BOOK XIV.
THE ANALYTICAL SCIENCE.
HISTORY OF CHEMISTRY.
. . . . . . . Soon had his crew
Opened into the hill a spacious wound,
And digged out ribs of gold . . . .
Anon out of the earth a fabric huge
Rose like an exhalation, with the sound
Of dulcet symphonies and voices sweet,
Built like a temple.
Milton. Paradise Lost, i.
CHAPTER I.
Improvement of the Notion of Chemical Analysis, and Recognition of it as the Spagiric Art.
THE doctrine of “the four elements” is one of the oldest monuments of man’s speculative nature; goes back, perhaps, to times anterior to Greek philosophy; and as the doctrine of Aristotle and Galen, reigned for fifteen hundred years over the Gentile, Christian, and Mohammedan world. In medicine, taught as the doctrine of the four “elementary qualities,” of which the human body and all other substances are compounded, it had a very powerful and extensive influence upon medical practice. But this doctrine never led to any attempt actually to analyse bodies into their supposed elements: for composition was inferred from the resemblance of the qualities, not from the separate exhibition of the ingredients; the supposed analysis was, in short, a decomposition of the body into adjectives, not into substances.
This doctrine, therefore, may be considered as a negative state, antecedent to the very beginning of chemistry; and some progress beyond this mere negation was made, as soon as men began to endeavor to compound and decompound substances by the use of fire or mixture, however erroneous might be the opinions and expectations which they combined with their attempts. Alchemy is a step in chemistry, so far as it implies the recognition of the work of the cupel and the retort, as the produce of analysis and synthesis. How perplexed and perverted were the forms in which this recognition was clothed,—how mixed up with mythical follies and extravagancies, we have [already] seen; and the share which Alchemy had in the formation of any sounder knowledge, is not such as to justify any further notice of that pursuit.
The result of the attempts to analyse bodies by heat, mixture, and the like processes, was the doctrine that the first principles of things are three, not four; namely, salt, sulphur, and mercury; and that, of these three, all things are compounded. In reality, the doctrine, as thus stated, contained no truth which was of any value; for, though the chemist could extract from most bodies portions which he called salt, [262] and sulphur, and mercury, these names were given, rather to save the hypothesis, than because the substances were really those usually so called: and thus the supposed analyses proved nothing, as Boyle justly urged against them.[1]
[1] Shaw’s Boyle. Skeptical Chymist, pp. 312, 313. &c.
The only real advance in chemical theory, therefore, which we can ascribe to the school of the three principles, as compared with those who held the ancient dogma of the four elements, is, the acknowledgment of the changes produced by the chemist’s operations, as being changes which were to be accounted for by the union and separation of substantial elements, or, as they were sometimes called, of hypostatical principles. The workmen of this school acquired, no doubt, a considerable acquaintance with the results of the kinds of processes which they pursued; they applied their knowledge to the preparation of new medicines; and some of them, as Paracelsus and Van Helmont, attained, in this way, to great fame and distinction: but their merits, as regards theoretical chemistry, consist only in a truer conception of the problem, and of the mode of attempting its solution, than their predecessors had entertained.
This step is well marked by a word which, about the time of which we speak, was introduced to denote the chemist’s employment. It was called the Spagiric art, (often misspelt Spagyric,) from two Greek words, (σπάω, ἀγείρω,) which mean to separate parts, and to unite them. These two processes, or in more modern language, analysis and synthesis, constitute the whole business of the chemist. We are not making a fanciful arrangement, therefore, when we mark the recognition of this object as a step in the progress of chemistry. I now proceed to consider the manner in which the conditions of this analysis and synthesis were further developed.
CHAPTER II.
Doctrine of Acid and Alkali.—Sylvius.
AMONG the results of mixture observed by chemists, were many instances in which two ingredients, each in itself pungent or destructive, being put together, became mild and inoperative; each [263] counteracting and neutralizing the activity of the other. The notion of such opposition and neutrality is applicable to a very wide range of chemical processes. The person who appears first to have steadily seized and generally applied this notion is Francis de la Boé Sylvius; who was born in 1614, and practised medicine at Amsterdam, with a success and reputation which gave great currency to his opinions on that art.[2] His chemical theories were propounded as subordinate to his medical doctrines; and from being thus presented under a most important practical aspect, excited far more attention than mere theoretical opinions on the composition of bodies could have done. Sylvius is spoken of by historians of science, as the founder of the iatro-chemical sect among physicians; that is, the sect which considers the disorders in the human frame as the effects of chemical relations of the fluids, and applies to them modes of cure founded upon this doctrine. We have here to speak, not of his physiological, but of his chemical views.
[2] Sprengel. Geschichte der Arzneykunde, vol. iv. Thomson’s History of Chemistry in the corresponding part is translated from Sprengel.
The distinction of acid and alkaline bodies (acidum, lixivum) was familiar before the time of Sylvius; but he framed a system, by considering them both as eminently acrid and yet opposite, and by applying this notion to the human frame. Thus[3] the lymph contains an acid, the bile an alkaline salt. These two opposite acrid substances, when they are brought together, neutralize each other (infringunt), and are changed into an intermediate and milder substance.
[3] De Methodo Medendi, Amst. 1679. Lib. ii. cap. 28, sects. 8 and 53.
The progress of this doctrine, as a physiological one, is an important part of the history of medical science in the seventeenth century; but with that we are not here concerned. But as a chemical doctrine, this notion of the opposition of acid and alkali, and of its very general applicability, struck deep root, and has not been eradicated up to our own time. Boyle, indeed, whose disposition led him to suspect all generalities, expressed doubts with regard to this view;[4] and argued that the supposition of acid and alkaline parts in all bodies was precarious, their offices arbitrary, and the notion of them unsettled. Indeed it was not difficult to show, that there was no one certain criterion to which all supposed acids conformed. Yet the general conception of such a combination as that of acid and alkali was supposed to [264] be, served so well to express many chemical facts, that it kept its ground. It is found, for instance, in Lemery’s Chemistry, which was one of those in most general use before the introduction of the phlogistic theory. In this work (which was translated into English by Keill, in 1698) we find alkalies defined by their effervescing with acids.[5] They were distinguished as the mineral alkali (soda), the vegetable alkali (potassa), and the volatile alkali (ammonia). Again, in Macquer’s Chemistry, which was long the text-book in Europe during the reign of phlogiston, we find acids and alkalies, and their union, in which they rob each other of their characteristic properties, and form neutral salts, stated among the leading principles of the science.[6]
[4] Shaw’s Boyle, iii. p. 432.
[5] Lemery, p. 25.
[6] Macquer, p. 19.
In truth, the mutual relation of acids to alkalies was the most essential part of the knowledge which chemists possessed concerning them. The importance of this relation arose from its being the first distinct form in which the notion of chemical attraction or affinity appeared. For the acrid or caustic character of acids and alkalies is, in fact, a tendency to alter the bodies they touch, and thus to alter themselves; and the neutral character of the compounds is the absence of any such proclivity to change. Acids and alkalies have a strong disposition to unite. They combine, often with vehemence, and produce neutral salts; they exhibit, in short, a prominent example of the chemical attraction, or affinity, by which two ingredients are formed into a compound. The relation of acid and base in a salt is, to this day, one of the main grounds of all theoretical reasonings.
The more distinct development of the notion of such chemical attraction, gradually made its way among the chemists of the latter part of the seventeenth and the beginning of the eighteenth century, as we may see in the writings of Boyle, Newton, and their followers. Beecher speaks of this attraction as a magnetism; but I do not know that any writer in particular, can be pointed out as the person who firmly established the general notion of chemical attraction.
But this idea of chemical attraction became both more clear and more extensively applicable, when it assumed the form of the doctrine of elective attractions, in which shape we must now speak of it. [265]
CHAPTER III.
Doctrine of Elective Attractions. Geoffroy. Bergman.
THOUGH the chemical combinations of bodies had already been referred to attraction, in a vague and general manner, it was impossible to explain the changes that take place, without supposing the attraction to be greater or less, according to the nature of the body. Yet it was some time before the necessity of such a supposition was clearly seen. In the history of the French Academy for 1718 (published 1719), the writer of the introductory notice (probably Fontenelle) says, “That a body which is united to another, for example, a solvent which has penetrated a metal, should quit it to go and unite itself with another which we present to it, is a thing of which the possibility had never been guessed by the most subtle philosophers, and of which the explanation even now is not easy.” The doctrine had, in fact, been stated by Stahl, but the assertion just quoted shows, at least, that it was not familiar. The principle, however, is very clearly stated[7] in a memoir in the same volume, by Geoffroy, a French physician of great talents and varied knowledge, “We observe in chemistry,” he says, “certain relations amongst different bodies, which cause them to unite. These relations have their degrees and their laws. We observe their different degrees in this;—that among different matters jumbled together, which have a certain disposition to unite, we find that one of these substances always unites constantly with a certain other, preferably to all the rest.” He then states that those which unite by preference, have “plus de rapport,” or, according to a phrase afterwards used, more affinity. “And I have satisfied myself,” he adds, “that we may deduce, from these observations, the following proposition, which is very extensively true, though I cannot enunciate it as universal, not having been able to examine all the possible combinations, to assure myself that I should find no exception.” The proposition which he states in this admirable spirit of philosophical caution, is this: “In all cases where two substances, [266] which have any disposition to combine, are united; if there approaches them a third, which has more affinity with one of the two, this one unites with the third and lets go the other.” He then states these affinities in the form of a Table; placing a substance at the head of each column, and other substances in succession below it, according to the order of their affinities for the substance which stands at the head. He allows that the separation is not always complete (an imperfection which he ascribes to the glutinosity of fluids and other causes), but, with such exceptions, he defends very resolutely and successfully his Table, and the notions which it implies.
