaction of its field will be — m f, where f stands for the acceleration and m is a positive constant, and we have the fundamental equation of dynamics

F − m f = 0

Hence, instead of admitting Kelvin’s contention that all physical phenomena must be given a mechanical explanation, it would seem more logical to assert that electrodynamics actually underlies mechanics.

Calculation shows the electromagnetic mass m to vary inversely with the radius of the charged particle. Now Thomson’s experiments made it possible to calculate the mass of an electron. Hence its radius can be computed, and is found to be about 2(10)^–13 part of a centimeter, or one fifty-thousandth part of the radius of the atom. Since numbers so small convey little meaning, consider the following illustration, due, in part, to Kelvin. Imagine a single drop of water to be magnified until it is as large as the earth. The individual atoms would then have the size of baseballs. Now magnify one of these atoms until it is comparable in size with St. Peter’s cathedral at Rome. The electrons within the atom would appear as a few grains of sand scattered about the nave. This separation between the constituent electrons of the atom,—so great in comparison with their dimensions,—explains how alpha particles can be shot by the billion through thin-walled glass tubing without leaving any holes behind or impairing in the slightest degree the high vacuum within the tube. The much smaller high speed beta particles pass through an average of ten thousand atoms without even coming near enough to one of the component electrons to detach it and form an ion.

Michelson-Morley Experiment.—In 1881 Michelson (22, 120, 1881) conceived an ingenious and bold method of measuring the orbital motion of the earth through the luminiferous ether. As the experiment was one involving considerable expense, Bell, the inventor of the telephone receiver, was appealed to successfully for the funds necessary to carry it through. Michelson’s experimental plan was as follows: A beam of light traveling in the direction of the earth’s motion strikes an unsilvered mirror m at an angle of 45°. Part of the light passes through, the rest being reflected at right angles to its original direction. Each ray is returned by a mirror at a distance l from m. On meeting again, the ray whose path has been at right angles to the direction of the earth’s motion passes on through the mirror, while the other ray is reflected so as to bring the two in line and form interference fringes. Now consider the effect of the earth’s motion on the paths of the two rays. In fig. 4 the earth is supposed to be moving to the right. The unsilvered mirror m bifurcates a beam of light coming from a source a. By the time the ray reflected from m has traveled to the mirror b and back, m will have moved forward to m’; a distance 2βl, where the small quantity β is the ratio of the earth’s velocity to the velocity of light. Hence the length of the path traversed by this ray is approximately

2l(1 + ½β²).

The other ray will reach the mirror c after the latter has moved forward a distance

^βl/_{(1 − β²)}

and on returning find m at m’. Hence its path has a length of roughly 2l(1 + β²). The difference in path of the two rays is β²l and consequently they should be a little out of phase on meeting at d. By rotating the apparatus clockwise through 90° the directions of the two rays relative to the earth’s motion are interchanged, and the interference fringes would be expected to shift an amount corresponding to a difference in path of 2β²l. This quantity is of course small,—β² is about one one hundred millionth,—but so sensitive are the methods of interferometry that Michelson felt confident that he would be able to detect the earth’s motion through the ether. The apparatus consisted of a table which could be rotated about a vertical axis in much the same way as a spectrometer table, and provided with arms a meter long to carry the mirrors b and c. With this length of arm the interference fringes from sodium light should shift by an amount corresponding to four hundredths of a wave length when the table is rotated through a right angle. When the experiment was first performed the apparatus was placed on a stone pier in the Physical Institute at Berlin. So sensitive was the instrument to outside vibrations that even after midnight it was found impossible to get consistent readings. Finally a satisfactory foundation was constructed in the cellar of the Astrophysical observatory at Potsdam. But what was the astonishment of the experimenters to find that the expected shift of the interference fringes did not exist!

Fig. 4.

The extreme delicacy of the experiment made it desirable to confirm the result by repeating it. This was done by Michelson and Morley (34, 333, 1887) in 1887. In place of a revolving table a massive slab of stone floating on mercury was used to carry the apparatus. This slab was kept in constant rotation, the observer following it around. Moreover, the precision of the experiment was greatly increased by reflecting each ray back and forth across the slab a number of times between leaving and returning to the mirror m. The accuracy attained was such as to justify Michelson in declaring that if the effect sought actually existed it could not be so great as one-twentieth of its calculated value. In 1905 Morley and Miller[^[166]] repeated the experiment for the second time and succeeded in increasing the sensitiveness of the apparatus to a point such that a motion through the ether of one-tenth of the earth’s orbital velocity could have been detected.

The displacement looked for in the Michelson-Morley experiment is known as a second-order effect in that it depends upon the square of the ratio of the velocity of the earth to that of light. Michelson at first considered that the negative result obtained confirmed a theory proposed by Stokes in which it was assumed that the ether inside and near its surface partakes of the motion of the earth, while that at a distance is practically quiescent. But there are many objections to Stokes’ theory, one of which was brought out by an experiment of Michelson’s (3, 475, 1897) in which he attempted by an interference method to detect a difference in the velocity of light at different levels above the earth’s surface. The negative result obtained led him to conclude that if Stokes’ theory were true the earth’s influence on the ether would have to extend to a distance above its surface comparable with its diameter. Meanwhile a more satisfactory explanation was forthcoming. It has been pointed out that a uniformly convected electric field is derivable from an electrostatic field by contracting dimensions in the direction of motion in the ratio

√(1 − β²) : 1.

Fitzgerald and Lorentz showed independently that if moving matter is distorted in this same way the result obtained by Michelson would be just that to be expected. For then the distance of the mirror c from m would be

l√(1 − β²)

instead of l, and the path of the ray moving parallel to the earth’s orbit

2l(1 + ½β²),

which is just that of the other ray. Of course when the apparatus is rotated through 90°, the distance of this mirror from m assumes its normal value again, and the distance of the other mirror becomes shortened. As all measurement consists in comparing the object to be measured with a standard this contraction could never be detected by experimental methods, for the measuring rod would contract in exactly the same ratio as the body to be measured.

In computing its electromagnetic mass Abraham had assumed the electron to be a uniformly charged rigid sphere which keeps its spherical form no matter how great a velocity it may be given. He found that the mass increases with the speed at very high velocities, becoming infinite as the velocity of light is approached, and that its value depends upon the direction of the applied force. After the Fitzgerald-Lorentz contraction was seen to be necessary in order to explain Michelson’s result, Lorentz calculated the electromagnetic mass of a charged sphere which is deformed into an oblate spheroid when set in motion. For this type of electron too, the mass approaches infinity for velocities as great as that of light, and is different for different directions. If a force is applied in the direction of motion the inertia to be overcome is a little greater than when the force is applied at right angles to this direction. Thus we have to distinguish between longitudinal and transverse masses. But the masses of Lorentz’s electron are not the same functions of its velocity as those of Abraham’s. Kaufmann and after him Bucherer tested experimentally the relation between transverse mass and velocity by observing the deflections produced by electric and magnetic fields in the paths of high speed beta particles. The latter’s work was such an ample confirmation of Lorentz’s formula that it may be considered as proven that a moving electron at least suffers contraction in the direction of motion in the ratio

√(1 − β²) : 1.

The electromagnetic theory of light had proved so successful when applied to bodies at rest that Lorentz was anxious to extend this theory to the optics of moving media. His problem was to find a group of homogeneous linear transformations that would leave the form of the electrodynamic equations unchanged. The Michelson-Morley experiment had shown that dimensions in the direction of motion must be contracted in the moving system, those at right angles remaining unaltered. But Lorentz soon found that it was also necessary to use a new unit of time in the moving system, and as this time was found to depend upon the position of the point at which it is to be determined, he called it the local time. Lorentz’s transformation is just that of the principle of relativity, but he did not succeed in expressing the electrodynamic equations in terms of the new coördinates and time in exactly the same form as for a system at rest, for the reason that he failed to endow these new units with sufficient reality to justify him in using them when it came to transforming the velocity term involved in an electric current.

Principle of Relativity.—In 1905 appeared in the Annalen der Physik[^[167]] a paper destined to alter entirely the point of view from which problems in light and electromagnetic theory are to be approached. The author was Albert Einstein, of Berne, Switzerland, a young man of twenty-six who had already made a number of notable contributions to theoretical physics.

The principle of relativity proposed by Einstein was by no means new to students of dynamics. Newton’s first two laws of motion express very clearly the fact that in mechanics all motion is relative. Force is proportional to acceleration, and the relation between the two is the same whether the motion under consideration is referred to fixed axes or to axes moving with a constant velocity. But in connection with the phenomena of light and electromagnetism the case seemed to be quite different. There everything was referred to a fixed ether, and even though Lorentz had found a set of transformations which left the electrodymanic equations practically unchanged, he continued to think in terms of an ether. So physicists were not a little startled when Einstein postulated that no experiment, practical or ideal, could ever distinguish between two systems in such a manner as to warrant the assertion that one of them is at rest and the other in motion. All motion is relative, and the laws governing physical, chemical and biological phenomena are the same in terms of the units of one system as in terms of those of any other.

Einstein next considers some very fundamental questions. What do we mean when we say that two events, one at A and the other at a point B far from A, occur at the same time? Obviously the expression has no significance unless synchronous clocks are stationed at the two points. But how is it to be determined whether or not these two clocks are synchronous? If instantaneous communication could be established between A and B the matter would be simple enough. Since no infinite velocity of transmission is available, however, let a light wave be sent from A to B and returned to A immediately upon its arrival. If the time indicated by the clock at B when the signal is received is half way between that at which it left A and the time at which it arrives on its return, then the two clocks may be considered synchronous. Now if it desired to measure the length of a bar which is moving parallel to the scale with which the measurement is to be made, it is necessary to note the positions of the two ends of the bar at the same instant. So even the measurement of the length of a moving body depends upon the condition of synchronism at different points in space.

The principle of relativity requires that the velocity of light shall be the same in one system as in another relative to which the first is in motion. Hence the definition of synchronism makes it possible to obtain a set of transformations connecting space and time measurement on one system with those on another. This group of transformations is exactly that which Lorentz had found would transform the electrodynamic equations into themselves. But Einstein’s point of view brought out a remarkable reciprocity which Lorentz had missed. If two parallel rods MN and OP are in motion relative to each other in the direction of their lengths, not only does OP appear shortened to an observer at rest with respect to MN, but MN appears shorter than normal in the same ratio to an observer who is moving along with the rod OP.

Einstein’s theory makes the velocity of light the maximum speed with which a signal can be transmitted. This leads to his celebrated addition theorem. Consider three observers A, B and C. Let B be moving relative to A with a velocity of nine-tenths the velocity of light, and C in the same direction with an equal velocity relative to B. In terms of old-fashioned notions of time and space, the velocity of C relative to A would be computed as one and eight-tenths the velocity of light. But the relativity theory gives it as ninety-nine hundredths the velocity of light. For the velocity of light can never be surpassed by that of any material object. This deduction from theory is most strikingly confirmed by the fact that although beta particles have been observed with velocities as high as ninety-nine hundredths that of light, the velocity of light is never quite equalled. It may be remarked in passing that the principle of relativity requires that the masses of all material bodies shall vary with the velocity in the same manner as Lorentz found to be the case for the electromagnetic mass of the deformable electron. In this connection Bumstead (26, 498, 1908) has devised an elegant method of deducing the ratio of longitudinal to transverse mass.

The close connection between electrodynamics and the principle of relativity is obvious from the fact that both lead to the same time and space transformations. Furthermore L. Page (37, 169, 1914) has shown that the electrodynamic equations can be derived exactly and in their entirety from nothing more than the kinematics of relativity and the assumption that every element of charge is a center of uniformly diverging lines of force. Hence it may safely be asserted that no purely electromagnetic phenomenon can ever come into contradiction with this principle. The simplicity thus introduced into the solution of a certain class of problems is enormous. As an example consider the question as to whether a moving star is retarded by the reaction of its own radiation. This purely electrodynamical problem is of such complexity that attempts to solve it have led to some controversy among mathematical physicists. The principle of relativity tells us without recourse to analysis that no retardation can exist.

Throughout the nineteenth century the ether has played a fundamental part in all important physical theories of light and electromagnetism. But if it is not possible for experiment to detect even the state of motion of the ether, why postulate the existence of such a medium? If it does not possess the most fundamental characteristic of matter, how can it possess such derived properties as density and elasticity,—properties which any conceivable mechanical medium must have in order to transmit transverse vibrations? The relativist does not deny the existence of an ether. To him the question has no more meaning than if he were asked to express an opinion as to the reality of parallels of latitude on the earth’s surface. As a convenient medium of expression in describing certain phenomena the ether has justified much of the use which has been made of it. But to attribute to it a degree of substantiality for which there is no warrant in experiment, is to change it from an aid into an obstacle to the progress of science. From the relativist point of view the distinction is very sharp between those motions of charged particles which are experimentally observable, and such geometrical conventions as electromagnetic fields, or analytical symbols as electric and magnetic intensities. These modes of representation have been and still are of the greatest use and importance, but their value in scientific description must not lead to lack of appreciation of their purely speculative character.

Finally attention must be drawn to the fact that the discoveries of inductive science, embodied in the great generalization we have just been discussing, have led to a more intimate knowledge of the nature of time and space than twenty centuries of introspection on the part of professional philosophers. Minskowski, whose promise of greater achievement was cut off by an untimely death, has shown that four dimensional geometry makes possible the representation with beautiful simplicity of the time and space relationships of this theory. The one time and three space dimensions merge in such a manner as to form a single whole with not a vestige of differentiation between these fundamental quantities. Wilson and Lewis[^[168]] have made this representation familiar to American readers through their admirable translation of Minskowski’s work into the notation of Gibbs’s vector analysis.

Aberration, the Doppler effect, anomalous dispersion, —indeed all known phenomena,—are found to be in accord with the principle of relativity. It must be borne in mind, however, that this principle applies only to systems moving relative to one another in straight lines with constant velocities. That there is something absolute about rotation has been recognized since Foucault performed his famous pendulum experiment in 1851. This experiment (C. S. Lyman, 12, 251 and 398, 1851) consisted in setting a pendulum composed of a heavy-brass ball suspended by a long wire into oscillation in such a way as to avoid appreciable ellipticity in its motion. Observation of the rate at which the ground rotates relative to the plane of vibration of the pendulum furnished a method of measuring the rotation of the earth about its axis without reference to celestial bodies. The gyroscopic compass in use to-day provides yet another terrestrial method of detecting this rotation.

The Future of Physics.—At times during the history of physics it has seemed as if the fundamental laws of this science had been so completely formulated that nothing remained to future generations beyond the routine of deducing to the full the consequences of these laws, and increasing the precision of the methods used to measure the constants appearing in them. That Laplace held this view has already been pointed out, and Maxwell, in his introductory lecture at the opening of the Cavendish laboratory in 1871, said, “This characteristic of modern experiments—that they consist principally of measurements—is so prominent, that the opinion seems to have gotten abroad that in a few years all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry on these measurements to another place of decimals.” That he himself did not entertain this view is made evident by a succeeding paragraph. “But we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continue to be poured. It may possibly be true that, in some of those fields of discovery which lie open to such rough observations as can be made without artificial methods, the great explorers of former times have appropriated most of what is valuable, and that the gleanings which remain are sought after rather for their abstruseness than for their intrinsic worth. But the history of science shows that even during that phase of her progress in which she devotes herself to improving the accuracy of the numerical measurement of quantities with which she has long been familiar, she is preparing the materials for the subjugation of new regions, which would have remained unknown if she had been contented with the rough methods of her early pioneers....”

That Maxwell’s forecast of the prospects of his science was no overestimate will be granted by those who have followed the progress of physics during the last twenty years. Yet the work accomplished in the past appears small compared to that which is left to the future. Many of the unsolved problems are matters of fitting together puzzling details, but there is at least one whose solution appears to demand a radical modification in our fundamental physical conceptions. This is the formulation of the laws which govern the motions of electrons and positively charged particles inside the atom.

Black Radiation.—The significance of the problem was first brought to light through the study of black radiation. By a black body is meant one whose distinguishing characteristic is that it emits and absorbs radiation of all frequencies, and black radiation is that which will exist in thermal equilibrium with such a body. The interest of this type of radiation lies in the fact, demonstrated by Kirchhoff, that its nature depends only upon the temperature of the black body with which it is in equilibrium, and on none of this body’s physical or chemical characteristics. Thus we may speak of the “temperature” of the radiation itself, meaning by this the temperature of the material body with which it would be in equilibrium.

The problem of black radiation is to find the distribution of energy among the waves of different frequencies at any given temperature. The first step toward a solution was made when Stefan showed experimentally, and Boltzmann as a deduction from thermodynamics and electrodynamics, that the total energy density summed up over all wave lengths varies with the fourth power of the absolute temperature. If the energy density is plotted as ordinate against the wave length as abscissa, the experimental curve for any one temperature rises from the axis of abscissas at the origin, reaches a maximum, and falls to zero again as the wave length becomes infinitely great. Now Wien’s displacement law, the second important step toward the determination of the form of this curve, shows that as the temperature is raised the wave length to which its highest point corresponds becomes shorter,—in fact this particular wave length varies inversely with the absolute temperature. This theoretical conclusion is entirely confirmed by experiment. (J. W. Draper, 4, 388, 1847.)