[7] Mém. Acad. Par. 1718, p. 202.
The value of such a tabulation was immense at the time, and is even still very great; it enabled the chemist to trace beforehand the results of any operation; since, when the ingredients were given, he could see which were the strongest of the affinities brought into play, and, consequently, what compounds would be formed. Geoffroy himself gave several good examples of this use of his table. It was speedily adopted into works on chemistry. For instance, Macquer[8] places it at the end of his book; “taking it,” as he says, “to be of great use at the end of an elementary tract, as it collects into one point of view, the most essential and fundamental doctrines which are dispersed through the work.”
[8] Pref., p. 13.
The doctrine of Elective Attraction, as thus promulgated, contained so large a mass of truth, that it was never seriously shaken, though it required further development and correction. In particular the celebrated work of Torbern Bergman, professor at Upsala, On Elective Attractions, published in 1775, introduced into it material improvements. Bergman observed, that not only the order of attractions, but the sum of those attractions which had to form the new compounds, must be taken account of, in order to judge of the result. Thus,[9] if we have a combination of two elements, P, s, (potassa and vitriolic acid), and another combination, L, m, (lime and muriatic acid,) though s has a greater affinity for P than for L, yet the sum of the attractions of P to m, and of L to s, is greater than that of the original compounds, and therefore if the two combinations are brought together, the new compounds, P, m, and L, s, are formed.
[9] Elect. Attract., p. 19.
The Table of Elective Attractions, modified by Bergman in pursuance of these views, and corrected according to the advanced knowledge of the time, became still more important than before. The next step [267] was to take into account the quantities of the elements which combined; but this leads us into a new train of investigation, which was, indeed, a natural sequel to the researches of Geoffroy and Bergman.
In 1803, however, a chemist of great eminence, Berthollet, published a work (Essai de Statique Chimique), the tendency of which appeared to be to throw the subject back into the condition in which it had been before Geoffroy. For Berthollet maintained that the rules of chemical combination were not definite, and dependent on the nature of the substances alone, but indefinite, depending on the quantity present, and other circumstances. Proust answered him, and as Berzelius says,[10] “Berthollet defended himself with an acuteness which makes the reader hesitate in his judgment; but the great mass of facts finally decided the point in favor of Proust.” Before, however, we trace the result of these researches, we must consider Chemistry as extending her inquiries to combustion as well as mixture, to airs as well as fluids and solids, and to weight as well as quality. These three steps we shall now briefly treat of.
[10] Chem. t. iii. p. 23.
CHAPTER IV.
Doctrine of Acidification and Combustion.—Phlogistic Theory.
PUBLICATION of the Theory by Beccher and Stahl.—It will be recollected that we are tracing the history of the progress only of Chemistry, not of its errors;—that we are concerned with doctrines only so far as they are true, and have remained part of the received system of chemical truths. The Phlogistic Theory was deposed and succeeded by the Theory of Oxygen. But this circumstance must not lead us to overlook the really sound and permanent part of the opinions which the founders of the phlogistic theory taught. They brought together, as processes of the same kind, a number of changes which at first appeared to have nothing in common; as acidification, combustion, respiration. Now this classification is true; and its importance remains undiminished, whatever are the explanations which we adopt of the processes themselves.
The two chemists to whom are to be ascribed the merit of this step, and the establishment of the phlogistic theory which they connected [268] with it, are John Joachim Beccher and George Ernest Stahl; the former of whom was professor at Mentz, and physician to the Elector of Bavaria (born 1625, died 1682); the latter was professor at Halle, and afterwards royal physician at Berlin (born 1660, died 1734). These two men, who thus contributed to a common purpose, were very different from each other. The first was a frank and ardent enthusiast in the pursuit of chemistry, who speaks of himself and his employments with a communicativeness and affection both amusing and engaging. The other was a teacher of great talents and influence, but accused of haughtiness and moroseness; a character which is well borne out by the manner in which, in his writings, he anticipates an unfavorable reception, and defies it. But it is right to add to this that he speaks of Beccher, his predecessor, with an ungrudging acknowledgment of obligations to him, and a vehement assertion of his merit as the founder of the true system, which give a strong impression of Stahl’s justice and magnanimity.
Beccher’s opinions were at first promulgated rather as a correction than a refutation of the doctrine of the three principles, salt, sulphur, and mercury. The main peculiarity of his views consists in the offices which he ascribes to his sulphur, these being such as afterwards induced Stahl to give the name of Phlogiston to this element. Beccher had the sagacity to see that the reduction of metals to an earthy form (calx), and the formation of sulphuric acid from sulphur, are operations connected by a general analogy, as being alike processes of combustion. Hence the metal was supposed to consist of an earth, and of something which, in the process of combustion, was separated from it; and, in like manner, sulphur was supposed to consist of the sulphuric acid, which remained after its combustion, and of the combustible part or true sulphur, which flew off in the burning. Beccher insists very distinctly upon this difference between his element sulphur and the “sulphur” of his Paracelsian predecessors.
It must be considered as indicating great knowledge and talent in Stahl, that he perceived so clearly what part of the views of Beccher was of general truth and permanent value. Though he[11] everywhere gives to Beccher the credit of the theoretical opinions which he promulgates, (“Beccheriana sunt quæ profero,”) it seems certain that he had the merit, not only of proving them more completely, and applying them more widely than his forerunner, but also of conceiving them [269] with a distinctness which Beccher did not attain. In 1697, appeared Stahl’s Zymotechnia Fundamentalis (the Doctrine of Fermentation), “simulque experimentum novum sulphur verum arte producendi.” In this work (besides other tenets which the author considered as very important), the opinion published by Beccher was now maintained in a very distinct form;—namely, that the process of forming sulphur from sulphuric acid, and of restoring the metals from their calces, are analogous, and consist alike in the addition of some combustible element, which Stahl termed phlogiston (φλογίστον, combustible). The experiment most insisted on in the work now spoken of,[12] was the formation of sulphur from sulphate of potass (or of soda) by fusing the salt with an alkali, and throwing in coals to supply phlogiston. This is the “experimentum novum.” Though Stahl published an account of this process, he seems to have regretted his openness. “He denies not,” he says, “that he should peradventure have dissembled this experiment as the true foundation of the Beccherian assertion concerning the nature of sulphur, if he had not been provoked by the pretending arrogance of some of his contemporaries.”
[11] Stahl, Præf. ad Specim. Becch. 1703.
[12] P. 117.
From this time, Stahl’s confidence in his theory may be traced becoming more and more settled in his succeeding publications. It is hardly necessary to observe here, that the explanations which his theory gives are easily transformed into those which the more recent theory supplies. According to modern views, the addition of oxygen takes place in the formation of acids and of calces, and in combustion, instead of the subtraction of phlogiston. The coal which Stahl supposed to supply the combustible in his experiment, does in fact absorb the liberated oxygen. In like manner, when an acid corrodes a metal, and, according to existing theory, combines with and oxidates it, Stahl supposed that the phlogiston separated from the metal and combined with the acid. That the explanations of the phlogistic theory are so generally capable of being translated into the oxygen theory, merely by inverting the supposed transfer of the combustible element, shows us how important a step towards the modern doctrines the phlogistic theory really was.
The question, whether these processes were in fact addition or subtraction, was decided by the balance, and belongs to a succeeding period of the science. But we may observe, that both Beccher and Stahl were aware of the increase of weight which metals undergo in [270] calcination; although the time had not yet arrived in which this fact was to be made one of the bases of the theory.
It has been said,[13] that in the adoption of the phlogistic theory, that is, in supposing the above-mentioned processes to be addition rather than subtraction, “of two possible roads the wrong was chosen, as if to prove the perversity of the human mind.” But we must not forget how natural it was to suppose that some part of a body was destroyed or removed by combustion; and we may observe, that the merit of Beccher and Stahl did not consist in the selection of one road or two, but in advancing so far as to reach this point of separation. That, having done this, they went a little further on the wrong line, was an error which detracted little from the merit or value of the progress really made. It would be easy to show, from the writings of phlogistic chemists, what important and extensive truths their theory enabled them to express simply and clearly.
[13] Herschel’s Introd. to Nat. Phil. p. 300.