Farther than this general thermodynamical principles are unable to go. Statistical mechanics, however, asserts that when a large number of like elements are in thermal equilibrium, the average kinetic energy associated with each degree of freedom is equal to a universal constant multiplied by the absolute temperature. This “principle of equi-partition of energy” has been applied in various ways to obtain a radiation law. The most straightforward method is based on the equilibrium which must ensue between radiation field and material oscillators when the latter emit, on the average, as much energy as they absorb. From whatever aspect the problem is treated, however, the radiation law obtained from the application of the equi-partition principle is the same. And while this law agrees well with the experimental curve for long wave lengths, it shows an energy density that becomes indefinitely great for extremely short waves, which is not only at variance with the facts, but actually leads to an infinite value of this quantity when integrated over the entire spectrum.

The Energy Quantum.—Now the principle of equi-partition of energy rests securely on most general dynamical principles. That these dynamical laws are inexact to any such extent as the divergence between theory and experiment would indicate, is inconceivable; that they are insufficient when applied to motions of electrons in such intense fields as occur within the atom seems no longer open to doubt. In order to obtain a radiation formula in accord with experiment Planck has found it necessary to extend the atomic idea to energy, which he conceives to exist in multiples of a fundamental quantum hν, ν being the frequency and h Planck’s constant. That some such hypothesis of discontinuity is essential in order to obtain any law that will even approximately fit the experimental facts has been proved by Poincaré. But the precise spot at which the quantum is introduced differs for every new derivation of Planck’s law. As deduced most recently by Planck himself, the quantum shows itself in connection with the emission of energy by the material oscillators with which the radiation field is in equilibrium. These oscillators are supposed to act quite normally in every respect except emission; here the radiation demanded by the electrodynamic equations is cast aside, and an oscillator is supposed to emit at once all its energy after it has accumulated an amount equal to some integral multiple of hν. A form of the theory which does not contain this improbable contradiction of the firmly established facts of electrodynamics introduces the quantum into the specification of the energy of vibration which is permitted to each oscillator. Here both emission and absorption follow the classical theory, but the motion of an emitting and absorbing linear oscillator of frequency ν is supposed to be stable only for those amplitudes for which the energy of its oscillations is an integral multiple of hν. In order to maintain the energy at these particular values, the oscillator may draw energy from, or deposit surplus energy with, other degrees of freedom which partake neither in emission nor absorption, but act merely as storehouses.

Photoelectric Effect.—When investigating the production of electromagnetic waves, Hertz had noticed that a spark passed more readily between the terminals of his oscillator when the negative electrode was illuminated by light from another spark. Further investigation by Hallwachs, Elster and Geitel, and others showed that this effect was due to the emission of electrons by a metal exposed to the influence of ultra-violet light. Lenard discovered that the energy with which a negatively charged particle is ejected is entirely independent of the intensity of the light, and further investigation showed it to depend only on the frequency. Einstein suggested that the electrons appearing in this so-called photo-electric effect start from within the metal with an initial energy hν. In passing through the surface a resistance is encountered, however, so he concluded that the energy with which the fastest moving electrons appear outside the metal should be equal to hν less the work done in overcoming this resistance. Recent experiments not only confirm this relation, but provide a most satisfactory method of determining the value of h. Millikan[^[169]] finds it to be 6·57(10)^–27 ergs sec., which gives the quantum for yellow light a value sixty times as great as the heat energy of a monatomic gas molecule at O°C. That this large amount of energy can be transferred from the incident light to the ejected electron is quite out of the question; it must come from within the atom. In this way some indication is obtained of how vast intra-atomic energies must be.

Structure of the Atom.—The generally accepted model of the atom is that due chiefly to Rutherford.[^[170]] He considers it to be constituted of electrons revolving about a positive nucleus either singly or grouped in concentric rings, in much the same manner as the planets revolve around the sun. Experiments on the scattering of alpha rays, however, show that the nucleus, while it must have a positive charge sufficient to neutralize the charges of all the electrons moving around it, cannot have a volume of an order of magnitude greater than that of the electron. The number of unit charges residing on it, except in the case of hydrogen, which is supposed to consist of a singly charged nucleus and only one electron, is found to be approximately half the atomic weight. Thus helium, with an atomic weight of about four, has a doubly charged nucleus with two electrons revolving about it, and lithium a triply charged nucleus and three electrons. The number of unit charges on the nucleus is supposed to correspond with the atomic number used by Moseley in interpreting the results of his experiment on the X-ray spectra of the elements.

Now the electron which is revolving around the positive nucleus of a hydrogen atom, must, according to electrodynamic laws, radiate energy. This radiation will act as a resistance to its motion, causing its orbit to become smaller and its frequency to increase. Hence luminous hydrogen would be expected to give off a continuous spectrum. The very fine lines actually found seem inexplicable on the classical dynamical and electrodynamical theories. These lines, and those of many other spectra, may even be grouped into series, and the relations between them expressed in mathematical form. Formulæ have been proposed by Balmer, Rydberg, Ritz and others, all of which contain a universal constant N as well as certain parameters which must be varied by unity in passing from one line of a series to the next.

In 1913 Bohr[^[171]] proposed anatomic theory which brings to light a remarkable numerical relationship between this quantity N and Planck’s constant h. He postulated that the electron in the hydrogen atom, for instance, cannot revolve in a circle of any arbitrary radius, but is confined to those orbits for which its kinetic energy is an integral multiple of ½hn, n being its orbital frequency. Now at times this electron is supposed to jump from an outer to an inner orbit, when the excess energy of the first orbit over the second is radiated away. But the energy emitted is also taken to be equal to hν, where ν is the frequency of the radiation. Hence ν can be determined, and the expression obtained for it is exactly that given long before by Balmer as an empirical law. The most remarkable thing about it, however, is that Bohr’s result contains a constant involving h and the electronic charge and mass which has precisely the value of the universal constant N of Balmer’s and Rydberg’s formulæ. In all, the theory accounts for three series of hydrogen, and yields satisfactory results for helium atoms which have lost an electron, or lithium atoms which have a double positive charge. But for atoms which retain more than a single electron it seems no longer to hold.

The three mentioned are only the most clearly defined of a growing group of phenomena in which the quantum manifests itself. Its significance and the alteration in our fundamental conceptions to which it seems to be leading is for the future to make clear. That it presents the most important and interesting problem as yet unsolved few physicists would deny.

American Physicists.—In attempting to cover the progress of physics during the last hundred years in the space of a few pages, many important developments of the subject have of necessity remained untouched, and the treatment of many others has been entirely inadequate. Among those appearing in the Journal of which no mention has been made are LeConte’s (25, 62, 1858) discovery of the sensitive flame and Rood’s (46, 173, 1893) invention of the flicker photometer. However, enough has been recounted to indicate the preeminent position in the history of physics in America occupied by four men: Joseph Henry, of the Albany Academy, Princeton, and the Smithsonian Institution; Henry Augustus Rowland, of Johns Hopkins University; Josiah Willard Gibbs, of Yale; and Albert Abraham Michelson, of the United States Naval Academy, Case School of Applied Science, Clark University, and the University of Chicago. Of these, the last named has the distinction of being the only American physicist to have received the Nobel prize, though there is little doubt that the other three would have been similarly honored had not their important work been published prior to the institution of this award. All four occupy high places in the ranks of the world’s great men of science, and the investigations carried out by them and their fellow workers in America have given to their country a position in the annals of physics which is by no means insignificant.

The Journal’s Part in Meteorology.

The meteorological investigations published in the early numbers of the Journal have played an important role in establishing a correct theory of storms. Before the origin of the United States Signal Service in 1871 no systematic weather reports were issued by any governmental agency in this country, and consequently the work of collecting as well as interpreting meteorological data rested entirely in the hands of interested individuals and institutions. The earliest important studies of storms to appear in the Journal were contributed by Redfield of New York, whose first paper (20, 17, 1831) treated in considerable detail a violent storm which passed over Long Island, Connecticut and Massachusetts in 1821. He concluded that “the direction of the wind at a particular place, forms no part of the essential character of a storm, but is only incidental to that particular portion ... of the track of the storm which may chance to become the point of observation, ... the direction of the wind being, in all cases, compounded of both the rotative and progressive velocities of the storm.” A few years later, analyses of twelve “gales and hurricanes of the Western Atlantic” (31, 115, 1837) led to the statement that the phenomena involved “are to be ascribed mainly to the mechanical gravitation of the atmosphere, as connected with the rotative and orbital movements of the earth’s surface.” In this paper is emphasized the fact that the wind may blow in diametrically opposite directions at points near the storm center. “While one vessel has been lying-to in a heavy gale of wind, another, not more than thirty leagues distant, has at the very same time been in another gale equally heavy, and lying-to with the wind in quite an opposite direction.” From an accompanying sketch showing wind directions, the reader would infer that, at this time, Redfield believed the motion of the air to be very nearly in circles about the storm center. The same idea is conveyed by a later paper (42, 112, 1842). Espy (39, 120, 1840) of Philadelphia, however, claimed that observation showed rather that the wind blew inwards toward a central point, if the storm were round in shape, or toward a central line, if it were oblong. This view Redfield (42, 112, 1842) contested, and brought forth much evidence to prove its falsity. A later statement (1, 1, 1846) of his own theory is as follows: “I have never been able to conceive, that the wind in violent storms moves only in circles. On the contrary, a vortical movement ... appears to be an essential element of their violent and long-continued action, of their increased energy towards the center or axis, and of the accompanying rain.... The degree of vorticular inclination in violent storms must be subject, locally, to great variations; but it is not probable that, on an average of the different sides, it ever comes near to forty-five degrees from the tangent of a circle,—and that such average inclination ever exceeds two points of the compass, may well be doubted.” A qualitative explanation of the effect of the earth’s rotation on the direction of the wind near the storm center had already been given by Tracy (45, 65, 1843), and this was followed some years later by Ferrel’s (31, 27, 1861) very thorough quantitative investigation of the dynamics of the atmosphere.

A number of individuals kept systematic records of meteorological observations, among whom was Loomis, whose storm analyses did much to settle the merits of the rival theories of Redfield and Espy. In studying the storm of 1836 (40, 34, 1841) he had drawn on the map lines through those points in the track of the storm where the barometer, at any given hour, is lowest. While this method revealed the general direction in which the storm was progressing, it failed to give much indication of its size or shape. In discussing the two tornadoes of February, 1842, one of which had already been described in the Journal (43, 278, 1842), he adopted a new and more illuminating graphical method. Instead of connecting points of lowest pressure, he drew a curve through all points where the barometer stood at its normal level, then one through those points at which the pressure was ²⁄₁₀ of an inch below normal, and so on. Temperature he treated in much the same way, and the strength and direction of the wind were indicated by arrows. This innovation gave to his storm analyses a significance which had been entirely lacking in those of his predecessors, and led to the familiar systems of isobars and isotherms in use on the daily charts issued by the Weather Bureau at the present time. Loomis advocated careful observations for one year at stations 50 miles apart all over the United States, so that sufficient data might be obtained to settle once for all the law of storms. His efforts, seconded by those of Henry, Bache, Pierce, Abbe, and Lapham, led eventually to the establishment of the Signal Service, and the publication of daily weather maps according to the plan advocated thirty years before. These maps afforded a basis for further analyses of storms, which he published in numerous “Contributions to Meteorology” (8, 1, 1874, et seq.) between 1874 and his death in 1890.

In addition to his work on storms, Loomis made a careful study of the earth’s magnetism (34, 290, 1838 et seq.), and of the aurora borealis (28, 385, 1859 et seq.). That a connection existed between sunspots, aurora, and terrestrial magnetism was already recognized. Loomis (50, 153, 1870 et seq.), however, showed that the periodicity of the aurora borealis, as well as of excessive disturbances in the earth’s magnetic field, corresponds very closely with that of sunspots.

Notes.

[154]. J. W. Gibbs, Trans. Conn. Acad. Arts and Sci., 3, 108 and 343. Abstract by the author, the Journal, 16, 441, 1878.

[155]. H. K. Onnes, Nature, 93, 481, 1914.

[156]. H. Hertz, Wied. Ann., 34, 551, 1888 et seq.

[157]. E. F. Nichols and G. F. Hull, Phys. Rev., 13, 307, 1901 et seq.

[158]. J. J. Thomson, Phil. Mag., 44, 293, 1897.

[159]. R. A. Millikan, Phys. Rev., 2, 109, 1913.

[160]. P. Zeeman, Phil. Mag., 43, 226, 1897.

[161]. H. A. Lorentz, Phil. Mag., 43, 232, 1897.

[162]. S. J. Barnett, Phys. Rev., 6, 239, 1915, and 10, 7, 1917.

[163]. W. C. Röntgen, Wied. Ann., 64, 1, 1898 et seq.

[164]. W. Friedrich, P. Knipping, and M. Laue, Ann. d. Phys., 41, 971, 1913.

[165]. H. G. J. Moseley, Phil. Mag., 26, 1024, 1913, and 27, 703, 1914.

[166]. E. W. Morley and D. C. Miller, Phil. Mag., 9, 680, 1905.

[167]. 17, 891, 1905.

[168]. E. B. Wilson and G. N. Lewis, Proc. Am. Acad., of Arts and Sci., 48, 389, 1912.

[169]. R. A. Millikan, Phys. Rev., 7, 355, 1916.

[170]. E. Rutherford, Phil. Mag., 21, 669, 1911.

[171]. N. Bohr, Phil. Mag., 26, 1, 1913 et seq.

XII
A CENTURY OF ZOOLOGY IN AMERICA

By WESLEY K. COE

This article is intended as a brief survey of the development of zoology in America, and no attempt is made to give a general history of the science. There are numerous accounts in several languages of zoological history in general, among them being W. A. Locy’s “Biology and its Makers.” Brief outlines of the history of zoology may be found in many zoological and biological text-books.

For the history of American zoology the reader is referred to Packard’s report on “A Century’s Progress in American Zoology,” published in the American Naturalist, (10, 591, 1876), to Packard’s “History of Zoology,” published in volume 1 of the Standard Natural History (pp. lxii to lxxii, 1885); to G. B. Goode’s “Beginnings of Natural History in America,”[^[172]] and “Beginnings of American Science,”[^[173]] and to H. S. Pratt’s Manual of the Common Invertebrate Animals (pp. 1–9), 1916. In Binney’s “Terrestrial Air-breathing Mollusks of the United States” (1851) is a chapter on the rise of scientific zoology in the United States which well describes the zoological conditions in the early part of the century, while numerous monographs and papers give the history of the investigations on the various groups of animals or on special fields of study.

Brief biographical sketches of the most distinguished of our older Naturalists—Wilson, Audubon, Agassiz, Wyman, Gray, Dana, Baird, Marsh, Cope, Goode and Brooks are given in “Leading American Men of Science,” edited by David Starr Jordan, 1910. More extensive biographies have been published separately, and the activities of a number of the more prominent American zoologists have been recorded in the Biographical Memoirs of the National Academy of Sciences.

The developmental history of zoology in America falls naturally into four fairly well marked periods, namely:—1, Period of descriptive natural history, previous to 1847, embracing the early studies on the classification and habits of animals, characteristic of the zoological work previous to the arrival of Louis Agassiz in America. 2, Period of morphology and embryology, 1847–1870, during which the influence of Agassiz directed the zoological studies toward problems concerning the relationships of animals as indicated by their structure and developmental history. 3, Period of evolution, 1870–1890, when the principle of natural selection received general recognition and the zoological studies were largely devoted to the applications of the theory to all groups of animals. 4, Period of experimental biology, since 1890, during which time have occurred the remarkable advances in our knowledge of the nature of organisms through the application of experimental methods in the various branches of the modern science of biology.

American Zoology in 1818.

At the beginning of the century which this volume commemorates, the accumulated biological knowledge of the world consisted mainly of what is to-day called descriptive natural history. The zoological treatises of the time were devoted to the names, distinguishing characters and habits of the species of animals and plants known to the naturalists of Europe either as native species or as the results of explorations in other parts of the world. This required little more than a superficial knowledge of their general anatomical structures.

The naturalists of those days had no conception of the life within the cell which we now know to form the basis of all the activities of animals and plants, nor had they even the necessary means of studying such life. The compound microscope, so necessary for the study of even the largest of the cells of the body, was not adapted to such use until 1835, although the instrument was invented in the seventeenth century. With the perfection of the microscope came a period of enthusiastic study of microscopic organisms and microscopic structures of higher animals and plants. It was not until twenty years after the founding of the Journal that the cell theory of structure and function in all organisms was established by the discoveries of Schleiden and Schwann.

The beginning of the nineteenth century saw great zoological activity in Europe, and particularly in France. Buffon’s great work on the Natural History of Animals had recently been completed, Cuvier had only one year before published his classic work in comparative anatomy, “Le Regne Animal,” and Lamarck’s “Philosophie Zoologique” had then aroused a new interest in classification and comparative anatomy from an evolutionary standpoint. E. Geoffroy St.-Hilaire was at the same time supporting an evolutionary theory based on embryonic influences resulting in sudden modifications of adult structure. These epoch-making discoveries and theories gained a considerable following in France, Germany and England, but seem to have had little influence on the zoological work of the following half century in America.

The science of zoology as understood to-day is commonly said to have been founded by Linnæus by the publication of the modern system of classification in the tenth edition of his “Systema Naturæ” in 1758. The influence of Linnæus aroused an interest in biological studies throughout Europe and stimulated new investigations in all groups of organisms. Such studies as related to animals naturally followed first the classification and relationship of species, that is, systematic zoology, and then led gradually into the development of the different branches of the subject, as morphology, comparative anatomy, physiology, and embryology, which eventually were recognized as almost independent sciences.

Of these sciences systematic zoology, which has come to mean the classification, structure, relationship, distribution and habits, or natural history, is the pioneer in any region. Thus we find in our new country at the time of the founding of the Journal in 1818, only sixty years after the publication of Linnæus’ great work, the beginning of American zoology taking the form of the collection and description of our native animals.

It is true that many of our more conspicuous and easily collected animals were described long before the opening of the nineteenth century, but this is to be credited mainly to the work of European naturalists who had made expeditions to this country for the purpose of studying and collecting. These collections were then taken to Europe and the results published there. We thus find in the 12th edition of Linnæus descriptions of over 500 American species, about half of which were birds. As an illustration of the extent to which some of these works covered the field even in those early days may be mentioned a monograph in two quarto volumes with many beautifully colored plates on the “Natural History of the rarer Lepidopterous Insects of Georgia.” This was published in London in 1797 by J. E. Smith from the notes and drawings of John Abbot, one of the keenest naturalists of any period.

During the early years of the nineteenth century, however, economic conditions in our country became such as to give opportunity for scientific thought. Educated men then formed themselves into societies for the discussion of scientific matters. This naturally led to the establishment of publications whereby the papers presented to the societies could be published and made available to the advancement of science generally. The most influential of these was the Journal of the Philadelphia Academy of Natural Science, which was established in 1817, and was devoted largely to zoological papers. The Annals of the New York Lyceum of Natural History date from 1823, and the Journal of the Boston Society of Natural History from 1834. The Transactions of the American Philosophical Society in Philadelphia and the Memoirs of the American Academy of Arts and Sciences in Boston also published many zoological articles.

In these publications and in the Journal, which was founded in 1818, appear the descriptions of newly discovered animal species, with observations on their habits.

The number of investigators in this field in the first quarter of the nineteenth century was but few, and most of these were compelled to take for the work such time as they could spare from their various occupations.

Gradually the workers became more numerous until about the middle of the century zoology was taught in all the larger colleges. The science thereby developed into a profession.

For some years the studies remained largely of a systematic nature, and embraced all groups of animals, but long before the close of the century the attention of the majority of the ever increasing group of zoologists was directed into more promising channels for research and there came the development of the sciences of comparative anatomy, physiology, embryology, experimental zoology, cytology, genetics, and the like, while the systematists became specialists in the various animal groups.

But the work in systematic zoology remains incomplete and many native species are still undescribed or imperfectly classified. It is perhaps fortunate that a few faithful systematists remain at their tasks and tend to keep the experimentalists from the disaster which might otherwise result from the confusion of the species under investigation.

Period of Descriptive Natural History.—Previous to 1847.

Of the few American naturalists whose writings were published toward the end of the eighteenth century and at the beginning of the nineteenth the names of William Bartram (1739–1823), Benjamin Barton (1766–1815), Samuel Mitchill (1764–1831), William Peck (1763–1822), and Thomas Jefferson (1743–1826), require special mention. Bartram’s entertaining volume describing his travels through the Carolinas, Georgia and Florida, published in 1793, contains a most interesting account of the birds and other animals which he found.

Barton wrote many charming essays on the natural history of animals, but was more particularly interested in botany. Mitchill’s most important works include a history of the fishes of New York (1814), and additions to an edition of Bewick’s General History of Quadrupeds. The latter, published in 1804, contains descriptions and figures of some American species and is the first American work on mammals.

Peck has the distinction of writing the first paper on systematic zoology published in America. This was a description of new species of fishes and was printed in 1794. He is also well known for his work on insects and fungi.

Jefferson in 1781 published an interesting book describing the natural history of Virginia, and during his presidency was of inestimable service to zoology through his support of scientific expeditions to the western portions of the country.

Previous to Agassiz’s introduction of laboratory methods of study in comparative anatomy and embryology in 1847, American naturalists generally confined their attention to the study of the classification and habits of the multitude of undescribed animals and plants of the region.

Such studies were naturally begun on the larger and more generally interesting animals such as the birds and mammals, and although many of these were fairly well described as to species before the opening of the nineteenth century, little was known of their habits. The natural history of our eastern birds first became well known through the accurate illustrations and exquisitely written descriptions of Alexander Wilson (in 1808–1813). Bonaparte’s continuation of Wilson’s work was published in four folio volumes beginning in 1826.

In 1828 appeared the first of Audubon’s magnificent folio illustrations of our birds. These were published in England, with later editions of smaller plates in America. Nuttall’s Manual of the Ornithology of the United States appeared in 1832–1834.

The second work on American mammals appeared in the second American edition of Guthrie’s Geography, published in 1815. The author is supposed to have been George Ord, although his name does not appear. In 1825 Harlan published his “Fauna Americana: Descriptions of the Mammiferous Animals inhabiting North America.” This was largely a compilation from European writers, particularly from Demarest’s Mammalogie, and had little value.

In 1826 Amos Eaton published a small “Zoological Text-book comprising Cuvier’s four grand divisions of Animals: also Shaw’s improved Linnean genera, arranged according to the classes and orders of Cuvier and Latreille. Short descriptions of some of the most common species are given for students’ exercises. Prepared for Rensselaer school and the popular class room.” “Four hundred and sixty-one genera are described in this text-book. They embrace every known species of the Animal Kingdom.” This is a compilation from European sources with a few American species of various groups included. On the other hand, Godman’s Natural History, in three volumes (1826–1828), was an illustrated and creditable work. Such was also the case with Sir John Richardson’s Fauna Boreali Americana of which the volume on quadrupeds was published in England in 1829. The other volumes on birds, fishes and insects appeared between 1827 and 1836. Audubon and Bachman’s beautifully illustrated “Quadrupeds of North America” was issued between 1841 and 1850.

About 1840 several of the states inaugurated natural history surveys and published catalogues of the local faunas. The reports on the animals of Massachusetts and New York are the most complete zoological monographs published in America up to that time. This is particularly true of DeKay’s Natural History of New York published between 1842 and 1844 in beautifully illustrated quarto volumes.

The leader in the systematic studies in the early part of the century was Thomas Say, who published descriptions of a large number of new species of animals, particularly reptiles, mollusks, crustacea and insects. Say’s conchology, printed in 1816 in Nicholson’s Cyclopedia, is the first American work of its kind. This was reprinted in 1819 under the title “Land and Fresh-water Shells of the United States.” In 1824–1828 appeared the three volumes of Say’s American Entomology.

The prominent position held by Say in the zoological work of this period is illustrated by the following paragraph from Eaton’s Zoological Text-book (1826, p. 133): “At present but a small proportion of American Animals, excepting those of large size, have been sought out ... And though Mr. Say is doing much; without assistance, his life must be protracted to a very advanced period to afford him time to complete the work. But if every student will contribute his mite, by sending Mr. Say duplicates of all undescribed species, we shall probably be in possession of a system, very nearly complete, in a few years.” How different is the attitude of the zoologist of to-day who sees the goal much further away after a century’s progress through the industry of hundreds of investigators.

During the period of Say’s most active work he is reported to have “slept in the hall of the Philadelphia Academy of Natural Sciences, where he made his bed beneath the skeleton of a horse and fed himself on bread and milk.”

Next to Say, the most active zoologist of the early part of the century was Charles Alexander Lesueur, who described and beautifully illustrated many new species of fishes, reptiles, and marine invertebrates. A memoir by George Ord, published in this Journal (8, 189, 1849), gives a full list of Lesueur’s papers.

One of the most prolific writers of the period was Constantine Rafinesque, a man of great brilliancy but one whose imagination so often dominated his observations that many of his descriptions of plants and animals are wholly unreliable.

United States Exploring Expedition.—In 1838 a fortunate circumstance occurred which eventually brought American systematic zoology into the front ranks of the science. This opportunity was offered by the United States Exploring Expedition under the command of Admiral Wilkes. With James D. Dana as naturalist, the expedition visited Madeira, Cape Verde Islands, eastern and western coasts of South America, Polynesia, Samoa, Australia, New Zealand, Fiji, Hawaiian Islands, west coast of United States, Philippines, Singapore, Cape of Good Hope, etc.

Of the extensive collections made on this four-years’ cruise, Dana had devoted particular attention to the study of the corals and allied animals (Zoophytes) and to the crustacea. In 1846 the report on the Zoophytes was published in elegant folio form with colored plates. Six years later the first volume of the report on Crustacea appeared, with a second volume after two additional years (1854). These reports describe and beautifully illustrate hundreds of new species, and include the first comprehensive studies of the animals forming well-known corals. They remain as the most conspicuous monuments in American invertebrate zoology. Unfortunately the very limited edition makes them accessible in only a few large libraries. The other, equally magnificent, volumes include: Mollusca and Shells, by A. A. Gould, 1856; Herpetology, by Charles Girard, 1858; Mammalogy and Ornithology, by John Cassin, 1858.

Principal investigators.—Of the many writers on animals at this period of descriptive natural history, the following were prominent in their special fields of study:

Ayres, Lesueur, Mitchill, Storer, Linsley, Wyman, DeKay, Smith, Kirtland, Rafinesque and Haldeman described the fishes.

Green, Barton, Harlan, Le Conte, Say, and especially Holbrook, studied the reptiles and amphibia. Holbrook’s great monograph of the reptiles (North American Herpetology) was published between 1834 and 1845.

Wilson, Audubon, Nuttall, Cooper, DeKay, Brewer, Ord, Baird, Gould, Bachman, Linsley and Fox were among the numerous writers on birds.

Godman, Ord, Richardson, Audubon, Bachman, DeKay, Linsley and Harlan published accounts of mammals.

On the invertebrates an important general work entitled “Invertebrata of Massachusetts; Mollusca, Crustacea, Annelida and Radiata” was published by A. A. Gould in 1841, which contains all the New England species of these groups known to that date.

Lea, Totten, Adams, Barnes, Gould, Binney, Conrad, Hildreth, Haldeman, were the principal writers on mollusks. The crustacea were studied by Say, Gould, Haldeman, Dana; the insects by Say, Melsheimer, Peck, Harris, Kirby, Herrick; the spiders by Hentz; the worms by Lee; the coelenterates and echinoderms by Say, Mantell and others.

The history of entomology in the United States previous to 1846 is given by John G. Morris in the Journal (1, 17, 1846). In this article F. V. Melsheimer is stated to be the father of American Entomology, while Say was the most prolific writer. Say’s entomological papers, edited by J. L. Le Conte, were completely reprinted with their colored illustrations in 1859. The first economic treatise is that by Harris on Insects Injurious to Vegetation; printed in 1841. This has had many editions.

Zoology in the American Journal of Science, 1818–1846.

The establishment of the Journal gave a further impetus to the scientific activities of Americans in furnishing a convenient means for publishing the results of their work. In the first volume of the Journal, for example, are two zoological articles by Say and a dozen short articles on various topics by Rafinesque, the latter being curious combinations of facts and fancy. Most of the zoological papers appearing in its first series of 50 volumes are characteristic of an undeveloped science in an undeveloped country. They deal, naturally, with observational studies on the structure and classification of species discovered in a virgin field, with notes on habits and life histories.

Many of the papers are purely systematic and include the first descriptions of numerous species of our mollusks, crustacea, insects, vertebrates and other groups. Of these, the writings of C. B. Adams, Barnes, A. A. Gould and Totten on mollusks, of J. D. Dana on corals and crustacea, of Harris on insects, of Harlan on reptiles, and of Jeffries Wyman and D. Humphreys Storer on fishes are representative and important.

The progress of zoology in America during the first twenty-eight years of the Journal’s existence, that is, up to the year 1846, is thus summarized by Professor Silliman in the preface to vol. 50 (page ix), 1847:

“Our zoology has been more fully investigated than our mineralogy and botany; but neither department is in danger of being exhausted. The interesting travels of Lewis and Clark have recently brought to our knowledge several plants and animals before unknown. Foreign naturalists are frequently visiting our territory; and, for the most part, convey to Europe the fruits of their researches, while but a small part of our own is examined and described by Americans: certainly this is little to our credit and still less to our advantage. Honorable exceptions to the truth of this remark are furnished by the exertions of some gentlemen in our principal cities, and in various other parts of the Union.”

During these 28 years the Journal had been of great service to zoology not only in the publication of the results of investigations but also in the review of important zoological publications in Europe as well as in America. There were also the reports of meetings of scientific societies. In fact all matters of zoological interest were brought to the attention of the Journal’s readers.

The Influence of Louis Agassiz.

At the time of the founding of the Journal and for nearly thirty years thereafter descriptive natural history constituted practically the entire work of American zoologists. In this respect American science was far behind that in Europe and particularly in France. It was not until the fortunate circumstances which brought the Swiss naturalist, Louis Agassiz, to our country in 1846 that the modern conceptions of biological science were established in America.

Agassiz was then 39 years of age and had already absorbed the spirit of generalization in comparative anatomy which dominated the work of the great leaders in Europe, and particularly in Paris. The influence of Leuckart, Tiedemann, Braun, Cuvier and Von Humboldt directed Agassiz’s great ability to similar investigations, and he was rapidly coming into prominence in the study of modern and fossil fishes when the opportunity to continue his research in America was presented. On arriving on our shores the young zoologist was so inspired with the opportunities for his studies in the new country that he decided to remain.

Bringing with him the broad conceptions of his distinguished European masters, he naturally founded a similar school of zoology in America. It is from this beginning that the present science of zoology with its many branches has developed.

It must be remembered in this connection that the great service which Agassiz rendered to American zoology consisted mainly in making available to students in America the ideals and methods of European zoologists. This he was eminently fitted to do both because of his European training and because of his natural ability as an inspiring leader.

The times in America, moreover, were fully ripe for the advent of European culture. There were already in existence natural history societies in many of our cities and college communities. These societies not only held meetings for the discussion of biological topics, but established museums open to the public, and to which the public was invited to contribute both funds and specimens. This led to a wide popular interest in natural history. It was therefore comparatively easy for such a man as Agassiz to develop this favorable public attitude into genuine enthusiasm.

The American Journal of Science announces the expected visit of Agassiz as a most promising event for American Zoology (1, 451, 1846): “His devotion, ability, and zeal—his high and deserved reputation and ... his amiable and conciliating character, will, without doubt, secure for him the cordial cooperation of our naturalists ... nor do we entertain a doubt that we shall be liberally repaid by his able review and exploration of our country.” We of to-day can realize how abundantly this prophecy was fulfilled.

In the succeeding volume (2, 440, 1846) occurs the record of Agassiz’s arrival. “We learn with pleasure that he will spend several years among us, in order thoroughly to understand our natural history.”

Immediately on reaching Boston, Agassiz began the publication of articles on our fauna, and the following year he was appointed to a professorship at Harvard. The Journal says (4, 449, 1847): “Every scientific man in America will be rejoiced to hear so unexpected a piece of good news.” The next year the Journal (5, 139, 1848) records Agassiz’s lecture courses at New York and Charleston, his popularity with all classes of the people and the gift of a silver case containing $250 in half eagles from the students of the College of Physicians and Surgeons.

The service of Agassiz to American zoology, therefore, consisted not only in the publication of the results of his researches and his philosophical considerations therefrom, but also, and perhaps in even greater degree, in the popularization of science. In the latter direction were his inspiring lectures before popular audiences and the early publication of a zoological text-book. This book, published in 1848, was entitled “Principles of Zoology, touching the Structure, Development, Distribution and Natural arrangement of the races of Animals, living and extinct, with numerous illustrations.” It was written with the cooperation of Augustus A. Gould. The review of this book in the Journal (6, 151, 1848) indicates clearly the broad modern principles underlying the new era which was beginning for American zoology.

“A work emanating from so high a source as the Principles of Zoology, hardly requires commendation to give it currency. The public have become acquainted with the eminent abilities of Prof. Agassiz through his lectures, and are aware of his vast learning, wide reach of mind, and popular mode of illustrating scientific subjects ... The volume is prepared for the student in zoological science; it is simple and elementary in style, full in its illustrations, comprehensive in its range, yet well considered and brought into the narrow compass requisite for the purpose intended.”

The titles of its chapters will show how little it differs in general subject matter from the most recent text-book in biology. Chapter I, The Sphere and fundamental principles of Zoology; II, General Properties of Organized Bodies; III, Organs and Functions of Animal Life; IV, Of Intelligence and Instinct; V, Of Motion (apparatus and modes); VI, Of Nutrition; VII, Of the Blood and Circulation; VIII, Of Respiration; IX, Of the Secretions; X, Embryology (Egg and its Development); XI, Peculiar Modes of Reproduction; XII, Metamorphoses of Animals; XIII, Geographical Distribution of Animals; XIV, Geological Succession of Animals, or their Distribution in Time.

A moment’s consideration of the fact that all these topics are excellently treated will show how great had been the progress of zoology in the first half of the nineteenth century. The sixty years that have elapsed since the publication of this book have served principally to develop these separate lines of biology into special fields of science without reorganization of the essential principles here recognized. This remained for many years the standard zoological and physiological text-book, and was republished in several editions here and in England. Another popular book is entitled “Methods of Study in Natural History” (1864).

More than 400 books and papers were written by Agassiz, over a third of which were published before he came to America. They cover both zoological and geological topics, including systematic papers on living and fossil groups of animals, but most important of all are his philosophical essays on the general principles of biology.

One of Agassiz’s greatest services to zoology was the publication of his “Bibliographia Zoologiæ et Geologiæ” by the Ray Society, beginning with 1848. The publication of the Lowell lectures in Comparative Embryology in 1849 gave wide audience to the general principles now recognized in the biogenetic law of ancestral reminiscence. As stated in the Journal (8, 157, 1849), the “object of the Lectures is to demonstrate that a natural method of classifying the animal kingdom may be attained by a comparison of the changes which are passed through by different animals in the course of their development from the egg to the perfect state; the change they undergo being considered as a scale to appreciate the relative position of the species.” These “principles of classification” are fully elucidated in a separate pamphlet, and are discussed at length in the Journal (11, 122, 1851).