That an enthusiastic temper is favorable to the production of great discoveries in science, is a rule which suffers no exception in the character of Beccher. In his preface[14] addressed “to the benevolent reader” of his Physica Subterranea, he speaks of the chemists as a strange class of mortals, impelled by an almost insane impulse to seek their pleasure among smoke and vapor, soot and flame, poisons and poverty. “Yet among all these evils,” he says, “I seem to myself to live so sweetly, that, may I die if I would change places with the Persian king.” He is, indeed, well worthy of admiration, as one of the first who pursued the labors of the furnace and the laboratory, without the bribe of golden hopes. “My kingdom,” he says, “is not of this world. I trust that I have got hold of my pitcher by the right handle,—the true method of treating this study. For the Pseudochymists seek gold; but the true philosophers, science, which is more precious than any gold.”
[14] Frankfort, 1681.
The Physica Subterranea made no converts. Stahl, in his indignant manner, says,[15] “No one will wonder that it never yet obtained a physician or a chemist as a disciple, still less as an advocate.” And again, “This work obtained very little reputation or estimation, or, to speak ingenuously, as far as I know, none whatever.” In 1671, Beccher published a supplement to his work, in which he showed how metal might be extracted from mud and sand. He offered to execute [271] this at Vienna; but found that people there cared nothing about such novelties. He was then induced, by Baron D’Isola, to go to Holland for similar purposes. After various delays and quarrels, he was obliged to leave Holland for fear of his creditors; and then, I suppose, came to Great Britain, where he examined the Scottish and Cornish mines. He is said to have died in London in 1682.
[15] Præf. Phys. Sub. 1703.
Stahl’s publications appear to have excited more notice, and led to controversy on the “so-called sulphur.” The success of the experiment had been doubted, which, as he remarks, it was foolish to make a matter of discussion, when any one might decide the point by experiment; and finally, it had been questioned whether the substance obtained by this process were pure sulphur. The originality of his doctrine was also questioned, which, as he says, could not with any justice be impugned. He published in defence and development of his opinion at various intervals, as the Specimen Beccherianum in 1703, the Documentum Theoriæ Beecherianæ, a Dissertation De Anatomia Sulphuris Artificialis; and finally, Casual Thoughts on the so-called Sulphur, in 1718, in which he gave (in German) both a historical and a systematic view of his opinions on the nature of salts and of his Phlogiston.
Reception and Application of the Theory.—The theory that the formation of sulphuric acid, and the restoration of metals from their calces, are analogous processes, and consist in the addition of phlogiston, was soon widely received; and the Phlogistic School was thus established. From Berlin, its original seat, it was diffused into all parts of Europe. The general reception of the theory may be traced, not only in the use of the term “phlogiston,” and of the explanations which it implies; but in the adoption of a nomenclature founded on those explanations, which, though not very extensive, is sufficient evidence of the prevalence of the theory. Thus when Priestley, in 1774, discovered oxygen, and when Scheele, a little later, discovered chlorine, these gases were termed dephlogisticated air, and dephlogisticated marine acid; while azotic acid gas, having no disposition to combustion, was supposed to be saturated with phlogiston, and was called phlogisticated air.
This phraseology kept its ground, till it was expelled by the antiphlogistic, or oxygen theory. For instance. Cavendish’s papers on the chemistry of the airs are expressed in terms of it, although his researches led him to the confines of the new theory. We must now give an account of such researches, and of the consequent revolution in the science. [272]
CHAPTER V.
Chemistry of Gases.—Black. Cavendish.
THE study of the properties of aëriform substances, or Pneumatic Chemistry, as it was called, occupied the chemists of the eighteenth century, and was the main occasion of the great advances which the science made at that period. The most material general truths which came into view in the course of these researches, were, that gases were to be numbered among the constituent elements of solid and fluid bodies; and that, in these, as in all other cases of composition, the compound was equal to the sum of its elements. The latter proposition, indeed, cannot be looked upon as a discovery, for it had been frequently acknowledged, though little applied; in fact, it could not be referred to with any advantage, till the aëriform elements, as well as others, were taken into the account. As soon as this was done, it produced a revolution in chemistry.
[2nd Ed.] [Though the view of the mode in which gaseous elements become fixed in bodies and determine their properties, had great additional light thrown upon it by Dr. Black’s discoveries, as we shall see, the notion that solid bodies involve such gaseous elements was not new at that period. Mr. Vernon Harcourt has shown[16] that Newton and Boyle admitted into their speculations airs of various kinds, capable of fixation in bodies. I have, in the succeeding chapter ([chap. vi.]), spoken of the views of Rey, Hooke, and Mayow, connected with the function of airs in chemistry, and forming a prelude to the Oxygen Theory.]
[16] Phil. Mag. 1846.
Notwithstanding these preludes, the credit of the first great step in pneumatic chemistry is, with justice, assigned to Dr. Black, afterwards professor at Edinburgh, but a young man of the age of twenty-four at the time when he made his discovery.[17] He found that the difference between caustic lime and common limestone arose from this, that the latter substance consists of the former, combined with a certain air, which, being thus fixed in the solid body, he called fixed air (carbonic [273] acid gas). He found, too, that magnesia, caustic potash, and caustic soda, would combine with the same air, with similar results. This discovery consisted, of course, in a new interpretation of observed changes. Alkalies appeared to be made caustic by contact with quicklime: at first Black imagined that they underwent this change by acquiring igneous matter from the quicklime; but when he perceived that the lime gained, not lost, in magnitude as it became mild, he rightly supposed that the alkalies were rendered caustic by imparting their air to the lime. This discovery was announced in Black’s inaugural dissertation, pronounced in 1755, on the occasion of his taking his degree of Doctor in the University of Edinburgh.
[17] Thomson’s Hist. Chem. i. 317.
The chemistry of airs was pursued by other experimenters. The Honorable Henry Cavendish, about 1765, invented an apparatus, in which aërial fluids are confined by water, so that they can be managed and examined. This hydro-pneumatic apparatus, or as it is sometimes called, the pneumatic trough, from that time was one of the most indispensable parts of the chemist’s apparatus. Cavendish,[18] in 1766, showed the identity of the properties of fixed air derived from various sources; and pointed out the peculiar qualities of inflammable air (afterwards called hydrogen gas), which, being nine times lighter than common air, soon attracted general notice by its employment for raising balloons. The promise of discovery which this subject now offered, attracted the confident and busy mind of Priestley, whose Experiments and Observations on different kinds of Air appeared in 1744–79. In these volumes, he describes an extraordinary number of trials of various kinds; the results of which were, the discovery of new kinds of air, namely, phlogisticated air (azotic gas), nitrous air (nitrous gas), and dephlogisticated air (oxygen gas).
[18] Phil. Trans. 1766.
But the discovery of new substances, though valuable in supplying chemistry with materials, was not so important as discoveries respecting their modes of composition. Among such discoveries, that of Cavendish, published in the Philosophical Transactions for 1784, and disclosing the composition of water by the union of two gases, oxygen and hydrogen, must be considered as holding a most distinguished place. He states,[19] that his “experiments were made principally with a view to find out the cause of the diminution which common air is well known to suffer, by all the various ways in which it is phlogisticated.” And, after describing various unsuccessful attempts, he finds [274] that when inflammable air is used in this phlogistication (or burning), the diminution of the common air is accompanied by the formation of a dew in the apparatus.[20] And thus he infers[21] that “almost all the inflammable air, and one-fifth of the common air, are turned into pure water.”
[19] Phil. Trans. 1784, p. 119.
[20] Phil. Trans. 1784, p. 128.
[21] Ib. p. 129.
Lavoisier, to whose researches this result was, as we shall soon see, very important, was employed in a similar attempt at the same time (1783), and had already succeeded,[22] when he learned from Dr. Blagden, who was present at the experiment, that Cavendish had made the discovery a few months sooner. Monge had, about the same time, made the same experiments, and communicated the result to Lavoisier and Laplace immediately afterwards. The synthesis was soon confirmed by a corresponding analysis. Indeed the discovery undoubtedly lay in the direct path of chemical research at the time. It was of great consequence in the view it gave of experiments in composition; for the small quantity of water produced in many such processes, had been quite overlooked; though, as it now appeared, this water offered the key to the whole interpretation of the change.
[22] A. P. 1781, p. 472
Though some objections to Mr. Cavendish’s view were offered by Kirwan,[23] on the whole they were generally received with assent and admiration. But the bearing of these discoveries upon the new theory of Lavoisier, who rejected phlogiston, was so close, that we cannot further trace the history of the subject without proceeding immediately to that theory.
[23] P. T. 1784, p. 154.
[2nd Ed.] [I have elsewhere stated,[24]—with reference to recent attempts to deprive Cavendish of the credit of his discovery of the composition of water, and to transfer it to Watt,—that Watt not only did not anticipate, but did not fully appreciate the discovery of Cavendish and Lavoisier; and I have expressed my concurrence with Mr. Vernon Harcourt’s views, when he says,[25] that “Cavendish pared off from the current hypotheses their theory of combustion, and their affinities of imponderable for ponderable matter, as complicating chemical with physical considerations; and he then corrected and adjusted them with admirable skill to the actual phenomena, not binding the facts to the theory, but adapting the theory to the facts.”
[24] Philosophy, b. vi. c. 4.
[25] Address to the British Association, 1839.