One of the most interesting of Agassiz’s numerous philosophical essays, originally contributed to the Journal (9, 369, 1850), discusses the “Natural Relations between Animals and the elements in which they live.” Another philosophical paper contributed to the Journal discusses the “Primitive diversity and number of Animals in Geological times” (17, 309, 1854). Of his systematic papers, those on the fishes of the Tennessee river, describing many new species, were published in the Journal (17, 297, 353, 1854).

Agassiz’s beautifully illustrated “Contributions to the Natural History of the United States” cover many subjects in morphology and embryology, which are treated with such thoroughness and breadth of view as to give them a place among the zoological classics. The Essay on Classification, the North American Testudinata, the Embryology of the turtle, and the Acalephs are the special topics. These are summarized and discussed at length in the Journal (25, 126, 202, 321, 342, 1858; 30, 142, 1860; 31, 295, 1861).

The volume on the “Journey in Brazil” (1868) in joint authorship with Mrs. Agassiz is a fascinating narrative of exploration.

The conceptions which Agassiz held as to the most essential aim of zoological study are well illustrated in his autobiographical sketch, where he writes:[^[174]]

“I did not then know how much more important it is to the naturalist to understand the structure of a few animals, than to command the whole field of scientific nomenclature. Since I have become a teacher, and have watched the progress of students, I have seen that they all begin in the same way; but how many have grown old in the pursuit, without ever rising to any higher conception of the study of nature, spending their life in the determination of species, and in extending scientific terminology!”

It is not surprising, then, that under such influence the older systematic studies should be replaced in large measure by those of a morphological and embryological nature.

The personal influence of Agassiz is still felt in the lives of even the younger zoologists of the present day. For the investigators of the present generation are for the most part indebted to one or another of Agassiz’s pupils for their guidance in zoological studies. These pupils include his son Alexander Agassiz, Allen, Brooks, Clarke, Fewkes, Goode, Hyatt, Jordan, Lyman, Morse, Packard, Scudder, Verrill, Wilder, and others—leaders in zoological work during the last third of the nineteenth century. Through such men as these the inspiration of Agassiz has been handed on in turn to their pupils and from them to the younger generation of zoologists.

The essential difference between the work of Agassiz and that of the American zoologists who preceded him was in his power of broad generalizations. To him the organism meant a living witness of some great natural law, in the interpretation of which zoology was engaged. The organism in its structure, in its development, in its habits furnished links in the chain of evidence which, when completed, would reveal the meaning of nature. Of all Agassiz’s pupils, probably William K. Brooks most fittingly perpetuated his master’s ideals.

Period of Morphology and Embryology, 1847–1870.

The new aspect of zoology which came as a result of the influence of Agassiz characterized the zoological work of the fifties and sixties, that is, until the significance of the natural selection theory of Darwin and Wallace became generally appreciated.

The work in these years and well into the seventies was largely influenced by the morphological, embryological and systematic studies of Louis Agassiz and his school. The structure, development, and homologies of animals as indicating their relationship and position in the scheme of classification was prominent in the work of this period. The adaptations of animals to their environment and the application of the biogenetic law to the various groups of animals were also favorite subjects of study.

The most successful investigators in this period on the different groups of animals include:—Louis Agassiz on the natural history and embryology of coelenterates and turtles; A. Agassiz, embryology of echinoderms and worms; H. J. Clark, embryology of turtles and systematic papers on sponges and coelenterates; E. Desor, echinoderms and embryology of worms; C. Girard, embryology, worms, and reptiles; J. Leidy, protozoa, coelenterates, worms, anatomy of mollusks; W. O. Ayres and T. Lyman, natural history of echinoderms; McCrady, development of acalephs; W. Stimpson, marine invertebrates; A. E. Verrill, coelenterates, echinoderms, worms; A. Hyatt, evolutionary theories, bryozoa and mollusks; Pourtales, deep sea fauna; C. B. Adams, A. and W. G. Binney, Brooks, Carpenter, Conrad, Dall, Jay, Lea, S. Smith, Tryon, mollusks; E. S. Morse, brachiopods, mollusks; J. D. Dana, coelenterates and Crustacea; Kirtland, Loew, Edwards, Hagen, Melsheimer, Packard, Riley, Scudder, Walsh, insects; Gill, Holbrook, Storer, fishes; Cope, evolutionary theories, fishes and amphibia; Baird, reptiles and birds; J. A. Allen, amphibia, reptiles and birds; Brewer, Cassin, Coues, Lawrence, birds; Audubon, Bachman, Baird, Cope, Wilder, mammals.

The progress of ornithology in the United States previous to 1876 is well described in a paper by J. A. Allen in the American Naturalist (10, 536, 1876). A sketch of the early history of conchology is given by A. W. Tryon in the Journal (33, 13, 1862).

Jeffries Wyman was the most prominent comparative anatomist of this period. His work includes classic papers on the anatomy and embryology of fishes, amphibia, and reptiles.

Zoology in the American Journal of Science, 1846–1870.

The fifty volumes of the second series of the Journal, including the years 1846 to 1870, cover approximately this period of morphology and embryology. During this period the Journal occupied a very important place in zoological circles, for J. D. Dana was for most of this period the editor-in-chief, while Louis Agassiz and Asa Gray were connected with it as associate editors. Moreover, in 1864 one of the most promising of Agassiz’s pupils, Addison E. Verrill, was called to Yale as professor of zoology and was made an associate editor in 1869.

In the Journal, therefore, may be found, in its original articles, together with its reports of meetings and addresses and its reviews of literature, a fairly complete account of the zoological activity of the period. The most important zoological researches, both in Europe and America, were reviewed in the bibliographic notices.

The most important series of zoological articles are by Dana himself. As his work on the zoophytes and crustacea of the U. S. Exploring Expedition continued, he published from time to time general summaries of his conclusions regarding the relationships of the various groups. Included among these papers are philosophical essays on general biological principles which must have had much influence on the biological studies of the time, and which form a basis for many of our present concepts.

The importance of these papers warrants the list being given in full. The titles are here in many cases abbreviated and the subjects consolidated.

General views on Classification, 1, 286, 1846.

Zoophytes, 2, 64, 187, 1846; 3, 1, 160, 337, 1847.

Genus Astraea, 9, 295, 1850.

Conspectus crustaceorum, 8, 276, 424, 1849; 9, 129, 1850; 11, 268, 1851.

Genera of Gammaracea, 8, 135, 1849; of Cyclopacea, 1, 225, 1846.

Markings of Carapax of Crabs, 11, 95, 1851.

Classification of Crustacea, 11, 223, 425; 12, 121, 238, 1851; 13, 119; 14, 297, 1852; 22, 14, 1856.

Geographical distribution of Crustacea, 18, 314, 1854; 19, 6; 20, 168, 349, 1855.

Alternation of Generations in Plants and Radiata, 10, 341, 1850.

Parthenogenesis, 24, 399, 1857.

On Species, 24, 305, 1857.

Classification of Mammals, 35, 65, 1863; 37, 157, 1864.

Cephalization, 22, 14, 1856; 36, 1, 321, 440, 1863; 37, 10, 157, 184, 1864; 41, 163, 1866; 12, 245, 1876.

Homologies of insectean and crustacean types, 36, 233, 1863; 47, 325, 1894.

Origin of life, 41, 389, 1866.

Relations of death to life in nature, 34, 316, 1862.

Of the above, the articles on cephalization as a fundamental principle in the development of the system of animal life have attracted much attention. The evidence from comparative anatomy, paleontology, and embryology alike supports the view that advance in the ontogenetic as well as in the phylogenetic stages is correlated with the unequal growth of the cephalic region as compared with the rest of the body. Dana shows that this principle holds good for all groups of animals. His homologies of the limbs of arthropods and vertebrates, however, do not accord with more modern views.

Other papers on the same and allied topics were published by Dana in other periodicals. His most conspicuous zoological works, however, are his reports on the Zoophytes and Crustacea of the United States Exploring Expedition, 1837–1842. The former consists of 741 quarto pages and 61 folio plates, describing over 200 new species, while the Crustacea report, in two volumes, has 1620 pages and 96 folio plates, with descriptions of about 500 new species. Each of these remains to-day as the most important contribution to the classification of the respective groups. The relationships of the species, genera and families were recognized with such remarkable judgment that Dana’s admirable system of classification has remained the basis for all subsequent work.

Dana’s critical reviews (25, 202, 321, 1858) of Agassiz’s “Contribution to the Natural History of the United States” are among the most interesting of his philosophical discussions concerning the relationships of animals as revealed by their structure, their embryology, and their geological history.

The remaining zoological articles in this series cover nearly the whole range of systematic zoology. Especially important are the articles by Verrill on coelenterates, echinoderms, worms and other invertebrates.

In the years following the publication of Darwin’s Origin of Species in 1859 occur many articles on the theory of natural selection. Some of the writers attack the theory, while others give it more or less enthusiastic support.

Experimental methods in solving biological problems were little used at this time, although a few articles of this nature appear in the Journal. Of these, a paper by W. C. Minor (35, 35, 1863) on natural and artificial fission in some annelids has considerable interest to-day.

Exploring Expeditions.

Of the important zoological expeditions the following may be selected as showing their influence on American Zoology:

The North Pacific Expedition, with William Stimpson as zoologist, returned in 1856 with much new information concerning the marine life of the coasts of Alaska and Japan and many new species of invertebrates.

In 1867–1869 the United States Coast Survey extended its explorations to include the deep sea marine life off the southeastern coasts and Gulf of Mexico under the leadership of Pourtales and Agassiz.

The Challenger explorations (1872–1876) added greatly to the knowledge of marine life off the American coast as well as in other parts of the world.

The explorations of the United States Fish Commission succeeded those of the Coast Survey in the collection of marine life off our coasts and in our fresh waters. These have continued since 1872 and have yielded most important results from both the scientific and economic standpoints.

Under the charge of Alexander Agassiz the Coast Survey Steamer “Blake,” in 1877 to 1880, was engaged in dredging operations in three cruises to various parts of the Atlantic. The U. S. Fish Commission Steamer “Albatross,” also in charge of Agassiz, made three expeditions in the tropical and other parts of the Pacific in the years from 1891 to 1905. The study of these collections has added greatly to our knowledge of systematic zoology and geographical distribution. The reports on some of the groups are still in course of preparation.

Period of Evolution, 1870–1890.

The time from 1870 to 1890 may be appropriately called the period of evolution, for although it commences eleven years after the publication of the Origin of Species, the importance of the natural selection theory was but slowly receiving general recognition. The hesitation in accepting this theory was due in no small degree to the opposition of Louis Agassiz. After the acceptance of evolution, although morphological and embryological studies continued as before, they were prosecuted with reference to their bearing on evolutionary problems.

Following closely the methods which had produced so much progress during the life of Agassiz, the field of zoology was now occupied by a new generation, among whom the pupils of Agassiz were the most prominent.

The teaching of biology at this time was also strongly influenced by Huxley, whose methods of conducting laboratory classes for elementary students were adopted in most of our large schools and colleges. This placed biology on the same plane with chemistry as a means for training in laboratory methods and discipline, with the added advantage that the subject of biology is much more intimately connected with the student’s everyday life and affairs.

This increasing demand for instruction in biology and the consequent necessity for more teachers brought an increasing number of investigators into this field.

Conspicuous in this period was the work of E. D. Cope, best known as a paleontologist, but whose work on the classification of the various groups of vertebrates stands preeminent, and whose philosophical essays on evolution had much influence on the evolutionary thought of the time. He was a staunch supporter of the Lamarckian doctrine. Alpheus Hyatt also maintained this theory, and brought together a great accumulation of facts in its support. He thereby contributed largely to our knowledge of comparative anatomy and embryology. A. S. Packard, whose publications cover a wide range of topics, was best known for his text-books of zoology and his manuals on insects.

W. K. Brooks was a leading morphologist and embryologist. S. F. Baird, for many years the head of the United States Fish Commission, was the foremost authority on fish and fisheries and is also noted for his work on reptiles, birds and mammals. The man of greatest influence, although by no means the greatest investigator, was C. O. Whitman. It is to him that we owe the inception of the Marine Biological Laboratory, the most potent influence in American zoology to-day; the organization of the American Morphological Society, the forerunner of the present American Society of Zoologists; and the establishment of the Journal of Morphology. G. B. Goode was distinguished for his work on fishes and for his writings on the history of science.

E. L. Mark, C. S. Minot, and Alexander Agassiz were acknowledged leaders in their special fields of research—Mark in invertebrate morphology and embryology, and Minot in vertebrate embryology, while Alexander Agassiz made many important discoveries in the systematic zoology and embryology of marine animals, and to him we owe in large measure our knowledge of the life in the oceans of nearly all parts of the world.

The knowledge of the representatives of the different divisions of the American fauna had now become sufficient to allow the publication of monographs on the various classes, orders and families. At this time also particular attention was given to the marine invertebrates of all groups.

Of the many investigators working on the various groups of animals at this time only a few may be mentioned. The protozoa were studied by Leidy, Clark, Ryder, Stokes; the sponges by Clark, Hyatt; the coelenterates by A. Agassiz, S. F. Clarke, Verrill; the echinoderms by A. Agassiz, Brooks, Kingsley, Fewkes, Lyman, Verrill; the various groups of worms by Benedict, Eisen, Silliman, Verrill, Webster, Whitman; the mollusks by A. and W. G. Binney, Tryon, Conrad, Dall, Sanderson Smith, Stearns, Verrill; the Brachiopods by Dall and Morse; the Bryozoa by Hyatt; the crustacea by S. I. Smith, Harger, Hagen, Packard, Kingsley, Faxon, Herrick; the insects by Packard, Horn, Scudder, C. H. Fernald, Williston, Norton, Walsh, Fitch, J. B. Smith, Comstock, Howard, Riley and many others; spiders by Emerton, Marx, McCook; tunicates by Packard and Verrill; fishes by Baird, Bean, Cope, Gilbert, Gill, Goode, Jordan, Putnam; amphibians and reptiles by Cope; birds by Baird, Brewer, Coues, Elliott, Henshaw, Allen, Merriam, Brewster, Ridgway; and the mammals by Allen, Baird, Cope, Coues, Elliott, Merriam, Wilder.

Interest in the evolutionary theory continued to increase and eventually developed into the morphological and embryological studies which reached their culmination between 1885 and 1890 under the guidance of Whitman, Mark, Minot, Brooks, Kingsley, E. B. Wilson and other famous zoologists of the time. In these years the Journal of Morphology was established and the American Morphological Society was formed.

The morphological, embryological and paleontological evidences of evolution as indicated by homologies, developmental stages and adaptations were the most absorbing subjects of zoological research and discussion.

Zoology in the American Journal of Science, 1870–1918.

The third series of the Journal (1870–1895), likewise including fifty volumes, embraces this period of zoological activity in morphological and embryological studies, culminating with the inception of the modern experimental methods.

In this period also occurred the greatest progress in marine systematic zoology, due to the explorations of the United States Fish Commission off the Atlantic Coast. The Journal had an important share in the zoological development of this period also, for A. E. Verrill, who was now an associate editor, was in charge of the collections of marine invertebrates. Consequently most of the discoveries in this field were published in the Journal in numerous original contributions by Verrill and his associates. The explorations of the U. S. Fish Commission Steamer “Albatross” are described from year to year by Verrill, with descriptions of the new species of invertebrates discovered.

The numerous original contributions by Verrill on subjects of general zoological interest as well as on those of a systematic nature give this third series of the Journal much zoological importance. Verrill’s papers cover almost the whole field of descriptive zoology, but are mainly devoted to marine invertebrates. Those which were originally contributed to the Journal or summarized by him in his literature reviews include the following topics:

Sponges, 16, 406, 1878.

Coelenterates, 37, 450, 1864; 44, 125, 1867; 45, 411, 186; 46, 143, 1868; 47, 282, 1869; 48, 116, 419, 1869; 49, 370, 1870; 3, 187, 432, 1872; 6, 68, 1873; 21, 508, 1881; 6, 493, 1898; 7, 41, 143, 205, 375, 1899; 13, 75, 1902.

Echinoderms, 44, 125, 1867; 45, 417, 1868; 49, 93, 101, 1870; 2, 430, 1871; 11, 416, 1876; 49, 127, 199, 1895; 28, 59, 1909; 35, 477, 1913; 37, 483, 1914; 38, 107, 1914; 39, 684, 1915.

Worms, 50, 223, 1870; 3, 126, 1872.

Mollusks, 49, 217, 1870; 50, 405, 1870; 3, 209, 281, 1872; 5, 465, 1873; 7, 136, 158, 1874; 9, 123, 177, 1875; 10, 213, 1875; 12, 236, 1876; 14, 425, 1877; 19, 284, 1880; 20, 250, 251, 1880; 2, 74, 91, 1896; 3, 51, 79, 162, 355, 1897.

Crustacea, 44, 126, 1867; 48, 244, 430, 1869; 25, 119, 534, 1908.

Ascidians, 1, 54, 93, 211, 288, 443, 1871; 20, 251, 1880.

Dredging operations and marine fauna, 49, 129, 1870; 2, 357, 1871; 5, 1, 98, 1873; 6, 435, 1873; 7, 38, 131, 405, 409, 498, 608, 1874; 9, 411, 1875; 10, 36, 196, 1875; 16, 207, 371, 1878; 17, 239, 258, 309, 472, 1879; 18, 52, 468, 1879; 19, 137, 187; 20, 390, 1880; 22, 292, 1881; 23, 135, 216, 309, 406, 1882; 24, 360, 477, 1882; 28, 213, 378, 1884; 29, 149, 1885.