I conceive that the discussion which the subject has recently received, has left no doubt on the mind of any one who has perused the [275] documents, that Cavendish is justly entitled to the honor of this discovery, which in his own time was never contested. The publication of his Journals of Experiments[26] shows that he succeeded in establishing the point in question in July, 1781. His experiments are referred to in an abstract of a paper of Priestley’s, made by Dr. Maty, the secretary of the Royal Society, in June, 1783. In June, 1783, also, Dr. Blagden communicated the result of Cavendish’s experiments to Lavoisier, at Paris. Watt’s letter, containing his hypothesis that “water is composed of dephlogisticated air and phlogiston deprived of part of their latent or elementary heat; and that phlogisticated or pure air is composed of water deprived of its phlogiston and united to elementary heat and light,” was not read till Nov. 1783; and even if it could have suggested such an experiment as Cavendish’s (which does not appear likely), is proved, by the dates, to have had no share in doing so.
[26] Appendix to Mr. V. Harcourt’s Address
Mr. Cavendish’s experiment was suggested by an experiment in which Warltire, a lecturer on chemistry at Birmingham, exploded a mixture of hydrogen and common air in a close vessel, in order to determine whether heat were ponderable.]
CHAPTER VI.
Epoch of the Theory of Oxygen.—Lavoisier.
Sect. 1.—Prelude to the Theory.—Its Publication.
WE arrive now at a great epoch in the history of Chemistry. Few revolutions in science have immediately excited so much general notice as the introduction of the theory of oxygen. The simplicity and symmetry of the modes of combination which it assumed; and, above all, the construction and universal adoption of a nomenclature which applied to all substances, and which seemed to reveal their inmost constitution by their name, naturally gave it an almost irresistible sway over men’s minds. We must, however, dispassionately trace the course of its introduction. [276]
Antoine Laurent Lavoisier, an accomplished French chemist, had pursued, with zeal and skill, researches such as those of Black, Cavendish, and Priestley, which we have described above. In 1774, he showed that, in the calcination of metals in air, the metal acquires as much weight as the air loses. It might appear that this discovery at once overturned the view which supposed the metal to be phlogiston added to the calx. Lavoisier’s contemporaries were, however, far from allowing this; a greater mass of argument was needed to bring them to this conclusion. Convincing proofs of the new opinion were, however, rapidly supplied. Thus, when Priestley had discovered dephlogisticated air, in 1774, Lavoisier showed, in 1776, that fixed air consisted of charcoal and the dephlogisticated or pure air; for the mercurial calx which, heated by itself, gives out pure air, gives out, when heated with charcoal, fixed air,[27] which has, therefore, since been called carbonic acid gas.
[27] Mém. Ac. Par. 1775.
Again, Lavoisier showed that the atmospheric air consists of pure or vital air, and of an unvital air, which he thence called azot. The vital air he found to be the agent in combustion, acidification, calcination, respiration; all of these processes were analogous: all consisted in a decomposition of the atmospheric air, and a fixation of the pure or vital portion of it.
But he thus arrived at the conclusion, that this pure air was added, in all the cases in which, according to the received theory, phlogiston was subtracted, and vice versâ. He gave the name[28] of oxygen (principe oxygène) to “the substance which thus unites itself with metals to form their calces, and with combustible substances to form acids.”
[28] Mém. Ac. Par. 1781, p. 448.
A new theory was thus produced, which would account for all the facts which the old one would explain, and had besides the evidence of the balance in its favor. But there still remained some apparent objections to be removed. In the action of dilute acids on metals, inflammable air was produced. Whence came this element? The discovery of the decomposition of water sufficiently answered this question, and converted the objection into an argument on the side of the theory: and thus the decomposition of water was, in fact, one of the most critical events for the fortune of the Lavoisierian doctrine, and one which, more than any other, decided chemists in its favor. In succeeding years, Lavoisier showed the consistency of his theory with [277] all that was discovered concerning the composition of alcohol, oil, animal and vegetable substances, and many other bodies.
It is not necessary for us to consider any further the evidence for this theory, but we must record a few circumstances respecting its earlier history. Rey, a French physician, had in 1630, published a book, in which he inquires into the grounds of the increase of the weight of metals by calcination.[29] He says, “To this question, then, supported on the grounds already mentioned, I answer, and maintain with confidence, that the increase of weight arises from the air, which is condensed, rendered heavy and adhesive, by the heat of the furnace.” Hooke and Mayow had entertained the opinion that the air contains a “nitrous spirit,” which is the supporter of combustion. But Lavoisier disclaimed the charge of having derived anything from these sources; nor is it difficult to understand how the received generalizations of the phlogistic theory had thrown all such narrower explanations into obscurity. The merit of Lavoisier consisted in his combining the generality of Stahl with the verified conjectures of Rey and Mayow.
[29] Thomson, Hist. Chem. ii. 95.
No one could have a better claim, by his early enthusiasm for science, his extensive knowledge, and his zealous labors, to hope that a great discovery might fall to his share, than Lavoisier. His father,[30] a man of considerable fortune, had allowed him to make science his only profession; and the zealous philosopher collected about him a number of the most active physical inquirers of his time, who met and experimented at his house one day in the week. In this school, the new chemistry was gradually formed. A few years after the publication of Priestley’s first experiments, Lavoisier was struck with the presentiment of the theory which he was afterwards to produce. In 1772, he deposited[31] with the secretary of the Academy, a note which contained the germ of his future doctrines. “At that time,” he says, in explaining this step, “there was a kind of rivalry between France and England in science, which gave importance to new experiments, and which sometimes was the cause that the writers of the one or other of the nations disputed the discovery with the real author.” In 1777, the editor of the Memoirs of the Academy speaks of his theory as overturning that of Stahl; but the general acceptance of the new opinion did not take place till later.
[30] Biogr. Univ. (Cuvier.)
Sect. 2.—Reception and Confirmation of the Theory of Oxygen.
The Oxygen Theory made its way with extraordinary rapidity among the best philosophers.[32] In 1785, that is, soon after Cavendish’s synthesis of water had removed some of the most formidable objections to it, Berthollet, already an eminent chemist, declared himself a convert. Indeed it was so soon generally adopted in France, that Fourcroy promulgated its doctrines under the name of “La Chimie Française,” a title which Lavoisier did not altogether relish. The extraordinary eloquence and success of Fourcroy as a lecturer at the Jardin des Plantes, had no small share in the diffusion of the oxygen theory; and the name of “the apostle of the new chemistry” which was at first given him in ridicule, was justly held by him to be a glorious distinction.[33]
[32] Thomson, ii. 130.
[33] Cuvier, Eloges, i. p. 20.
Guyton de Morveau, who had at first been a strenuous advocate of the phlogistic theory, was invited to Paris, and brought over to the opinions of Lavoisier; and soon joined in the formation of the nomenclature founded upon the theory. This step, of which we shall shortly speak, fixed the new doctrine, and diffused it further. Delametherie alone defended the phlogistic theory with vigor, and indeed with violence. He was the editor of the Journal de Physique, and to evade the influence which this gave him, the antiphlogistians[34] established, as the vehicle of their opinions, another periodical, the Annales de Chimie.
[34] Thomson, ii. 133.
In England, indeed, their success was not so immediate. Cavendish,[35] in his Memoir of 1784, speaks of the question between the two opinions as doubtful. “There are,” he says, “several Memoirs of M. Lavoisier, in which he entirely discards phlogiston; and as not only the foregoing experiments, but most other phenomena of nature, seem explicable as well, or nearly as well, upon this as upon the commonly believed principle of phlogiston,” Cavendish proceeds to explain his experiments according to the new views, expressing no decided preference, however, for either system. But Kirwan, another English chemist, contested the point much more resolutely. His theory identified inflammable air, or hydrogen, with phlogiston; and in this view, he wrote a work which was intended as a confutation of [279] the essential part of the oxygen theory. It is a strong proof of the steadiness and clearness with which the advocates of the new system possessed their principles, that they immediately translated this work, adding, at the end of each chapter, a refutation of the phlogistic doctrines which it contained. Lavoisier, Berthollet, De Morveau, Fourcroy, and Monge, were the authors of this curious specimen of scientific polemics. It is also remarkable evidence of the candor of Kirwan, that notwithstanding the prominent part he had taken in the controversy, he allowed himself at last to be convinced. After a struggle of ten years, he wrote[36] to Berthollet in 1796, “I lay down my arms, and abandon the cause of phlogiston.” Black followed the same course. Priestley alone, of all the chemists of great name, would never assent to the new doctrines, though his own discoveries had contributed so much to their establishment. “He saw,” says Cuvier,[37] “without flinching, the most skilful defenders of the ancient theory go over to the enemy in succession; and when Kirwan had, almost the last of all, abjured phlogiston, Priestley remained alone on the field of battle, and threw out a new challenge, in a memoir addressed to the principal French chemists.” It happened, curiously enough, that the challenge was accepted, and the arguments answered by M. Adet, who was at that time (1798,) the French ambassador to the United States, in which country Priestley’s work was published. Even in Germany, the birth-place and home of the phlogistic theory, the struggle was not long protracted. There was, indeed, a controversy, the older philosophers being, as usual, the defenders of the established doctrines; but in 1792, Klaproth repeated, before the Academy of Berlin, all the fundamental experiments; and “the result was a full conviction on the part of Klaproth and the Academy, that the Lavoisierian theory was the true one.”[38] Upon the whole, the introduction of the Lavoisierian theory in the scientific world, when compared with the great revolution of opinion to which it comes nearest in importance, the introduction of the Newtonian theory, shows, by the rapidity and temper with which it took place, a great improvement, both in the means of arriving at truth, and in the spirit with which they were used.