Miscellaneous, 39, 221, 1865; 41, 249, 268, 1866; 44, 126, 1867; 48, 92, 1869; 3, 386, 1872; 7, 134, 1847; 10, 364, 1875; 16, 323, 1878; 20, 251, 1880; 3, 132, 135, 1897; 9, 313, 1900; 12, 88, 1901; 13, 327, 1902; 14, 72, 1902; 15, 332, 1903; 24, 179, 1907; 29, 561, 1910.

S. I. Smith describes the metamorphosis of the Crustacea (3, 401, 1872; 6, 67, 1873), species of crustacea (3, 373, 1872; 7, 601, 1874; 9, 476, 1875), and dredging operations in Lake Superior (2, 373, 448, 1871). In this series occurs also a series of papers on comparative anatomy and embryology from the Chesapeake Zoological Laboratory in charge of W. K. Brooks. In the 39th and 40th volumes of the third series (1890) occur several papers on evolutionary topics by John T. Gulick (39, 21; 40, 1, 437) which have attracted much attention.

Before the end of this period, however, the Journal was relieved from the necessity of publishing zoological articles by the establishment of several periodicals devoted especially to the various fields of zoology. We find, therefore, but few exclusively zoological papers after 1885, although articles of a general biological interest and the reviews of zoological books continue.

In the fourth series of the Journal, beginning in 1896, occur also a number of articles on systematic zoology by Verrill and others and several papers having a general biological interest. Brief reviews of a small number of zoological books are still continued, but at the present day the Journal, which played so important a part in the early development of American zoology, has been given over to the geological and physical sciences in harmony with the modern demand for specialization.

Period of Experimental Biology, since 1890.

Zoological studies remained in large measure observational and comparative until about 1890 when the experimental methods of Roux, Driesch and others came into prominence. Interest then turned from the accumulation of facts to an analysis of the underlying principles of biological phenomena. The question now was not so much what the organism does as how it does what is observed, and this question could be answered only by the experimental control of the conditions. These experimental studies met with such remarkable success that in a few years the older morphological studies were largely abandoned, the Morphological Society changed its name to the Society of Zoologists, and in 1904 the Journal of Experimental Zoology was established. The experimental methods were applied to all branches of biological science, and while it must be freely admitted that little progress has been made toward an understanding of the ultimate causes which underlie biological phenomena, a great advance has been made in the elucidation of the general principles involved.

Experimental embryology, histology, regeneration, comparative physiology, neurology, cytology, and heredity have in recent years successfully adopted an experimental aspect and have made significant progress thereby. Biology has now taken its place beside chemistry and physics as an experimental science.

The latest great advance in biology has been in the field of heredity. The rediscovery of the Mendelian principles of heredity in 1900 brought to light the most important generalization in biology in recent times. The new science of genetics is essentially the experimental study of heredity.

We are at the moment in the midst of an effort to establish in biology a few relatively simple laws by using for the purpose the vast accumulations of observational data gathered in past years, supplemented by such experimental data as have been provided by these more recent investigations. Such hypotheses as have been formulated are for the most part only tentatively held, for their validity is generally incapable of a critical test. But wherever such tests have been possible, the laws of mathematics, physics and chemistry are found applicable to biological phenomena.

The number of investigators has now become so great and their activities so prolific that the list and synopses of the zoological publications each year cover upwards of 1000 to 1500 pages in the International Catalogue of Scientific Literature.

American Leadership.—During the first half of the century the progress of zoology in America remained distinctly behind that of Europe. At the beginning of the century the science was farthest developed by the French and English, although Linnæus was a Swede and took his degree in Holland. Under the influence of Von Baer and his monumental treatise on embryology (Ueber Entwicklungsgeschichte der Thiere, 1828), and supported later by the great physiologist, Johannes Müller, whose “Physiologie des Menschen” (1846) forms the basis of modern physiology, the German school forged rapidly ahead and eventually assumed the leadership in zoology, as in several other branches of science.

In the latter half of the century the influence of the German universities dominated in a large measure the zoological investigations in America. The reason for this is partly due to the fact that many of our young zoologists, after finishing their college course, completed their preparation for research by a year or more at a German university. The more mature zoologists, too, looked forward with keen anticipation to spending their summer vacations and sabbatical years in research in a German laboratory or at the famous Naples station in which the German influence was dominant.

With the rise of experimental biology since 1890, however, the American zoologists have shown so high a degree of originality in devising experiments, so much skill in performing them, and such keenness in analyzing the results, that they have assumed the world leadership in several of the special fields into which the science of zoology is now divided.

Biological Periodicals.

Perhaps in no better way can the progress of biology in America be illustrated than by a brief survey of the origin and development of the more important biological journals. For it will be seen that these publications have become more numerous and more specialized as the science has advanced in specialization.

The early publications—which as is well known, treated mainly of the birds, mammals and other vertebrates, and of insects, crustacea and shells—consisted mainly of separate books or pamphlets, published by private subscription. After the establishment of the so-called Academies of Science, or of Arts and Sciences, toward the end of the eighteenth and in the first quarter of the nineteenth century, the reports of the meetings began to be published as periodical Journals, supported by the academies. In these publications, and in the Journal which was founded at the same time, appear papers on all branches of science, including zoology. As soon as zoology in America assumed its modern aspects through the influence of Louis Agassiz and his followers the earliest strictly zoological journals were established.

It should be noted, however, that the journals of the scientific and natural history societies were more or less fully devoted to zoological topics according to the nature of the activities of the members and correspondents. After the establishment of the Museum of Comparative Zoology by Louis Agassiz came the founding in 1863 of its Bulletin and later its Memoirs. These publications have continued to the present day as a standard of excellence for the reports of zoological investigations. In connection with the systematic work on mollusks, the American Journal of Conchology was established in 1865. The American Naturalist was founded in 1867 by four of Louis Agassiz’s pupils, Hyatt, Morse, Packard and Putnam. It was later edited by Cope as a leading periodical for the publication of biological papers, particularly those relating to evolution, and is at present devoted to evolutionary topics. It is now in the 52d volume of its new series.

With the awakened interest in comparative anatomy and embryology came the need for an American journal which should supply a means of publication for the reports of researches accomplished by the increasing number of workers in these fields. This need was fully met by the establishment of the Journal of Morphology in 1887. This publication, now in its 30th volume, has equalled the best European journals in the character of its papers. A few years later (1891) came the Journal of Comparative Neurology for the publication of investigations relating to the morphology and physiology of the nervous system and to nervous and allied phenomena in all groups of organisms. Twenty-eight volumes of this journal have been completed. The Zoological Bulletin was started under the auspices of the Marine Biological Laboratory in 1897 for the publication of papers of a less extensive nature and which could be more promptly issued than those in the Journal of Morphology where elaborate plates were required. After two years the scope of the Bulletin was enlarged to include botanical and physiological subjects. The name was correspondingly changed to the Biological Bulletin. Of this important periodical 33 volumes have been issued.

For the publication of papers on human and comparative anatomy and embryology, the American Journal of Anatomy was established in 1901, and is now in its twenty-third volume.

Meanwhile the trend of zoological interest was toward topics connected with the ultimate nature of biological phenomena. The meaning of these phenomena could be determined only by the experimental method. Researches in this field became more prominent and the adequate publication of the numerous papers required the establishment of a new journal in 1904. This was named the Journal of Experimental Zoology. It immediately took its place in the front rank of American zoological periodicals. Twenty-four volumes have been published.

In spite of the constantly increasing number of journals, the science grew faster than the means of publication. So crowded did the American journals become that long delays often resulted before the results of an investigation could be issued. This condition was met in part by the sending of many papers to be published in European journals (a necessity most discreditable to American zoology) and in part by the establishment of additional means of publication. Of the latter the Anatomical Record, now in its fourteenth volume, was begun in 1906 for the prompt publication of briefer papers on vertebrate anatomy, embryology and histology and for preliminary reports and notes on technique.

During the past few years has come a great advance in the experimental breeding of plants and animals. Problems in heredity and evolution have taken on a new interest since the importance and validity of Mendel’s discovery have been recognized. To meet this development of biology the journal Genetics was begun in 1916 for the publication of technical papers, while the Journal of Heredity, modified from the American Breeders Magazine, is devoted to popular articles on animal and plant breeding, and Eugenics.

On the whole, the science of zoology is now assuming a closer relation to practical affairs. Entomology, for example, is now represented by the Journal of Economic Entomology, of which 10 volumes have been issued since 1907. The Journal of Animal Behavior covers another practical field of research. The Proceedings of the Society for Experimental Biology and Medicine, starting in 1903, the American Journal of Physiology, and several other publications cover the physiological field. The Journal of Parasitology, established 1914, now in its fourth volume, is devoted to the interests of medical zoology. The Auk, now in the 34th volume of its new series (42d of old series), is the official organ of the American Ornithologists Union and is devoted to the dissemination of knowledge concerning bird life. The Annals of the Entomological Society of America, established in 1908, and now in its 10th volume, is one of several important entomological journals. The Nautilus, of which 28 volumes have been issued, is one of the more successful journals devoted to conchology. This list might be extended to include numerous other periodicals of importance, both technical and popular, which have been of great service in the various fields of biology.

In addition to these are the many volumes of systematic papers in the Proceedings of the United States National Museum, the practical reports in the Bulletin of the United States Fish Commission, the vast literature issued yearly by the various divisions of the United States Department of Agriculture, Public Health Service and other Governmental departments, while the list of publications by scientific societies, museums, and other institutes is constantly increasing and covers all fields of biological research.

At the present time facilities for the publication of research on any branch of zoology are as a rule entirely adequate. For this highly satisfactory condition the science is indebted to the support given five of its most important journals by the Wistar Institute of Anatomy and Biology.

Biological Associations.

An important light on the history of biology in America can be thrown by a glance at the rise and development of societies or associations for the report and discussion of papers relating to that branch of science. In the first half of the nineteenth century natural history societies were formed in most cities and centers of learning. These were very important factors in the promotion of scientific research as well as in the diffusion of popular knowledge of living things. The aims and activities of twenty-nine such scientific societies, many of which were devoted especially to natural history, are described in one of the early volumes of the Journal (10, 369, 1826). The Connecticut Academy of Arts and Sciences, dating from 1799, the Philadelphia Academy of Natural Sciences from 1812, and the New York Lyceum of Natural History (in 1876 name changed to New York Academy of Sciences) from 1817 are among the oldest of those which still exist.

Of national institutions the American Philosophical Society was founded in 1743, the American Academy of Arts and Sciences in 1780, and the National Academy of Sciences in 1863.

The American Association for the Advancement of Science, with its thousands of members, now has separate sections for each of the special branches of science. This great association was organized in 1848, as the successor of the Association of American Geologists and Naturalists. This was itself a revival of the American Geological Society which first met at Yale in 1819. Its meetings have given a great support to the scientific work of the country.

The American Society of Naturalists was founded in 1883. The original plan of the society was for the discussion of methods of investigation, administration and instruction in the natural sciences, but its program is now entirely devoted to discussions and papers of a broad biological interest. It also arranges for an annual dinner of the several biological societies and an address on some general biological topic.

In 1890, toward the end of the period in which morphological studies were being emphasized, the professional zoologists of the eastern states founded the American Morphological Society. This association held annual meetings during the Christmas holidays for the presentation of zoological papers. This name became less appropriate after a few years because of the gradual decrease in the proportion of morphological investigations owing to the greater attention being directed to problems in experimental zoology and physiology. Consequently the name was changed to the American Society of Zoologists. To be eligible for membership in this society a person must be an active investigator in some branch of zoology, as indicated by the published results.

The American Association of Anatomists includes in its membership investigators and teachers in comparative anatomy, embryology, and histology as well as in human anatomy. Many professional zoologists and experimental biologists present their papers before this society, or at the meetings of the American Physiological Society. The Entomological Society of America and the American Association of Economic Entomologists are large and active societies.

These national societies have been of great service in fostering a high standard of zoological research. A still more important service, though generally less conspicuous, is rendered by the journal clubs in connection with all the larger zoological laboratories, and by local scientific societies which are now maintained in all the larger centers of learning throughout the country. There are also specific societies for some of the different fields of biological work.

Biological Stations.

No insignificant factor in the development of biological science has been the establishment of biological stations where investigators, teachers and students meet in the Summer vacation for special studies, discussions and research. The most successful of these laboratories have been located on the seashore and here the study of marine life in Summer supplements the work of the school or university biological courses. The famous Naples Station was founded in 1870, and was shortly after followed by several others. Similar biological stations are now supported on almost every coast in Europe and in several inland localities.

The first such American school was established by Louis Agassiz at the island of Penikese on the coast of Massachusetts in 1873, succeeding his private laboratory at Nahant. During that Summer more than forty students gained enthusiasm for the work of future years. Unfortunately the laboratory so auspiciously started was of brief duration, for the death of Agassiz occurred in December of the same year, and the laboratory was discontinued at the end of the following Summer. Shortly afterward Alexander Agassiz equipped a small private laboratory at Newport, Rhode Island, and W. K. Brooks established the Chesapeake Bay Zoological Laboratory.

At this time the United States Fish Commission was engaged under the direction of Spencer F. Baird in a survey of the marine life of the waters off the Eastern Coast. Between 1881 and 1886 the Commission established the splendidly equipped biological station at Woods Hole, Massachusetts. Both here and at the Fish Commission Laboratory at Beaufort, North Carolina, much work in general zoology as well as in economic problems is accomplished. These laboratories are designed particularly for specialists engaged in researches connected with the work of the Fish Commission.

A need was soon felt for a marine laboratory along broader lines, and one available to the students and teachers of the schools and colleges. To meet these requirements the Woods Hole Marine Biological Laboratory was started in 1887, as the successor to an earlier laboratory at Annisquam, and has since become a great Summer congress for biologists from all parts of the country. It is safe to say that no other institution has been of equal service in securing for biology the high plane it now occupies in American science. The leading spirit in the establishment of this laboratory and its director for many years was Charles O. Whitman.

Successful marine laboratories are located also at Cold Spring Harbor, Long Island; at Harpswell, Maine; and at Bermuda. The Carnegie Institution maintains a laboratory at Tortugas Island, Florida, for the investigation of tropical marine life.

On the Pacific Coast marine laboratories are located at Pacific Grove and at La Jolla, California, and at Friday Harbor, Washington. Several other biological laboratories are open each Summer on our coasts, as well as a number of fresh-water laboratories on the interior lakes. There are also several mountain laboratories. The influence of these laboratories on American biology is immeasurable.

Natural History Museums.

Museums of Natural History or “Cabinets of Natural Curios” as they were sometimes called, were established in the first half of the nineteenth century in connection with the various natural history societies. These were of much service in stimulating the collection of zoological “specimens” and in arousing a popular interest in natural history.

The zoological museum of earlier days consisted of rows on rows of systematically arranged specimens, each carefully labelled with scientific name, locality, date of collection and donor—much like the pages of a catalogue. All this has now been changed; the bottles of specimens have been relegated to the storeroom, and the great plate glass cases of the modern museum represent individual studies in the various fields of modern zoological research, or individual chapters in the latest biological text-books. Often the talent of the artist and the skill of the taxidermist are cunningly combined to produce most realistic bits of nature.

The United States National Museum, the American Museum of Natural History, the Field Columbian Museum and the Museum of Comparative Zoology are among the finest museums of the world, while many of the states, cities, and universities maintain public museums as a part of their educational systems.

Systematic Zoology and Taxonomy.

The work in systematic zoology is now mainly carried on by specialists in relatively small groups of animals. This is necessitated both by the increasingly large number of species known to science and by the completeness and exactness with which species must now be defined. The majority of systematic workers are now connected with museums where the large collections furnish material for comparative studies.

Prominent in this field is the United States National Museum, the publications of which are mainly taxonomic and zoogeographic, and cover every group of organism. The adequacy of this great museum for such studies may be illustrated by the collection of mammals. This museum has the types of 1135 of the 2138 forms (including species and subspecies) of North American mammals recognized in Miller’s list,[^[175]] and less than 200 forms lack representatives among the 120,000 specimens of mammals. Systematic monographs of several of the orders of mammals have been published.

Systematic study of the birds has brought the number of species and subspecies known to inhabit North and Middle America to above 3000. The most comprehensive systematic treatise is the still incomplete report of Ridgeway[^[176]] of which seven large volumes have already been issued.

On the reptiles, the most complete monograph is that by Cope[^[177]] entitled “The Crocodilians, Lizards and Snakes of North America.”

The Amphibia have also been studied by Cope, whose report on the Batrachia of North America[^[178]] is the standard taxonomic work.

The most comprehensive systematic work on fishes is the “Descriptive Catalogue of the Fishes of North and Middle America” by Jordan and Evermann.[^[179]]

The invertebrate groups have been in part similarly monographed by the members of the U. S. National Museum staff and others, and further studies are in progress. Other taxonomic monographs published by this museum include the various groups of animals from many different parts of the world.

A number of the larger State, municipal, and university museums publish bulletins on special groups represented in their collections as well as articles of general zoological interest.

Expeditions, subsidized by museum and private funds, are from time to time sent to various parts of the world and their results are often published in sumptuous manner.

The total number of living species of animals is unknown, but considering that about a quarter of a million new species have been described during the past thirty years, it is probable that several million species are in existence to-day. More than half a million have been described. These are probably but a small fraction of the number that have existed in past geological ages.

Thus, in spite of all the work that has been done in systematic zoology and as the number of known species continues to increase, there still remain many groups of animals, some of which are by no means rare or minute, in which probably only a small proportion of the species are as yet capable of identification.

It is only since the publication of Ward and Whipple’s “Fresh-water Biology” within the past year that the amateur zoologist could hope to find even the names of all the organisms which may be collected from a single pool of water. And in many cases he will still meet with disappointment, for many of our protozoa and other fresh-water organisms have not yet been described as species.

During the past few years there has been a tendency on the part of some of our biologists engaged in experimental work to disparage the studies of the systematists. It must be granted, however, that both lines of work are essential to the sound development of zoological science, for experimental investigations in which the accurate diagnosis of species is ignored always result in confusion.

Ecology.—The marvelous modifications in structure and instincts by which the various animals are adapted to their surroundings now forms a special topic in biological research and one of the most fascinating. The adaptations in habitat, time, behavior, appearance and even in structure are found capable of a certain individual modification when studied experimentally.

Zoogeography.—Closely associated with systematic zoology, and indeed a part of the subject in its broader sense, is the study of the geographical distribution of animal species and larger groups.

Paleontology.—The geological succession of organisms embraces a field where zoologist and geologist meet. The wonderful progress made by American investigators is well described in the preceding chapters on Historical Geology and Vertebrate Paleontology.

Biometry.

Since Darwin’s theory of evolution postulated the origin of new species by means of natural selection, it was obviously necessary in order to apply a critical test to determine the precise limits of a species. It was, therefore, proposed to subject a given species to a strict examination by the application of statistical methods to determine the range of variation of its members and the extent to which the species intergrades with others. Other problems, particularly those concerning heredity, were treated in similar manner. This branch of biological science was particularly developed by the English School, led by Sir Francis Galton, followed by Karl Pearson and William Bateson.

In America the methods of biometry have been utilized extensively by Charles B. Davenport, Raymond Pearl, H. S. Jennings and others in the solution of problems in genetics and evolution. Their work shows the great value of critical statistical analysis in the interpretation of biological data. A thorough training in mathematics is now found to be hardly less important for the biologist than is a knowledge of physics and chemistry, for the science of biometry has become one of the most important adjuncts to the study of genetics.

Comparative Anatomy and Embryology.

Comparative Anatomy.—Upon the foundations laid down by Cuvier a century ago the present elaborate structure of comparative anatomy of animals, both vertebrate and invertebrate, has been developed. Vast as is the present accumulation of facts and theories many important problems still await their solution. Jeffries Wyman was long a leader in this field, where many workers are now engaged.

Embryology.—The embryological studies, so brilliantly begun by Von Baer early in the nineteenth century, are still in progress. They have now been extended to the groups more difficult of investigation and into the earliest stages of fertilization and implantation in the mammals. Artificial cultural methods have yielded important results. Louis and Alexander Agassiz, Mark, Minot, Brooks, Whitman, Conklin and E. B. Wilson have taken prominent parts in this work.

In the early nineties embryological studies were directed to the arrangement of cells in the dividing egg, and there was much discussion of “cell lineage” in development. Valuable as were these studies they threw comparatively little light on the general problems of evolution.

Experimental Embryology.—A more fertile field, developed at the same period and a little later, was found in experimental embryology. The discoveries made by Driesch and others in shaking apart the cells of the dividing egg or by destroying one or more of these cells gave a new insight into the potency of cells for compensatory and regenerative processes. These studies attracted many able investigators, who made still further advance by subjecting the germ cells, developing eggs, embryos, and developing organs to a great variety of artificial conditions.

Artificial Parthenogenesis.—Another question concerns the nature of the process of fertilization and the agencies which cause the fertilized egg to develop into an embryo. In 1899 Jacques Loeb succeeded in causing development in unfertilized sea-urchin eggs by subjecting them to concentrated sea water for a period and then returning them to their normal environment. To this promising field of experimental work came many of the foremost biologists both in America and Europe. It was soon found that the eggs of most groups of animals except the higher vertebrates could be made to develop into more or less perfect embryos and larval forms by treatment with a great variety of chemical substances, by increased temperature, by mechanical stimuli and by other means. This artificial parthenogenesis, as it is called, has also been successful in plants (Fucus), and recently Loeb has reared several frogs to sexual maturity by merely puncturing with a sharp needle the eggs from which they were derived. Loeb, then, maintains that “the egg is the future embryo and animal; and that the spermatozoon, aside from its activating effect, only transmits Mendelian characters to the egg.”[^[180]]

Further experimental analyses of the nature of the fertilization mechanism have recently been made by Morgan, Conklin, F. R. Lillie, and others.

Germinal Localization.—The question as to whether the egg contains localized organ-forming substances has been studied experimentally particularly by means of the centrifuge. The results indicate that neither of the older opposing theories of “performation” or “epigenesis” is applicable to all eggs, but that in certain organisms the eggs possess a well marked differentiation while in others each part of the egg is essentially, although probably not absolutely, equipotential.

The Germplasm Cycle.—Since Weismann’s postulation of the independence of soma and germplasm in 1885 many attempts have been made to trace the path of the hereditary substance from one generation to the next. A recent book by Hegner[^[181]] summarizes the success attained in various groups of animals.

Cytology.

Another important field of investigation which has attracted many workers is that which pertains to the life of the cell—the science of cytology. Although the celltheory was established as early as 1839, little advance was made in this subject in America before 1880. Since that time, however, Americans have been so successful in cytological discoveries that they are now among the world’s leaders in this field.

These studies have been followed along both descriptive and experimental lines. The most prominent of the early workers in this field are E. L. Mark and E. B. Wilson. Mark’s description of the maturation, fecundation, and segmentation of the egg is the most accurate and complete of the early cytological studies. Wilson’s discoveries concerning the details of fertilization and his “Atlas of Fertilization and Karyokinesis,” published in 1895, have now become classic. Wilson, too, has published the only American text-book on cytology,[^[182]] and has more recently taken the lead in studies concerning the relation between the chromosomes and sex. Besides Wilson, Montgomery, Mark, McClung, Morgan, Miss Stevens, Conklin and their associates and students have now furnished conclusive evidence that the sex of an organism is determined by, or associated with, the nuclear constitution of the fertilized egg. This constitution is moreover shown to be dependent upon the chromosomes received from the germ cells.

This explanation is in strict accordance with the results of experimental breeding. It is also quite in harmony with the Mendelian law of inheritance, and in fact forms one of the strongest supports for the view that all Mendelian factors are resident in the chromosomes. Recent work has also discovered the mechanism which governs the complicated conditions of sex which occur in those animals which exhibit alternating sexual and parthenogenetic generations. These remarkable processes are in all cases found to depend upon a definite distribution of the chromosomes.

Other recent experimental work has shown that while the sex is thus normally determined in the fertilized egg, it is in some animals not irrevocably fixed, and the normal effect of the sex chromosomes may be inhibited by abnormal conditions in the developing embryo, as is demonstrated by the recent work of Lillie and others.

The cytological basis for Mendelian inheritance has been very extensively studied by Morgan and his pupils in connection with their work on inheritance in the common fruit fly Drosophila. The evidence supports Weismann’s earlier hypothesis that the chromosomes are the bearers of the heritable factors, and that these are arranged in a series in the different chromosomes. This theory is shown to be in such strict accord with both the cytological studies and the results of experimental breeding that Morgan has ventured to indicate definite points in particular chromosomes as the loci of definite heritable factors, or genes.

Confirmation of this view is furnished by the behavior of the so-called sex-linked characters, the genes for which are situated in the same chromosome as that which carries the sex factor. Many ingenious breeding experiments indicate further that all the hereditary characters in Drosophila are borne in four great linkage groups corresponding with the four pairs of chromosomes which the cells of this fly possess.

Comparative Physiology.

None of the experimental fields has been of greater importance in zoological progress than that which concerns the functions of the various organs. Without this companion science morphology and comparative anatomy would have become unintelligible. American investigators, among whom G. H. Parker stands prominent, have taken a leading part in this field also.

Neurology.—The physiological analysis of the components of the nervous system, both in vertebrates and invertebrates, is another important branch of experimental biology. The 28 volumes of the Journal of Comparative Neurology attest the large influence that American investigators have had in the development of this science.

Regeneration.—Experimental studies on the powers of regeneration in plants and animals have been made from the earliest times. During the past few years, however, there has been made a concerted attempt to analyze the factors which determine the amount and rate of regeneration. Much progress has been made toward the postulation of definite laws applicable to the regenerative processes of the parts of each organism. The critical analyses of Morgan, Loeb and Child have been particularly stimulating.

Tissue Culture.—Another line of experimental work which has been developed within the past few years by Harrison, Carrell, and others is the culture of body tissues in artificial media. These experiments have included the cultivation in tubes or on glass slides of the various tissues of numerous species of animals. They have yielded much information regarding the structure, growth and multiplication of cells, the formation of tissues, and the healing of wounds.

Transplantation and Grafting.—Closely associated experiments consist in the transplantation of organs or other portions of the body to abnormal positions, to the bodies of other animals of the same species or of other species. In this way much has been learned about the potentiality of organs for self-differentiation, for regulation, for regeneration and for compensatory adaptations. The experiments have shown, further, the independence of soma and germplasm and have revealed the nature of certain organs whose functions were previously obscure.

Tropisms and Instincts.—Another field of experimental biology concerns the analysis of behavior of organisms in response to various forms of stimuli. These studies are being prosecuted on all groups of organisms, including the larval stages of many animals, and are yielding most remarkable results. The success in this field of research is largely due to stimulating influence of Jacques Loeb, Parker, Jennings, and their co-workers.

Biological Chemistry.—Still another experimental field which has developed into one of the most important of the biological sciences relates to the fundamental chemical and physical changes which underlie all organic phenomena. A knowledge of both physiological and physical chemistry is to-day essential for all advanced biological work. The peculiar nature of life itself, of growth, disease, old-age, degeneration, death and dissolution are presumably only manifestations of chemical and physical laws. The ultimate goal of all experimental biology, therefore, will be reached only when the basic physico-chemical properties of life are understood. At that time only will the perennial controversy between vitalism and mechanism be ended.

Economic Zoology.

A moment’s reflection will show that economic biology is the most essential of all sciences to the human welfare and progress. For man’s relation to his environment is such that the penalty for ignorance or neglect of the biological principles involved in the struggle for existence quickly overwhelms him with a horde of parasites or other enemies.

It is only by the intelligent application of biological knowledge that our food supplies, our forests, our domesticated animals and our bodies can be protected from the ever ravenous organisms which surround us.

The losses to food supplies and other products by insects alone amounts to 100 millions of dollars a month in the United States. And the parasites cause losses in sickness and premature deaths each year of many millions more. Then there are the destructive rodents and other animals which add largely to our burdens of support. These enemies next to wars and fungi are the most destructive agencies on earth. Could they but be eliminated man’s struggle against opposing forces would be in large measure overcome. The results of recent work in economic zoology, both in regard to the destruction of enemies and protection of useful mammals, birds and fishes, furnish a bright outlook for the future.

Protozoology.—Partly as an experimental field for the solution of general biological problems and partly because of its practical applications the study of protozoa has now developed into a special science.

The results of the investigations of Calkins, Woodruff, Jennings and others have greatly supplemented our understanding of the signification of such important biological phenomena as reproduction, sexual differentiation, conjugation, tropisms, and metabolism.

From an economic standpoint the protozoa have recently been shown to be of the greatest importance because of the human and animal diseases for which they are responsible.

Parasitology.—The animal parasites of man, domesticated animals and plants include numerous species of protozoa, worms, and insects. Together with the bacteria and a few higher fungi they cause all communicable diseases. When we consider that not only our health but also our entire food supply is dependent upon the elimination of these organisms we must admit that parasitology is the most important economically of all the sciences.

The reports of the investigations of Stiles and his associates in the Hygienic Laboratory and of Ransom and his staff in the Bureau of Animal Industry are widely distributed by the federal government. The systematic studies so ably begun by Joseph Leidy in the middle of the last century have been continued by Ward, Linton, Pratt, Curtis and others on the parasites of many groups of animals.

Economic Entomology.—Another extremely important biological science, the practical applications of which are second only to those of parasitology in importance, is entomology. In the last few years economic entomology has exceeded any of the other branches of biology in the number of its investigators. The American Association of Economic Entomologists has a membership of about five hundred. The work of most of these is supported by appropriations from the State and federal governments, and the results of their investigations are widely published.

It is now well known that some of the protozoon parasites are conveyed from man to man only through the bites of insects. The local eradication of several of our most fatal diseases has recently been brought about by the application of measures to destroy such insects. This is the greatest triumph of economic zoology.

Economic Ichthyology.—The U. S. Fish Commission has for many years been actively engaged in investigations on the food fishes, including methods for increasing the food supply by suitable protection and artificial propagation. The work includes also edible and otherwise useful mollusks and crustacea. Their marine and fresh-water laboratories have also been of great service to general biological science.

Economic Ornithology and Mammalogy.—In addition to the local bird clubs and the American Ornithologists Union for the study and preservation of bird and mammal life, the Bureau of Biological Survey has for some years conducted investigations on the economic importance of the various species. The publications of this Bureau are of great value both in determining the economic status of our birds and mammals, and also in recommending means for the protection of the beneficial species and the destruction of the injurious. Several of the States issue similar publications.

Genetics.

One of the most interesting chapters in biology relates to the development of the modern science of heredity, or genetics.

Previous to the year 1900, when the Mendelian principle of inheritance was re-discovered, the relative importance of heredity and of environment in the development of an organism was little understood. It is true that Weismann had insisted on the independence of soma and germplasm some years earlier (1883), but the body of the individual was still generally considered the key to its inheritance.

The recognition of the general application of Mendel’s discovery gave a great impetus to experimental breeding both in plants and animals. While heretofore it had been necessary to depend upon the somatic characters as evidence of the hereditary constitution of an individual, it now became possible, knowing the hereditary constitution of the parents of any pair of individuals, to predict with almost mathematical certainty the characters of their possible offspring.

In general, the laws of possible chance combinations of any group of characters determine the probability of any particular offspring possessing one or many of those characters. The physical basis for such Mendelian inheritance is evidently the chance combinations of chromosomes which result from the processes of maturation and union of the germ cells.

Certain limitations to the law are met with because the relatively small number of chromosomes involves linkage of genes, because of the occasional interchange of groups of genes between homologous chromosomes, and because the relative activity or potency of any particular gene may differ in different races, and, finally, because the normal activity of any given gene may be modified or inhibited by the action of other genes. It is by no means certain, however, that all inheritance is Mendelian, for there still remains much evidence that the hereditary basis of certain characters may be resident in the cytoplasm, rather than in the chromosomes. A recent book by Morgan, Sturtevant, Müller and Bridges (1915), entitled “the mechanism of Mendelian heredity” gives the cytological explanation of Mendelian inheritance.

Americans have from the first taken a leading part in this field of research and have been quick to recognize its practical applications to the improvement of breeds in both animals and plants. This prominent position is largely due to the experimental work of Castle, Davenport, Morgan, Jennings, Pearl, and their co-workers on animals and that of East, Emerson, Davis, Hayes and Shull on plants.

The geneticist now realizes that the appearance of the body (phenotype) gives but little clue to the inheritance (genotype). That two white flowers produce only purple offspring, or two white fowls only deeply colored chickens, or that a pair of guinea pigs, one of which is black and the other white, have only gray agouti offspring, while other apparently similar white flowers or white animals produce offspring like themselves, is now readily comprehensible and mathematically predictable.

The most important application of our newly acquired knowledge of inheritance is in the improvement of the human race. The wonderful opportunity in this direction must be apparent to all. The welfare of humanity depends upon the immediate adoption of eugenic principles. The Eugenics Record Office has secured many of the essential data.

With the destruction of the world’s best germ plasm at a rate never equalled before, the outlook for the future race would be appalling were it not for the hope that with the advent of a righteous peace will come a realization of the necessity of applying these new biological discoveries to improving the races of men. That the discoveries have been made too late in the world’s history to be of such use to humanity must not be thought possible.

Evolution.

Previous to the publication of Darwin’s “Origin of Species” in 1859, American zoologists were generally inclined toward special creation, in spite of the evidences for evolution which had been presented by Erasmus Darwin, Buffon, Lamarck, and Geoffroy St.-Hilaire. This attitude of mind continued for some years after the publication of the natural selection theory of Darwin and Wallace. This was in part due to the powerful influence of Louis Agassiz and others who bitterly opposed the Darwinian theory. The influence of Asa Gray in gaining a general acceptance for this theory is explained in the following chapter.

A modified Lamarckian doctrine was widely accepted in the last quarter of the century, due largely to the influence of Cope, Hyatt and Packard. The inheritance of “acquired characters” demanded by this theory seems incompatible with the discoveries of recent times, so that “to-day the theory has few followers amongst trained investigators, but it still has a popular vogue that is wide-spread and vociferous.”[^[183]]

The origin of new varieties and species by accidental and fortuitous modifications (mutations) of the germplasm is now the most widely accepted theory of evolution.

Some of the most important discoveries regarding the origin of new forms have been recently made by Morgan and his pupils. From a stock of the common fruit fly (Drosophila ampelophila) more than 125 new types have arisen within six years. Each of these types breeds true. “Each has arisen independently and suddenly. Every part of the body has been affected by one or another of these mutations.” To arrange these mutations arbitrarily into graded series would give the impression of an evolutionary series, but this is directly contrary to the known facts concerning their origin, for each mutation “originated independently from the wild type.” “Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the individual.” This evolutionary process is usually accompanied by the elimination of those forms which have remained stable or which have developed adverse mutations.