[35] Phil. Trans. 1784, p. 150.
[36] Pref. to Fourcroy’s Chemistry, xiv.
[37] Cuvier, Eloge de Priestley, p. 208.
[38] Thomson, vol. ii. p. 136.
Some English writers[39] have expressed an opinion that there was [280] little that was original in the new doctrines. But if they were so obvious, what are we to say of eminent chemists, as Black and Cavendish, who hesitated when they were presented, or Kirwan and Priestley, who rejected them? This at least shows that it required some peculiar insight to see the evidence of these truths. To say that most of the materials of Lavoisier’s theory existed before him, is only to say that his great merit was, that which must always be the great merit of a new theory, his generalization. The effect which the publication of his doctrines produced, shows us that he was the first person who, possessing clearly the idea of quantitative composition, applied it steadily to a great range of well-ascertained facts. This is, as we have often had to observe, precisely the universal description of an inductive discoverer. It has been objected, in like manner, to the originality of Newton’s discoveries, that they were contained in those of Kepler. They were so, but they needed a Newton to find them there. The originality of the theory of oxygen is proved by the conflict, short as it was, which accompanied its promulgation; its importance is shown by the changes which it soon occasioned in every part of the science.
[39] Brande, Hist. Diss. in Enc, Brit. p. 182. Lunn, Chem. in Enc. Met. p. 596.
Thus Lavoisier, far more fortunate than most of those who had, in earlier ages, produced revolutions in science, saw his theory accepted by all the most eminent men of his time, and established over a great part of Europe within a few years from its first promulgation. In the common course of events, it might have been expected that the later years of his life would have been spent amid the admiration and reverence which naturally wait upon the patriarch of a new system of acknowledged truths. But the times in which he lived allowed no such euthanasia to eminence of any kind. The democracy which overthrew the ancient political institutions of France, and swept away the nobles of the land, was not, as might have been expected, enthusiastic in its admiration of a great revolution in science, and forward to offer its homage to the genuine nobility of a great discoverer. Lavoisier was thrown into prison on some wretched charge of having, in the discharge of a public office which he had held, adulterated certain tobacco; but in reality, for the purpose of confiscating his property.[40] In his imprisonment, his philosophy was his resource; and he employed himself in the preparation of his papers for printing. When he was brought before the revolutionary tribunal, he begged for a respite of a few days, in order to complete some researches, the results of which [281] were, he said, important to the good of humanity. The brutish idiot, whom the state of the country at that time had placed in the judgment-seat, told him that the republic wanted no sçavans. He was dragged to the guillotine, May the 8th, 1794, and beheaded, in the fifty-second year of his age; a melancholy proof that, in periods of political ferocity, innocence and merit, private virtues and public services, amiable manners and the love of friends, literary fame and exalted genius, are all as nothing to protect their possessor from the last extremes of violence and wrong, inflicted under judicial forms.
[40] Biog. Univ. (Cuvier.)
Sect. 3.—Nomenclature of the Oxygen Theory.
As we have already said, a powerful instrument in establishing and diffusing the new chemical theory, was a Systematic Nomenclature founded upon it, and applicable to all chemical compounds, which was soon constructed and published by the authors of the theory. Such a nomenclature made its way into general use the more easily, in that the want of such a system had already been severely felt; the names in common use being fantastical, arbitrary, and multiplied beyond measure. The number of known substances had become so great, that a list of names with no regulative principle, founded on accident, caprice, and error, was too cumbrous and inconvenient to be tolerated. Even before the currency which Lavoisier’s theory obtained, these evils had led to attempts towards a more convenient set of names. Bergman and Black had constructed such lists; and Guyton de Morveau, a clever and accomplished lawyer of Dijon, had formed a system of nomenclature in 1782, before he had become a convert to Lavoisier’s theory, in which task he had been exhorted and encouraged by Bergman and Macquer. In this system,[41] we do not find most of the characters of the method which was afterwards adopted. But a few years later, Lavoisier, De Morveau, Berthollet and Fourcroy, associated themselves for the purpose of producing a nomenclature which should correspond to the new theoretical views. This appeared in 1787, and soon made its way into general use. The main features of this system are, a selection of the simplest radical words, by which substances are designated, and a systematic distribution of terminations, to express their relations. Thus, sulphur, combined with oxygen in two different proportions, forms two acids, the [282] sulphurous and the sulphuric; and these acids form, with earthy or alkaline bases, sulphides and sulphates; while sulphur directly combined with another element, forms a sulphuret. The term oxyd (now usually written oxide) expressed a lower degree of combination with oxygen than the acids. The Méthode de Nomenclature Chimique was published in 1787; and in 1789, Lavoisier published a treatise on chemistry in order further to explain this method. In the preface to this volume, he apologizes for the great amount of the changes, and pleads the authority of Bergman, who had exhorted De Morveau “to spare no improper names; those who are learned will always be learned, and those who are ignorant will thus learn sooner.” To this maxim they so far conformed, that their system offers few anomalies; and though the progress of discovery, and the consequent changes of theoretical opinion, which have since gone on, appear now to require a further change of nomenclature, it is no small evidence of the skill with which this scheme was arranged, that for half a century it was universally used, and felt to be far more useful and effective than any nomenclature in any science had ever been before.
[41] Journal de Physique, 1782, p. 370.
CHAPTER VII.
Application and Correction of the Oxygen Theory.
SINCE a chemical theory, as far as it is true, must enable us to obtain a true view of the intimate composition of all bodies whatever, it will readily be supposed that the new chemistry led to an immense number of analyses and researches of various kinds. These it is not necessary to dwell upon; nor will I even mention the names of any of the intelligent and diligent men who have labored in this field. Perhaps one of the most striking of such analyses was Davy’s decomposition of the earths and alkalies into metallic bases and oxygen, in 1807 and 1808; thus extending still further that analogy between the earths and the calces of the metals, which had had so large a share in the formation of chemical theories. This discovery, however, both in the means by which it was made, and in the views to which it led, bears upon subjects hereafter to be treated of.
The Lavoisierian theory also, wide as was the range of truth which it embraced, required some limitation and correction. I do not now [283] speak of some erroneous opinions entertained by the author of the theory; as, for instance, that the heat produced in combustion, and even in respiration, arose from the conversion of oxygen gas to a solid consistence, according to the doctrine of latent heat. Such opinions not being necessarily connected with the general idea of the theory, need not here be considered. But the leading generalization of Lavoisier, that acidification was always combination with oxygen, was found untenable. The point on which the contest on this subject took place was the constitution of the oxymuriatic and muriatic acids;—as they had been termed by Berthollet, from the belief that muriatic acid contained oxygen, and oxymuriatic a still larger dose of oxygen. In opposition to this, a new doctrine was put forward in 1809 by Gay-Lussac and Thenard in France, and by Davy in England;—namely, that oxymuriatic acid was a simple substance, which they termed chlorine, and that muriatic acid was a combination of chlorine with hydrogen, which therefore was called hydrochloric acid. It may be observed, that the point in dispute in the controversy on this subject was nearly the same which had been debated in the course of the establishment of the oxygen theory; namely, whether in the formation of muriatic acid from chlorine, oxygen is subtracted, or hydrogen added, and the water concealed.
In the course of this dispute, it was allowed on both sides, that the combination of dry muriatic acid and ammonia afforded an experimentum crucis; since, if water was produced from these elements, oxygen must have existed in the acid. Davy being at Edinburgh in 1812, this experiment was made in the presence of several eminent philosophers; and the result was found to be, that though a slight dew appeared in the vessel, there was not more than might be ascribed to unavoidable imperfection in the process, and certainly not so much as the old theory of muriatic acid required. The new theory, after this period, obtained a clear superiority in the minds of philosophical chemists, and was further supported by new analogies.[42]
[42] Paris, Life of Davy, i. 337.
For, the existence of one hydracid being thus established, it was found that other substances gave similar combinations; and thus chemists obtained the hydriodic, hydrofluoric, and hydrobromic acids. These acids, it is to be observed, form salts with bases, in the same manner as the oxygen acids do. The analogy of the muriatic and fluoric compounds was first clearly urged by a philosopher who was [284] not peculiarly engaged in chemical research, but who was often distinguished by his rapid and happy generalizations, M. Ampère. He supported this analogy by many ingenious and original arguments, in letters written to Davy, while that chemist was engaged in his researches on fluor spar, as Davy himself declares.[43]
[43] Paris, Life of Davy, i. 370.
Still further changes have been proposed, in that classification of elementary substances to which the oxygen theory led. It has been held by Berzelius and others, that other elements, as, for example, sulphur, form salts with the alkaline and earthy metals, rather than sulphurets. The character of these sulpho-salts, however, is still questioned among chemists; and therefore it does not become us to speak as if their place in history were settled. Of course, it will easily be understood that, in the same manner in which the oxygen theory introduced its own proper nomenclature, the overthrow or material transformation of the theory would require a change in the nomenclature; or rather, the anomalies which tended to disturb the theory, would, as they were detected, make the theoretical terms be felt as inappropriate, and would suggest the necessity of a reformation in that respect. But the discussion of this point belongs to a step of the science which is to come before us [hereafter].
It may be observed, that in approaching the limits of this part of our subject, as we are now doing, the doctrine of the combination of acids and bases, of which we formerly traced the rise and progress, is still assumed as a fundamental relation by which other relations are tested. This remark connects the stage of chemistry now under our notice with its earliest steps. But in order to point out the chemical bearing of the next subjects of our narrative, we may further observe, that metals, earths, salts, are spoken of as known classes of substances; and in like manner the newly-discovered elements, which form the last trophies of chemistry, have been distributed into such classes according to their analogies; thus potassium, sodium, barium, have been asserted to be metals; iodine, bromine, fluorine, have been arranged as analogical to chlorine. Yet there is something vague and indefinite in the boundaries of such classifications and analogies; and it is precisely where this vagueness falls, that the science is still obscure or doubtful. We are led, therefore, to see the dependence of Chemistry upon Classification; and it is to Sciences of Classification which we shall next proceed; as soon as we have noticed the most general views [285] which have been given of chemical relations, namely, the views of the electro-chemists.
But before we do this, we must look back upon a law which obtains in the combination of elements, and which we have hitherto not stated; although it appears, more than any other, to reveal to us the intimate constitution of bodies, and to offer a basis for future generalizations. I speak of the Atomic Theory, as it is usually termed; or, as we might rather call it, the Doctrine of Definite, Reciprocal, and Multiple Proportions.
CHAPTER VIII.
Theory of Definite, Reciprocal, and Multiple Proportions.
Sect. 1.—Prelude to the Atomic Theory, and its Publication by Dalton.
THE general laws of chemical combination announced by Mr. Dalton are truths of the highest importance in the science, and are now nowhere contested; but the view of matter as constituted of atoms, which he has employed in conveying those laws, and in expressing his opinion of their cause, is neither so important nor so certain. In the place which I here assign to his discovery, as one of the great events of the history of chemistry, I speak only of the law of phenomena, the rules which govern the quantities in which elements combine.
This law may be considered as consisting of three parts, according to the above description of it;—that elements combine in definite proportions;—that these determining proportions operate reciprocally;—and that when, between the same elements, several combining proportions occur, they are related as multiples.
That elements combine in certain definite proportions of quantity, and in no other, was implied, as soon as it was supposed that chemical compounds had any definite properties. Those who first attempted to establish regular formulæ[44] for the constitution of salts, minerals, and [286] other compounds, assumed, as the basis of this process, that the elements in different specimens had the same proportion. Wenzel, in 1777, published his Lehre von der Verwandschaft der Körper; or, Doctrine of the Affinities of Bodies; in which he gave many good and accurate analyses. His work, it is said, never grew into general notice. Berthollet, as we have [already] stated, maintained that chemical compounds were not definite; but this controversy took place at a later period. It ended in the establishment of the doctrine, that there is, for each combination, only one proportion of the elements, or at most only two or three.
[44] Thomson, Hist. Chem. vol. ii. p. 279.
Not only did Wenzel, by his very attempt, presume the first law of chemical composition, the definiteness of the proportions, but he was also led, by his results, to the second rule, that they are reciprocal. For he found that when two neutral salts decompose each other, the resulting salts are also neutral. The neutral character of the salts shows that they are definite compounds; and when the two elements of the one salt, P and s, are presented to those of the other, B and n, if P be in such quantity as to combine definitely with n, B will also combine definitely with s.[45]
[45] I am told that Wenzel (whose book I have not seen), though he adduces many cases in which double decomposition gives neutral salts, does not express the proposition in a general form, nor use letters in expressing it.
Views similar to those of Wenzel were also published by Jeremiah Benjamin Richter[46] in 1792, in his Anfangsgründe der Stöchyometrie, oder Messkunst Chymischer Elemente, (Principles of the Measure of Chemical Elements) in which he took the law, just stated, of reciprocal proportions, as the basis of his researches, and determined the numerical quantities of the common bases and acids which would saturate each other. It is clear that, by these steps, the two first of our three rules may be considered as fully developed. The change of general views which was at this time going on, probably prevented chemists from feeling so much interest as they might have done otherwise, in these details; the French and English chemists, in particular, were fully employed with their own researches and controversies.
[46] Thomson, Hist. Chem. vol. ii. p. 283.
Thus the rules which had already been published by Wenzel and Richter had attracted so little notice, that we can hardly consider Mr. Dalton as having been anticipated by those writers, when, in 1803, he began to communicate his views on the chemical constitution of [287] bodies; these views being such as to include both these two rules in their most general form, and further, the rule, at that time still more new to chemists, of multiple proportions. He conceived bodies as composed of atoms of their constituent elements, grouped, either one and one, or one and two, or one and three, and so on. Thus, if C represent an atom of carbon and O one of oxygen, O C will be an atom of carbonic oxide, and O C O an atom of carbonic acid; and hence it follows, that while both these bodies have a definite quantity of oxygen to a given quantity of carbon, in the latter substance this quantity is double of what it is in the former.
The consideration of bodies as consisting of compound atoms, each of these being composed of elementary atoms, naturally led to this law of multiple proportions. In this mode of viewing bodies, Mr. Dalton had been preceded (unknown to himself) by Mr. Higgins, who, in 1789, published[47] his Comparative View of the Phlogistic and Antiphlogistic Theories. He there says,[48] “That in volatile vitriolic acid, a single ultimate particle of sulphur is united only to a single particle of dephlogisticated air; and that in perfect vitriolic acid, every single particle of sulphur is united to two of dephlogisticated air, being the quantity necessary to saturation;” and he reasons in the same manner concerning the constitution of water, and the compounds of nitrogen and oxygen. These observations of Higgins were, however, made casually, and not followed out, and cannot affect Dalton’s claim to original merit.
[47] Turner’s Chem. p. 217.
[48] P. 36 and 37.
Mr. Dalton’s generalization was first suggested[49] during his examination of olefiant gas and carburetted hydrogen gas; and was asserted generally, on the strength of a few facts, being, as it were, irresistibly recommended by the clearness and simplicity which the notion possessed. Mr. Dalton himself represented the compound atoms of bodies by symbols, which professed to exhibit the arrangement of the elementary atoms in space as well as their numerical proportion; and he attached great importance to this part of his scheme. It is clear, however, that this part of his doctrine is not essential to that numerical comparison of the law with facts, on which its establishment rests. These hypothetical configurations of atoms have no value till they are confirmed by corresponding facts, such as the optical or crystalline properties of bodies may perhaps one day furnish.
[49] Thomson, vol. ii. p. 291. [288]
Sect. 2.—Reception and Confirmation of the Atomic Theory.
In order to give a sketch of the progress of the Atomic Theory into general reception, we cannot do better than borrow our information mainly from Dr. Thomson, who was one of the earliest converts and most effective promulgators of the doctrine. Mr. Dalton, at the time when he conceived his theory, was a teacher of mathematics at Manchester, in circumstances which might have been considered narrow, if he himself had been less simple in his manner of life, and less moderate in his worldly views. His experiments were generally made with apparatus of which the simplicity and cheapness corresponded to the rest of his habits. In 1804, he was already in possession of his atomic theory, and explained it to Dr. Thomson, who visited him at that time. It was made known to the chemical world in Dr. Thomson’s Chemistry, in 1807; and in Dalton’s own System of Chemistry (1808) the leading ideas of it were very briefly stated. Dr. Wollaston’s memoir, “on superacid and subacid salts,” which appeared in the Philosophical Transactions for 1808, did much to secure this theory a place in the estimation of chemists. Here the author states, that he had observed, in various salts, the quantities of acid combined with the base in the neutral and in the superacid salts to be as one to two: and he says that, thinking it likely this law might obtain generally in such compounds, it was his design to have pursued this subject, with the hope of discovering the cause to which so regular a relation may be ascribed. But he adds, that this appears to be superfluous after the publication of Dalton’s theory by Dr. Thomson, since all such facts are but special cases of the general law. We cannot but remark here, that the scrupulous timidity of Wollaston was probably the only impediment to his anticipating Dalton in the publication of the rule of multiple proportions; and the forwardness to generalize, which belongs to the character of the latter, justly secured him, in this instance, the name of the discoverer of this law. The rest of the English chemists soon followed Wollaston and Thomson, though Davy for some time resisted. They objected, indeed, to Dalton’s assumption of atoms, and, to avoid this hypothetical step, Wollaston used the phrase chemical equivalents, and Davy the word proportions, for the numbers which expressed Dalton’s atomic weights. We may, however, venture to say that the term “atom” is the most convenient, and it need not be understood as claiming our assent to the hypothesis of indivisible molecules. [289]
As Wollaston and Dalton were thus arriving independently at the same result in England, other chemists, in other countries, were, unknown to each other, travelling towards the same point.