A question that is being vigorously debated at this time concerns the possible effects of selection on the hereditary factors. Are the genes fixed both qualitatively and quantitatively or does a given gene vary in potency under different conditions and in different individuals? In the former case selection can only separate the existing genes into separate pure strains. But if the gene be quantitatively variable, then selection will result in the establishment of new types.

Castle has long stoutly maintained the effect of such selection, and his forces have recently been augmented by Jennings. The experimental work now in process will doubtless yield a decisive answer.

Conclusion.

A comparison of the simple descriptive natural history of a century ago with the foregoing manifold developments of modern biology will indicate the wonderful progress which has occurred during this period. The path has led from the crude methods of the almost unaided eye and hand to the applications of the most delicate experimental apparatus. For the marvelous success which zoology has attained has been possible only by the skillful use of scalpel, microscope, microtome and other mechanical devices and by the refined methods of the chemist and physicist.

The central truth to which all these discoveries consistently point is the unity and harmony of all biological phenomena, and indeed of all nature. No longer does the zoologist find any demarcated line separating his field of research from that of the botanist or the chemist or even of the physicist, for all the natural sciences obviously deal with closely associated phenomena. The aim of the future will be both to complete fields of study already marked out and to derive a comprehensive explanation of the general principles involved.

Notes.

[172]. Proc. Biol. Soc. Washington, 3, 35, 1886.

[173]. Ibid., 4, 9, 1888. Both of these papers are reprinted in Ann. Rept. Smithsonian Inst., 1897, U. S. Nat. Mus., Pt. 2, pp. 357–466, 1901.

[174]. Louis Agassiz: his Life and Correspondence, by Elizabeth Carey Agassiz, p. 145, 1885.

[175]. List of North American Land Mammals in the United States National Museum, 1911. Bull. 79, U. S. Nat. Mus., 1912.

[176]. Birds of North and Middle America, Bull. 50, parts I-VII, U. S. Nat. Mus., 1901–1916.

[177]. Report U. S. Nat. Mus. for 1898, pp. 153–1270, 1900.

[178]. Bull. 34, U. S. Nat. Mus., 1889.

[179]. Bull. 47, parts I-IV, U. S. Nat. Mus., 1896–1900.

[180]. J. Loeb, The Organism as a Whole, p. 126, 1916.

[181]. The Germ-cell Cycle in Animals, 1914.

[182]. The Cell in Development and Inheritance, 1896; second edition, 1900.

[183]. Morgan, T. H. A critique of the theory of evolution, p. 32, 1916.

XIII
THE DEVELOPMENT OF BOTANY SINCE 1818

By GEORGE L. GOODALE

“Our Botany, it is true, has been extensively and successfully investigated, but this field is still rich, and rewards every new research with some interesting discovery.”

Such are the words with which the sagacious and far-sighted founder of the American Journal of Science and Arts, in his general introduction to the first volume, alludes to the study of plants. It is plain that the editor, embarking on this new enterprise, appreciated the attractions of this inviting field and sympathetically recognized the good work which was being done in it. It is not surprising, therefore, to find that he welcomed to the pages of his initial number contributions to botany.

Early Botanical Works.—The collections of dried and living North American plants, which had been carried from time to time to botanists in Europe, had been eagerly studied, and the results had been published in accessible treatises. Besides these general treatises, there had been issued certain works, wholly devoted to the American Flora. Among these latter may be mentioned Pursh’s “Flora” (1814) and Nuttall’s “Genera” (1818). There were also a few works which were rather popular in their character, such as Amos Eaton’s “Manual of Botany for North America” (1817), and Bigelow’s “Collection of the Plants of Boston and environs” (1814). These handbooks were convenient, and possessed the charm of not being exhaustive; consequently a botanist, whether professional or amateur, was stimulated to feel that he had a good chance of enriching the list of species and adding to the next edition.

The Early Years of Botany in the Journal.

At that time, the botanists had no journal in this country devoted to their science. Here and there they found opportunity for publishing their discoveries in some medical periodical or in a local newspaper. Hence American botanists availed themselves of the welcome extended by Silliman to botanical contributors to place their results on record in a magazine devoted to science in its wide sense. Specialization and subdivision of science had not then begun to dissociate allied subjects, and, consequently, botanists felt that they would be at home in this journal conducted by a chemist. Botanists responded promptly to this invitation with interesting contributions.

It is well to remember that the appliances at the command of naturalists at the date when the Journal began its service, were imperfect and inadequate. The botanist did not possess a convenient achromatic microscope, and he was not in possession of the chemical aids now deemed necessary in even the simplest research. Hence, attention was given almost wholly to such matters as the forms of plants and the more obvious phenomena of plant-life. In view of the poverty of instrumental aids in research, the results attained must be regarded as surprising.

In the very first volume of the Journal, bearing the date of 1818, there are descriptions of four new genera and of four new species of plants; certainly a large share to give to systematic botany. Besides these articles, there are some instructive notes concerning a few plants, which up to that time had been imperfectly understood. There are four Floral Calendars which give details in regard to the blossoming and the fruiting of plants in limited districts, a botanical subject of some importance but likely to become tedious in the long run. Just here, the skill of the editor in limiting undesirable contributions is shown by his tactful remark designed to soothe the feelings of a prolix writer whose too long list of plants in a floral calendar he had editorially cut down to reasonable limits. The editor remarks, “such extended observations are desirable, but it may not always be convenient to insert very voluminous details of daily floral occurrence.” It is convenient to consider by themselves some of the botanical contributions published in the first series of volumes of the Journal during a period of twenty years, the period before Asa Gray became actively and constantly associated with the Journal.

In systematic and geographical botany one finds communications from Douglass and Torrey (4, 56, 1822) on the plants of what was then the Northwest; Lewis C. Beck (10, 257, 1826; 11, 167, 1826; 14, 112, 1828) contributed valuable papers on the botany of Illinois and Missouri; there is a literal translation by Dr. Ruschenberger (19, 63, 299, 1831; 20, 248, 1831; 23, 78, 250, 1833) of a very long list of the plants of Chili; Wolle and Huebener (37, 310, 1839) gave an annotated catalogue of botanical specimens collected in Pennsylvania; Tuckerman (45, 27, 1843) presented communications in regard to numerous species which he had examined critically; Darlington (41, 365, 1841) published his lecture on grasses; Asa Gray (40, 1, 1841) gave an instructive account of European herbaria visited by him, and he contributed also a charming account (42, 1, 1842) of a botanical journey to the mountains of North Carolina. The most extensive series of botanical communication at this time was the Caricography by Professor Dewey of Williams College, presented in many numbers of the Journal; the first of these in 7, pp. 264–278, 1824. There were also descriptions of certain new genera, and species, and critical studies in synonyms.

Cryptogamic botany is represented in the first series of volumes of the Journal by L. C. Beck’s (15, 287, 1829) study of ferns and mosses, by Bailey’s (35, 113, 1839) histology of the vascular system of ferns, by Fries’ Systema mycologicum (12, 235, 1829), and by De Schweinitz (9, 397, 1825) and Halsey, who had in hand a cryptogamic manual. There are two important papers by Alexander Braun, translated by Dr. George Engelmann, one on the Equisetaceæ of North America (46, 81, 1844) and the other on the Characeæ (46, 92, 1844).

Vegetable paleontology had begun to attract attention in many places in this country, and therefore the translated contributions by Brongniart on fossil plants were given space in the Journal. Plant-physiology received a good share of attention either in short notices or in longer articles. Such titles appear as, the respiration of plants, the circulation of sap, the excrementitious matter thrown off by plants, the effects of certain gases and poisons on plants, and the relations of plants to different colored light. One of the most important of the notes is that in which is described the discovery by Robert Brown (19, 393, 1831) of the constant movement of minute particles suspended in a liquid, first detected by him in the fovilla of pollen grains, and now known as the Brownian (or Brunonian) movement. The heading under which this note appears is of interest, “The motion of living particles in all kinds of matter.”

One side of botany touches agriculture and economics. That side was represented even in the first volume of the Journal by a study of “the comparative quantity of nutritious matter which may be obtained from an acre of land when cultivated with potatoes or wheat.” Succeeding volumes in this series likewise present phases which are of special interest regarded from the point of view of economics; for example, those which treat of rotation of crops and of enriching the soil. Probably the economic paper which may be regarded as the most important, in fact epoch-making, is the full account of the invention by Appert of a method for preserving food indefinitely (13, 163, 1828). We all know that Appert’s process has revolutionized the preservation of foods, and in its modern modification underlies the vast industry of canned fruits, vegetables and so on. There are suggestions, also, as to the utilization of new foods, or of old foods in a new way, which resemble the suggestions made in these days of food conservation. For example, it is shown that flour can be made from leguminous seeds by steaming and subsequent drying, and pulverizing. There are excellent hints as to the best ways of preparing and using potatoes, and also for preserving them underground, where they will remain good for a year or two. It is shown that potato flour can be made into excellent bread. Another method of making bread, namely from wood, is described, but it does not seem quite so practicable. There are interesting notes on the sugar-beet as a source of sugar, and here appears one of the earliest accounts of the Assam tea-plant, which was destined to revolutionize the tea industry throughout the world. Cordage and textile fibers of bark and of wood should be utilized in the manufacture of paper. In fact one comes upon many such surprises in economic botany as the earlier volumes of the Journal are carefully examined.

Early numbers of the Journal present with sufficient fullness accounts of the remarkable discovery by Daguerre and others of a process for taking pictures by light, on a silver plate or upon paper (37, 374, 1839; 38, 97, 1840, etc.). Before many years passed, the Journal had occasion to show that these novel photographic delineations could be made useful in the investigation of problems in botany. In the pages of the Journal it would be easily possible to trace the development of this art in its relations to natural history. Silliman possessed great sagacity in selecting for his enterprise all the novelties which promised to be of service in the advancement of science. In 1825 (9, 263) the Journal republished from the Edinburgh Journal of Science an essay by Dr. (afterwards Sir) William Jackson Hooker, on American Botany. In this essay the author states that “the various scientific Journals” which “are published in America, contain many memoirs upon the indigenous plants. Among the first of these in point of value, and we think also the first with regard to time, we must name Silliman’s Journal of Science.” The author enumerates some of the contributors to the Journal and the titles of their papers.

It has been a useful practice of the Journal, almost from the first, to transfer to its pages memoirs which would otherwise be likely to escape the notice of the majority of American botanists. The book notices and the longer book reviews covered so wide a field that they placed the readers of the Journal in touch with nearly all of the current botanical literature both here and abroad. These critical notices did much towards the symmetrical development of botany in the United States. And as we shall now see, the Journal notices and reviews in the hands of Asa Gray continued to be one of the most important factors in the advancement of American botany.

Asa Gray and the Journal.

In 1834 there appears in the Journal (25, 346) a “Sketch of the Mineralogy of a portion of Jefferson and St. Lawrence Counties, New York, by J. B. Crawe of Watertown and A. Gray of Utica, New York.” This appears to be the first mention in the Journal of the name of Dr. Asa Gray, who, shortly after that date, became thoroughly identified with its botanical interests. In the early part of his career both before and immediately after graduating in medicine, Gray gave much attention to the different branches of natural history in its wide sense. He not only studied but taught “chemistry, geology, mineralogy, and botany,” the latter branch being the one to which he devoted most of his attention. Among his early guides in the pursuit of botany may be mentioned Dr. Hadley, “who had learned some botany from Dr. Ives of New Haven,” and Dr. Lewis C. Beck of Albany, author of Botany of the United States North of Virginia. At that period he made the acquaintance of Dr. John Torrey of New York, with whom he later became associated in most important descriptive work. During the years between his graduation in medicine and 1842, the year when he came to Harvard College, his activities were diverse and intense; so that his preparation for his distinguished career was very broad and thorough. His first visit to Europe, in 1838, brought him into personal relations with a large number of the botanists of Great Britain and the Continent. This extensive acquaintance, added to his broad training, enabled him even from the outset to exert a profound influence upon the progress of his favorite science. He made the Journal tributary to this development. His name first appears as associate editor in 1853, but there are articles in the Journal from his pen which bear an earlier date. The first of these early botanical papers is the following: “A Translation of a memoir entitled ‘Beiträge zur Lehre von der Befruchtung der Pflanzen,’ (contributions to the doctrine of the impregnation of plants, by A. J. C. Corda:) with prefatory remarks on the progress of discovery relative to vegetable fecundation; by Asa Gray, M. D.” (31, 308, 1837). Dr. Gray says that he made the translation from the German for his own private use, but thinking that it might be interesting to the Lyceum, he brought it before the Society, with “a cursory account of the progress of discovery respecting the fecundation of flowering plants, for the purpose of rendering the memoir more generally intelligible to those who are not particularly conversant with the present state of botanical science.” The translation occupies six pages of the Journal, while the prefatory remarks fill nine pages. The prefatory remarks constitute an exhaustive essay on the subject, embodied in attractive and perfectly clear language. The translator shows complete familiarity with the matter in hand and gives an adequate account of all the work done on the subject up to the date of M. Corda’s paper. A second important paper by him near this period is his review of “A Natural System of Botany: or a systematic view of the Organization, Natural Affinities, and Geographical Distribution of the whole Vegetable Kingdom; together with the use of the more important species in Medicine, the Arts, and rural and domestic economy, by John Lindley. Second edition, with numerous additions and corrections, and a complete list of genera and their synonyms. London: 1836” (32, 292, 1837). A very brief notice of this work in the first part of the volume for 1837 closes with the words, “A more extended notice of the work may be expected in the ensuing number of the Journal.” The extended notice proved to be a critical study of the work, signed by the initials A. G. which later became so familiar to readers of the Journal. Citation of a few of its sentences will indicate the strong and quiet manner in which Dr. Gray, even at the outset, wrote his notices of books. In speaking of the second edition of Professor Lindley’s work, he says:

“It is not necessary to state that a treatise of this kind was greatly needed, or to allude to the peculiar qualifications of the learned and industrious author for the accomplishment of the task, or the high estimation in which the work is held in Europe. But we may properly offer our testimony respecting the great and favorable influence which it has exerted upon the progress of botanical science in the United States. Great as the merits of the work undoubtedly are, we must nevertheless be excused from adopting the terms of extravagant and sometimes equivocal eulogy employed by a popular author, who gravely informs his readers that no book, since printed Bibles were first sold in Paris by Dr. Faustus, ever excited so much surprise and wonder as did Dr. Torrey’s edition of Lindley’s Introduction to the Natural System of Botany. Now we can hardly believe that either the author or the American editor of the work referred to was ever in danger, as was honest Dr. Faustus, of being burned for witchcraft, neither do we find anything in its pages calculated to produce such astonishing effects, except, perhaps, upon the minds of those botanists, if such they may be called, who had never dreamed of any important changes in the science since the appearance of good Dr. Turton’s translation of the Species Plantarum, and who speak of Jussieu as a writer who has greatly improved the natural orders of Linnæus.”

In the Journal for 1840 there is a large group of unsigned book reviews under the heading, “Brief notices of recent Botanical works, especially those most interesting to the student of North American Botany.” The first of these short reviews deals with the second section of Part VII of De Candolle’s “Prodromus.” In 1847 the consideration of the “Prodromus” is resumed by the same author and the initials of A. G. are appended. This indicates that Dr. Gray was probably the writer of some of the unsigned book reviews which had appeared in the Journal between 1837 and 1840. Doubtless Silliman availed himself of the assistance of his associates, Eli Ives and others, in New Haven, in the examination of current botanical literature, and it is extremely probable that he early secured help from young Dr. Gray, who had shown himself to be a keen critic as well as a pleasing writer. The notices of botanical works from 1840 bear marks of having been from the same hand. They cover an extremely wide range of subjects. While they are good-tempered they are critical, and they had much to do with the development of botany, in this country, along safe lines.

Gray as Editor.—Gray’s name as associate editor of the Journal appears in 1853. He had been a welcome contributor, as we have seen, for many years. His influence upon the progress of botany in the United States was largely due to his connection with the Journal. His reviews extended over a very wide range, and supplemented to a remarkable degree his other educational work. It must be permitted to allude here to his sagacity as a writer of educational treatises. In his first elementary text-book, published in 1836, he expressed wholly original views in regard to certain phases of structure and function in plants, which became generally adopted at a later date. His Manual of Botany was constructed, and subsequent editions were kept, on a plan which made no appeal to those who wanted to work on lines of least resistance; in fact he had no patience with those who desired merely to ascertain the name of a plant. In the Journal he emphasizes the desirability of learning all the affinities of the plant under consideration. At a later period, when entirely new chapters had been opened in the life of plants, he sought by his contributions in the Journal to interest students in this wider outlook.

Professor C. S. Sargent has selected with good judgment some of the more important scientific papers by Professor Gray and has republished them in a convenient form.[^[184]] Many of these papers were contributed to the Journal in the form of reviews. These reviews touch nearly every branch of the science of botany. As Sargent justly says, “Many of the reviews are filled with original and suggestive observations, and taken together, furnish the best account of the development of botanical literature during the last fifty years that has yet been written.” In these longer reviews in the Journal, Gray was wont to take a book under review as affording an opportunity to illustrate some important subject, and many of the reviews are crowded with his expositions. For example, in his examination of vonMohl’s “Vegetable Cell” (15, 451, 1853) he takes up the whole subject of microscopic structure, so far as it was then understood, and he points out the probable errors of some of Mohl’s contemporaries, showing what and how great were Mohl’s own contributions to histology. Such a review is a landmark in the science. The physiology of the cell and the nutrition of the plant were favorite topics with Professor Gray, and he brought much of his knowledge in regard to them into such a review as that of Boussingault (25, 120, 1858) on the “Influence of nitrates on the production of vegetable matter.”