In 1807, Berzelius,[50] intending to publish a system of chemistry, went through several works little read, and among others the treatises of Richter. He was astonished, he tells us, at the light which was there thrown upon composition and decomposition, and which had never been turned to profit. He was led to a long train of experimental research, and, when he received information of Dalton’s ideas concerning multiple proportions, he found, in his own collection of analyses, a full confirmation of this theory.
[50] Berz. Chem. B. iii. p. 27.
Some of the Germans, indeed, appear discontented with the partition of reputation which has taken place with respect to the Theory of Definite Proportions. One[51] of them says, “Dalton has only done this;—he has wrapt up the good Richter (whom he knew; compare Schweigger, T, older series, vol. x., p. 381;) in a ragged suit, patched together of atoms; and now poor Richter comes back to his own country in such a garb, like Ulysses, and is not recognized.” It is to be recollected, however, that Richter says nothing of multiple proportions.
[51] Marx. Gesch. der Cryst. p. 202.
The general doctrine of the atomic theory is now firmly established over the whole of the chemical world. There remain still several controverted points, as, for instance, whether the atomic weights of all elements are exact multiples of the atomic weight of hydrogen. Dr. Prout advanced several instances in which this appeared to be true, and Dr. Thomson has asserted the law to be of universal application. But, on the other hand, Berzelius and Dr. Turner declare that this hypothesis is at variance with the results of the best analyses. Such controverted points do not belong to our history, which treats only of the progress of scientific truths already recognized by all competent judges.
Though Dalton’s discovery was soon generally employed, and universally spoken of with admiration, it did not bring to him anything but barren praise, and he continued in the humble employment of which we have spoken, when his fame had filled Europe, and his name become a household word in the laboratory. After some years he was appointed a corresponding member of the Institute of France; which may be considered as a European recognition of the importance [290] of what he had done; and, in 1826, two medals for the encouragement of science having been placed at the disposal of the Royal Society by the King of England, one of them was assigned to Dalton, “for his development of the atomic theory.” In 1833, at the meeting of the British Association for the Advancement of Science, which was held in Cambridge, it was announced that the King had bestowed upon him a pension of 150l.; at the preceding meeting at Oxford, that university had conferred upon him the degree of Doctor of Laws, a step the more remarkable, since he belonged to the sect of Quakers. At all the meetings of the British Association he has been present, and has always been surrounded by the reverence and admiration of all who feel any sympathy with the progress of science. May he long remain among us thus to remind us of the vast advance which Chemistry owes to him!
[2nd Ed.] [Soon after I wrote these expressions of hope, the period of Dalton’s sojourn among us terminated. He died on the 27th of July, 1844, aged 78.
His fellow-townsmen, the inhabitants of Manchester, who had so long taken a pride in his residence among them, soon after his death came to a determination to perpetuate his memory by establishing in his honor a Professor of Chemistry at Manchester.]
Sect. 3.—The Theory of Volumes.—Gay-Lussac.
The atomic theory, at the very epoch of its introduction into France, received a modification in virtue of a curious discovery then made. Soon after the publication of Dalton’s system, Gay-Lussac and Humboldt found a rule for the combination of substances, which includes that of Dalton as far as it goes, but extends to combinations of gases only. This law is the theory of volumes; namely, that gases unite together by volume in very simple and definite proportions. Thus water is composed exactly of 100 measures of oxygen and 200 measures of hydrogen. And since these simple ratios 1 and 1, 1 and 2, 1 and 3, alone prevail in such combinations, it may easily be shown that laws like Dalton’s law of multiple proportions, must obtain in such cases as he considered.
[2nd Ed.] [M. Schröder, of Mannheim, has endeavored to extend to solids a law in some degree resembling Gay-Lussac’s law of the volumes of gases. According to him, the volumes of the chemical equivalents [291] of simple substances and their compounds are as whole numbers.[52] MM. Kopp, Playfair, and Joule have labored in the same field.]
[52] Die molecular-volume der Chemischen Verbindungen in festen und flüssingen Zustande, 1843.
I cannot now attempt to trace other bearings and developments of this remarkable discovery. I hasten on to the last generalization of chemistry; which presents to us chemical forces under a new aspect, and brings us back to the point from which we departed in commencing the history of this science.
CHAPTER IX.
Epoch of Davy and Faraday.
Sect. 1.—Promulgation of the Electro-chemical Theory by Davy.
THE reader will recollect that the History of Chemistry, though highly important and instructive in itself, has been an interruption of the History of Electro-dynamic Research:—a necessary interruption, however; for till we became acquainted with Chemistry in general, we could not follow the course of Electro-chemistry: we could not estimate its vast yet philosophical theories, nor even express its simplest facts. We have now to endeavor to show what has thus been done, and by what steps;—to give a fitting view of the Epoch of Davy and Faraday.
This is, doubtless, a task of difficulty and delicacy. We cannot execute it at all, except we suppose that the great truths, of which the discovery marks this epoch, have already assumed their definite and permanent form. For we do not learn the just value and right place of imperfect attempts and partial advances in science, except by seeing to what they lead. We judge properly of our trials and guesses only when we have gained our point and guessed rightly. We might personify philosophical theories, and might represent them to ourselves as figures, all pressing eagerly onwards in the same [292] direction, whom we have to pursue: and it is only in proportion as we ourselves overtake those figures in the race, and pass beyond them, that we are enabled to look back upon their faces; to discern their real aspects, and to catch the true character of their countenances. Except, therefore, I were of opinion that the great truths which Davy brought into sight have been firmly established and clearly developed by Faraday, I could not pretend to give the history of this striking portion of science. But I trust, by the view I have to offer of these beautiful trains of research and their result, to justify the assumption on which I thus proceed.
I must, however, state, as a further appeal to the reader’s indulgence, that, even if the great principles of electro-chemistry have now been brought out in their due form and extent, the discovery is but a very few years, I might rather say a few months, old, and that this novelty adds materially to the difficulty of estimating previous attempts from the point of view to which we are thus led. It is only slowly and by degrees that the mind becomes sufficiently imbued with those new truths, of which the office is, to change the face of a science. We have to consider familiar appearances under a new aspect; to refer old facts to new principles; and it is not till after some time, that the struggle and hesitation which this employment occasions, subsides into a tranquil equilibrium. In the newly acquired provinces of man’s intellectual empire, the din and confusion of conquest pass only gradually into quiet and security. We have seen, in the history of all capital discoveries, how hardly they have made their way, even among the most intelligent and candid philosophers of the antecedent schools: we must, therefore, not expect that the metamorphosis of the theoretical views of chemistry which is now going on, will be effected without some trouble and delay.
I shall endeavor to diminish the difficulties of my undertaking, by presenting the earlier investigations in the department of which I have now to speak, as much as possible according to the most deliberate view taken of them by the great discoverers themselves, Davy and Faraday; since these philosophers are they who have taught us the true import of such investigations.
There is a further difficulty in my task, to which I might refer;—the difficulty of speaking, without error and without offence, of men now alive, or who were lately members of social circles which exist still around us. But the scientific history in which such persons play a part, is so important to my purpose, that I do not hesitate to incur [293] the responsibility which the narration involves; and I have endeavored earnestly, and I hope not in vain, to speak as if I were removed by centuries from the personages of my story.
The phenomena observed in the Voltaic apparatus were naturally the subject of many speculations as to their cause, and thus gave rise to “Theories of the Pile.” Among these phenomena there was one class which led to most important results: it was discovered by Nicholson and Carlisle, in 1800, that water was decomposed by the pile of Volta; that is, it was found that when the wires of the pile were placed with their ends near each other in the fluid, a stream of bubbles of air arose from each wire, and these airs were found on examination to be oxygen and hydrogen: which, as we have had to narrate, had already been found to be the constituents of water. This was, as Davy says,[53] the true origin of all that has been done in electro-chemical science. It was found that other substances also suffered a like decomposition under the same circumstances. Certain metallic solutions were decomposed, and an alkali was separated on the negative plates of the apparatus. Cruickshank, in pursuing these experiments, added to them many important new results; such as the decomposition of muriates of magnesia, soda, and ammonia by the pile; and the general observation that the alkaline matter always appeared at the negative, and the acid at the positive, pole.
[53] Phil. Trans. 1826, p. 386.
Such was the state of the subject when one who was destined to do so much for its advance, first contributed his labors to it. Humphry Davy was a young man who had been apprenticed to a surgeon at Penzance, and having shown an ardent love and a strong aptitude for chemical research, was, in 1798, made the superintendent of a “Pneumatic Institution,” established at Bristol by Dr. Beddoes, for the purpose of discovering medical powers of factitious airs.[54] But his main attention was soon drawn to galvanism; and when, in consequence of the reputation he had acquired, he was, in 1801, appointed lecturer at the Royal Institution in London (then recently established), he was soon put in possession of a galvanic apparatus of great power; and with this he was not long in obtaining the most striking results.