As a systematic botanist, Gray was naturally much interested in the vexed question of nomenclature of plants. One of his most important communications to the Journal is his review, in the volume for 1883 (26, 417), of DeCandolle’s work on the subject. He deals with this strictly technical matter much as he did in a contribution to the Journal which he made in 1868 (46, 63). In both of these papers he states with clearness the general features of the code of nomenclature. He says explicitly that the code does not make, but rather declares, the common law of botanists. The treatment of the subject at his hands would rightly impress a general reader as showing a strong desire to have common sense applied to doubtful cases, instead of insisting on inflexible rules. For this reason, his rule of practice was not always acceptable to those who were anxious to secure conformity to arbitrary rules at whatever cost. As he said in a paper published in the Journal in 1847 (3, 302), “The difficulty of a reform increases with its necessity. It is much easier to state the evils than to relieve them; and the well-meant endeavors that have recently been made to this end, are, some of them, likely, if adopted, to make confusion worse confounded.” This feeling led him to be very conservative in the matter of reform in nomenclature.

This subject of botanical nomenclature illustrates a method frequently employed by Professor Gray to elucidate a difficult matter. He would find in the treatise under review a text, or texts, on which he would build a treatise of his own, and in this way he made clear his own views relative to most of the important phases of botany. When he faced controverted matters, his attitude still remained judicial. While he was tolerant of opinions which clashed with his own, he was always severe upon charlatanism and impatient of inaccuracy. The pages of the Journal contain many severe criticisms at his hands, but an unprejudiced person would say that the severity is merited.

Sometimes, however, instead of reviewing a book or an address, he would follow the custom inaugurated early in the history of the Journal, of making copious extracts, and thus give to its readers an opportunity of examining materials which otherwise might not fall in their way.

Gray’s contributions to the Journal comprise more than one thousand titles, without counting the memorial notices and the shorter obituary notes. In these notices he sums up in a few well-chosen words the contributions made to botany by his contemporaries. Even in the few instances in which he felt obliged to note with disapproval some of the work, he expressed himself with personal friendliness. The necrology, as it appeared from month to month, was a labor of love. All of the longer memorial notices are what it is the fashion now-a-days to call appreciations, and these are so happily phrased that it would seem as if the writer in many a case asked himself, “Would my friend, about whom I am now writing, make any change in this sketch?”

Gray on Darwinism.—In October, 1859, Darwin’s epoch-making work, “The Origin of Species,” was published. An early copy was sent to the editor of the Journal, Professor James D. Dana. This arrived in New Haven on December 21, but it was preceded by a personal letter which is of so much interest that it is here transcribed in full. It should be added that Dana was at this time in Europe where he was spending a year in the search for health after a serious nervous breakdown. In his absence the book was noticed by Gray as stated below. The letter is, as follows:

Down, Bromley, Kent.

Nov. 11th, 1859.

My dear Sir,

I have sent you a copy of my Book (as yet only an abstract) on the Origin of Species. I know too well that the conclusion, at which I have arrived, will horrify you, but you will, I believe and hope, give me credit for at least an honest search after the truth. I hope that you will read my Book, straight through; otherwise from the great condensation it will be unintelligible. Do not, I pray, think me so presumptuous as to hope to convert you; but if you can spare time to read it with care, and will then do what is far more important, keep the subject under my point of view for some little time occasionally before your mind, I have hopes that you will agree that more can be said in favour of the mutability of species, than is at first apparent. It took me many long years before I wholly gave up the common view of the separate creation of each species. Believe me, with sincere respect and with cordial thanks for the many acts of scientific kindness which I have received from you,

My dear Sir,

Yours very sincerely,

Charles Darwin.

In March, 1860 (29, 153), Gray published in the Journal an elaborate and cautious review of Darwin’s work. He alluded to the absence of the chief editor of the Journal in the following words:

“The duty of reviewing this volume in the American Journal of Science would naturally devolve upon the principal editor whose wide observation and profound knowledge of various departments of natural history, as well as of geology, particularly qualify him for the task. But he has been obliged to lay aside his pen to seek in distant lands the entire repose from scientific labor so essential to the restoration of his health, a consummation devoutly to be wished and confidently to be expected. Interested as Mr. Dana would be in this volume, he could not be expected to accept its doctrine. Views so idealistic as those upon which his ‘Thoughts upon Species’ are grounded, will not harmonize readily with a doctrine so thoroughly naturalistic as that of Mr. Darwin.... Between the doctrines of this volume and those of the great naturalist whose name adorns the title page of this Journal [Mr. Agassiz] the widest divergence appears.”

Gray then proceeds to contrast the two views of Darwin and Agassiz, “for this contrast brings out most prominently and sets in strongest light and shade the main features of the theory of the origination of species by means of Natural Selection.” He then states both sides with great fairness, and proceeds:

“Who shall decide between such extreme views so ably maintained on either hand, and say how much truth there may be in each. The present reviewer has not the presumption to undertake such a task. Having no prepossession in favor of naturalistic theories, but struck with the eminent ability of Mr. Darwin’s work, and charmed with its fairness, our humbler duty will be performed if, laying aside prejudice as much as we can, we shall succeed in giving a fair account of its method and argument, offering by the way a few suggestions such as might occur to any naturalist of an inquiring mind. An editorial character for this article must in justice be disclaimed. The plural pronoun is employed not to give editorial weight, but to avoid even the appearance of egotism and also the circumlocution which attends a rigorous adherence to the impersonal style.”

In this review he moves slowly and thoughtfully, but not timidly, over the new paths. There is no clear indication in the review that he has yet made up his mind as to the validity of Darwin’s hypothesis. But, in a second article appearing in the Journal for September of the same year (30, 226), under the title “Discussion between two readers of Darwin’s treatise on the origin of species upon its natural theology” Gray plainly begins to incline to take a very favorable view of the Darwinian theory, and makes use of the following ingenious illustration to show that it is not inconsistent with theistic design. A few paragraphs here quoted show the felicity of his style in a controverted matter:

“Recall a woman of a past generation and show her a web of cloth; ask her how it was made, and she will say that the wool or cotton was carded, spun, and woven by hand. When you tell her it was not made by manual labor, that probably no hands have touched the materials throughout the process, it is possible that she might at first regard your statement as tantamount to the assertion that the cloth was made without design. If she did, she would not credit your statement. If you patiently explained to her the theory of carding-machines, spinning-jennies, and power-looms, would her reception of your explanation weaken her conviction that the cloth was the result of design? It is certain that she would believe in design as firmly as before, and that this belief would be attended by a higher conception and reverent admiration of a wisdom, skill, and power greatly beyond anything she had previously conceived possible.”

By this review Gray disarmed hostility to such an extent that some persons who had been antagonistic to Darwinism accepted it with only slight reservation. It may be fairly claimed that the Journal bore a leading part in influencing the views of naturalists in America in regard to the Darwinian theory.

Dr. Gray soon put the Darwinian hypothesis to a severe test. In the Journal for 1840 he had called attention to the remarkable similarity which exists between the flora of Japan and a part of the temperate portion of North America. The first notice of this subject by him occurs in a short review of Dr. Zuccarini’s “Flora Japonica,” a work based on material furnished by Dr. Siebold, who had long lived in Japan. In this review (39, 175, 1840), he enumerates certain plants common to the two regions, and says, “It is interesting to remark how many of our characteristic genera are reproduced in Japan, not to speak of striking analogous forms.” In a subsequent paper (28, 187, 1859), he recurs to this subject, and, after alluding to geological data furnished by J. D. Dana, he says:

“I cannot resist the conclusion that the extant vegetable kingdom has a long and eventful history, and that the explanation of apparent anomalies in the geographical distribution of species may be found in the various and prolonged climatic or other vicissitudes to which they have been subject in earlier times; that the occurrence of certain species, formerly supposed to be peculiar to North America, in a remote or antipodal region, affords in itself no presumption that they were originated there, and that interchange of plants between eastern North America and eastern Asia is explicable upon the most natural and generally received hypothesis (or at least offers no greater difficulty than does the arctic flora, the general homogeneousness of which round the world has always been thought compatible with local origin of the species) and is perhaps not more extensive than might be expected under the circumstances. That the interchange has mainly taken place in high northern latitudes, and that the isothermal lines have in earlier times turned northward on our eastern and southward on our northwest coast, as they do now, are points which go far towards explaining why eastern North America, rather than Oregon and California, has been mainly concerned in this interchange, and why the temperate interchange, even with Europe, has principally taken place through Asia.”

From “Life and Letters of Charles Darwin” by Francis Darwin.

This paper was communicated in 1859, on the eve of the publication of Darwin’s “Origin of Species.” At a later date he applied the Darwinian theory to the possible solution of the problem, and came to the conclusion that the two floras had a common origin in the Arctic zone, during the Tertiary period, or the Cretaceous which preceded it, and the descendants had made their way down different lines toward the south, the species varying under different climatic conditions, and thus exhibiting similarity but not absolute identity of form. Before the American Association for the Advancement of Science, in his Presidential address, in 1872, he used the following language:

“According to these views, as regards plants at least, the adaptation to successive times and changed conditions has been maintained, not by absolute renewals, but by gradual modifications. I, for one, cannot doubt that the present existing species are the lineal successors of those that garnished the earth in the old time before them, and that they were as well adapted to their surroundings then, as those which flourish and bloom around us are to their conditions now. Order and exquisite adaptation did not wait for man’s coming, nor were they ever stereotyped. Organic Nature—by which I mean the system and totality of living things, and their adaptation to each other and to the world—with all its apparent and indeed real stability, should be likened, not to the ocean, which varies only by tidal oscillations from a fixed level to which it is always returning, but rather to a river, so vast that we can neither discern its shores nor reach its sources, whose onward flow is not less actual because too slow to be observed by the ephemeræ which hover over its surface, or are borne upon its bosom.”

Gray’s active interest in the Journal continued until the very end of his life. There were many critical notices from his pen in 1887. His last contribution to its pages was the botanical necrology, which appeared posthumously in volume 35, of the third series (1888). His connection with the Journal covered, therefore, a period of more than a half a century of its life.[^[185]]

The changes that were wrought in botany by the application of Darwinism were far reaching. Attempts were promptly made to reconstruct the system of botanical classification on the basis of descent. The more successful of these endeavors met with welcome, and now form the groundwork of arrangement of families, genera, and species, in the Herbaria in this country, in the manuals of descriptive botany, and in the text-books of higher grade. This overturn did not take place until after Gray’s death, although he foresaw that the revolution was impending.

One of the most obvious changes was that which gave a high degree of prominence in American school treatises to the study of the lower instead of the higher or flowering plants, these latter being treated merely as members in a long series, and with scant consideration. But of late years, there has been a renewed popular interest in the phænogamia, leading to a more thorough investigation of local floras, and also to the examination of the relations of plants to their surroundings. The results of a large part of this technical work are published in strictly botanical periodicals and now-a-days seldom find a place in the pages of a general journal of science.

Cryptogamic Botany in the Journal since 1846.

In glancing rapidly at the First Series it has been seen that a fair share of attention was early paid by the Journal to the flowerless plants. So far as the means and methods of the time permitted, the ferns, mosses, lichens, and the larger algæ and fungi of America were studied assiduously and important results were published, chiefly on the side of systematic botany.

The Second Series comprises the years between 1846 and 1871. In this series one finds that the range of cryptogamic botany is much widened. Besides interesting book notices relative to these plants, there are a good many papers on the larger fungi, on the algæ, and mosses. Here are contributions by Curtis, by Ravenel, by Bailey, and by Sullivant. The lichens are treated of in detail by Tuckerman, and there are some excellent translations by Dr. Engelmann of papers by Alexander Braun. Some of the destructive fungi are considered, as might well be the case in the period of the potato famine. It is in these years that one first finds the name of Daniel Cady Eaton, who later had so much to do with developing an interest in the subject of ferns in this country. He was a frequent contributor of critical notices.

Cryptogamic Botany, as it is now understood, is a comparatively modern branch of science. The appliances and the methods for investigating the more obscure groups, and especially for revealing the successive stages of their development, were unsatisfactory until the latter half of the last century. Gray recognized this condition of affairs, and appreciated the importance of the new methods and the better appliances. Therefore he viewed with satisfaction the pursuit of these studies abroad by one of his students and assistants, William G. Farlow. Dr. Farlow carried to his studies under DeBary and others unusual powers of observation and great industry. He speedily became an accomplished investigator in cryptogamic botany and enriched the science by notable discoveries, one of which to-day bears his name in botanical literature. On his return to the United States, Farlow entered at once upon a successful career as an inspiring teacher and a fruitful investigator. He became a frequent contributor to the Journal, keeping its readers in touch with the more important additions to cryptogamic botany. He had wisely chosen to deal with the whole field, and consequently he has been able to preserve a better perspective than is kept by the extreme specialist. The greater number of cryptogamic botanists in this country have been under Professor Farlow’s instruction.

Systematic and Geographical Botany of Late Years.

The usefulness of the Journal in descriptive systematic botany of phanerogams is shown not only by its acceptance of the leading features of DeCandolle’s Phytography, where very exact methods are inculcated, but by the very numerous contributions by Sereno Watson and others at the Harvard University Herbarium, as well as from private systematists. It is in the pages of the Journal that one finds the record of much of the critical work of Tuckerman and of Engelmann, in interesting Phanerogamia. Of late years the Journal has had the privilege, of publishing a good deal of the careful work of Theo Holm, in the difficult groups of Cyperaceæ, and also his admirable studies in the morphology and the anatomy of certain interesting plants of higher orders.

Attention was called, in passing, to Gray’s deep interest in geographical botany. In this important branch, besides his contributions, one finds, among many others, such papers as LeConte’s “Flora of the Coast Islands of California in Relation to Recent Changes of Physical Geography” (34, 457, 1887), and Sargent’s “Forests of Central Nevada” (17, 417, 1879). Examination reveals a surprising number of communications which bear indirectly upon this subject.

Paleontological Botany.

When the Journal began its career, the subject of fossil plants was very obscure. Brongniart’s papers, especially the Journal translations, enabled the students in America to undertake the investigation of such fossils and the results were to a considerable extent published in the Journal. Since the subject belongs as much to geology as to botany, it finds its appropriate home in the pages of the Journal. The recent papers on this topic show how great has been the advance in methods and results since the early days of the Journal’s century. Under the care of George E. Wieland, the communications and the bibliographical notices of paleontological treatises show the progress which he and others are making in this attractive field.

Economic Botany, Plant Physiology, etc.

At the outset, the Journal, as we have seen, devoted much attention to certain phases of economic botany, and, even down to the present, it has maintained its hold upon the subject. The correspondence of Jerome Nicklès from 1853 to 1867 brought before its readers a vast number of valuable items which would not in any other way have been known to them. And the Journal dealt wisely with the scientific side of agriculture, under the hands of S. W. Johnson and J. H. Gilbert, and others, placing it on its proper basis. This work was supplemented by Norton’s remarkable work in the chemistry of certain plants, the oat, for example, and certain plant-products. In fact it might be possible to construct from the pages of the Journal a fair synopsis of the important principles of agronomy.

Physiology has been represented not only by the studies which had been inaugurated and stimulated by the Darwinian theory, such as the cross-fertilization and the close-fertilization of plants, plant-movements, and the like, but there have been a good many special communications, such as Dandeno on toxicity, Plowman on electrical relations, and ionization, and W. P. Wilson on respiration.

There are many broad philosophical questions which have found an appropriate home in the Journal, such as “The Plant-individual in its relation to the species” (Alexander Braun, 19, 297, 1855; 20, 181, 1855), and “The analogy between the mode of reproduction in plants and the alternation of generations observed in some radiata” (J. D. Dana, 10, 341, 1850). Akin to these are many of the reflections which one finds scattered throughout the pages of the Journal, frequently in minor book notices. As might be expected, some attention has been paid to the very special branch of botany which is strictly called medical. For example, early in its history, the Journal published a long treatise by Dr. William Tully (2, 45, 1820), on the ergot of rye. This is considered from a structural as well as from a medical point of view and is decidedly ahead of the time in which it was written. There are a few references to vegetable poisons, and there is a fascinating account of the effect of the common white ash on the activities of the rattlesnake. In short it may be said that the editor did much towards making the Journal readable as well as strictly scientific.

The list of reviewers who have been permitted to use the pages of the Journal for notices of botanical and allied books in recent years is pretty long. One finds the initials of Wesley R. Coe, George P. Clinton, Arthur L. Dean, Alexander W. Evans, William G. Farlow, George L. Goodale, Arthur H. Graves, Herbert E. Gregory, Lafayette B. Mendel, Leo F. Rettger, Benjamin L. Robinson, George R. Wieland, and others.

At the present time, in the biological sciences, as in every department of thought, there is great specialization, and each specialty demands its own private organ of publication. Naturally this has led to a falling off in the botanical communications to the Journal, but it cannot be forgotten that the history of North American Botany has been largely recorded in its pages.

Notes.

[184]. Scientific Papers of Asa Gray. Selected by Charles Sprague Sargent. Two volumes, Boston, 1889 (see notice in vol. 38, 419, 1889).

[185]. A notice of Gray’s life and works is given by his life-long friend, J. D. Dana, in the Journal in 1888 (35, 181–203).

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TRANSCRIBER’S NOTES

1. Typos fixed; non-standard spelling and dialect retained.
2. Used numbers for footnotes.