[54] Paris, Life of Davy, i. 58.
His first paper on the subject[55] is sent from Bristol, in September, 1800; and describes experiments, in which he had found that the decompositions observed by Nicholson and Carlisle go on, although the [294] water, or other substance in which the two wires are plunged, be separated into two portions, provided these portions are connected by muscular or other fibres. This use of muscular fibres was, probably, a remnant of the original disposition, or accident, by which galvanism had been connected with physiology, as much as with chemistry. Davy, however, soon went on towards the conclusion, that the phenomena were altogether chemical in their nature. He had already conjectured,[56] in 1802, that all decompositions might be polar; that is, that in all cases of chemical decomposition, the elements might be related to each other as electrically positive and negative; a thought which it was the peculiar glory of his school to confirm and place in a distinct light. At this period such a view was far from obvious; and it was contended by many, on the contrary, that the elements which the voltaic apparatus brought to view, were not liberated from combinations, but generated. In 1806, Davy attempted the solution of this question; he showed that the ingredients which had been supposed to be produced by electricity, were due to impurities in the water, or to the decomposition of the vessel; and thus removed all preliminary difficulties. And then he says,[57] “referring to my experiments of 1800, 1801, and 1802, and to a number of new facts, which showed that inflammable substances and oxygen, alkalies and acids, and oxidable and noble metals, were in electrical relations of positive and negative, I drew the conclusion, that the combinations and decompositions by electricity were referrible to the law of electrical attractions and repulsions,” and advanced the hypothesis, “that chemical and electrical attractions were produced by the same cause, acting in the one case on particles, in the other on masses; . . . and that the same property, under different modifications, was the cause of all the phenomena exhibited by different voltaic combinations.”
[55] Nicholson’s Journal, 4to. iv. 275.
[56] Phil. Trans. 1826.
[57] Ib. 1826, p. 389.
Although this is the enunciation, in tolerably precise terms, of the great discovery of his epoch, it was, at the period of which we speak, conjectured rather than proved; and we shall find that neither Davy nor his followers, for a considerable period, apprehended it with that distinctness which makes a discovery complete. But in a very short time afterwards, Davy drew great additional notice to his researches by effecting, in pursuance, as it appeared, of his theoretical views, the decomposition of potassa into a metallic base and oxygen. This was, as he truly said, in the memorandum written in his journal at the [295] instant, “a capital experiment.” This discovery was soon followed by that of the decomposition of soda; and shortly after, of other bodies of the same kind; and the interest and activity of the whole chemical world were turned to the subject in an intense degree.
At this period, there might be noticed three great branches of speculation on this subject; the theory of the pile, the theory of electrical decomposition, and the theory of the identity of chemical and electrical forces; which last doctrine, however, was found to include the other two, as might have been anticipated from the time of its first suggestion.
It will not be necessary to say much on the theories of the voltaic pile, as separate from other parts of the subject. The contact-theory, which ascribed the action to the contact of different metals, was maintained by Volta himself; but gradually disappeared, as it was proved (by Wollaston[58] especially,) that the effect of the pile was inseparably connected with oxidation or other chemical changes. The theories of electro-chemical decomposition were numerous, and especially after the promulgation of Davy’s Memoir in 1806; and, whatever might be the defects under which these speculations for a long time labored, the subject was powerfully urged on in the direction in which truth lay, by Davy’s discoveries and views. That there remained something still to be done, in order to give full evidence and consistency to the theory, appears from this;—that some of the most important parts of Davy’s results struck his followers as extraordinary paradoxes;—for instance, the fact that the decomposed elements are transferred from one part of the circuit to another, in a form which escapes the cognizance of our senses, through intervening substances for which they have a strong affinity. It was found afterwards that the circumstance which appeared to make the process so wonderful, was, in fact, the condition of its going on at all. Davy’s expressions often seem to indicate the most exact notions: for instance, he says, “It is very natural to suppose that the repellent and attractive energies are communicated from one particle to another of the same kind, so as to establish a conducting chain in the fluid; and that the locomotion takes place in consequence;”[59] and yet at other times he speaks of the element as attracted and repelled by the metallic surfaces which form the poles;—a different, and, as it appeared afterwards, an untenable view. Mr. Faraday, who supplied what was wanting, justly notices this vagueness. [296] He says,[60] that though, in Davy’s celebrated Memoir of 1806, the points established are of the utmost value, the mode of action by which the effects take place is stated very generally; so generally, indeed, that probably a dozen precise schemes of electro-chemical action might be drawn up, differing essentially from each other, yet all agreeing with the statement there given.” And at a period a little later, being reproached by Davy’s brother with injustice in this expression, he substantiated his assertion by an enumeration of twelve such schemes which had been published.
[58] Phil. Trans. 1801, p. 427.
[59] Paris, i. 154.
[60] Researches, 482.
But yet we cannot look upon this Memoir of 1806, otherwise than as a great event, perhaps the most important event of the epoch now under review. And as such it was recognized at once all over Europe. In particular, it received the distinguished honor of being crowned by the Institute of France, although that country and England were then engaged in fierce hostility. Buonaparte had proposed a prize of sixty thousand francs “to the person who by his experiments and discoveries should advance the knowledge of electricity and galvanism, as much as Franklin and Volta did;” and “of three thousand francs for the best experiment which should be made in the course of each year on the galvanic fluid;” the latter prize was, by the First Class of the Institute, awarded to Davy.
From this period he rose rapidly to honors and distinctions, and reached a height of scientific fame as great as has ever fallen to the lot of a discoverer in so short a time. I shall not, however, dwell on such circumstances, but confine myself to the progress of my subject.
Sect. 2.—Establishment of the Electro-chemical Theory by Faraday.
The defects of Davy’s theoretical views will be seen most clearly by explaining what Faraday added to them. Michael Faraday was in every way fitted and led to become Davy’s successor in his great career of discovery. In 1812, being then a bookseller’s apprentice, he attended the lectures of Davy, which at that period excited the highest admiration.[61] “My desire to escape from trade,” Mr. Faraday says, “which I thought vicious and selfish, and to enter into the service of science, which I imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to Sir H. Davy.” He was favorably received, and, in the next year, became [297] Davy’s assistant at the Institution; and afterwards his successor. The Institution which produced such researches as those of these two men, may well be considered as a great school of exact and philosophical chemistry. Mr. Faraday, from the beginning of his course of inquiry, appears to have had the consciousness that he was engaged on a great connected work. His Experimental Researches, which appeared in a series of Memoirs in the Philosophical Transactions, are divided into short paragraphs, numbered into a continued order from 1 up to 1160, at the time at which I write;[62] and destined, probably, to extend much further. These paragraphs are connected by a very rigorous method of investigation and reasoning which runs through the whole body of them. Yet this unity of purpose was not at first obvious. His first two Memoirs were upon subjects which we have already treated of (B. xiii. [c. 5] and [c. 8]), Voltaic Induction, and the evolution of Electricity from Magnetism. His “Third Series” has also been [already] referred to. Its object was, as a preparatory step towards further investigation, to show the identity of voltaic and animal electricity with that of the electrical machine; and as machine electricity differs from other kinds in being successively in a state of tension and explosion, instead of a continued current, Mr. Faraday succeeded in identifying it with them, by causing the electrical discharge to pass through a bad conductor into a discharging-train of vast extent; nothing less, indeed, than the whole fabric of the metallic gas-pipes and water-pipes of London. In this Memoir[63] it is easy to see already traces of the general theoretical views at which he had arrived; but these are not expressly stated till his “Fifth Series;” his intermediate Fourth Series being occupied by another subsidiary labor on the conditions of conduction. At length, however, in the Fifth Series, which was read to the Royal Society in June, 1833, he approaches the theory of electro-chemical decomposition. Most preceding theorists, and Davy amongst the number, had referred this result to attractive powers residing in the poles of the apparatus; and had even pretended to compare the intensity of this attraction at different distances from the poles. By a number of singularly beautiful and skilful experiments, Mr. Faraday shows that the phenomena can with no propriety be [298] ascribed to the attraction of the poles.[64] “As the substances evolved in cases of electro-chemical decomposition may be made to appear against air,[65] which, according to common language, is not a conductor, nor is decomposed; or against water,[66] which is a conductor, and can be decomposed; as well as against the metal poles, which are excellent conductors, but undecomposable; there appears but little reason to consider this phenomenon generally as due to the attraction or attractive powers of the latter, when used in the ordinary way, since similar attractions can hardly be imagined in the former instances.”
[61] Paris, ii. 3.
[62] December, 1835. (At present, when I am revising the second edition, September, 1846, Dr. Faraday has recently published the “Twenty-first Series” of his Researches ending with paragraph 2453.)
[63] Phil. Trans. 1833.
[64] Researches, Art. 497
[65] Researches, Arts. 465, 469.
[66] 495.
Faraday’s opinion, and, indeed, the only way of expressing the results of his experiments, was, that the chemical elements, in obedience to the direction of the voltaic currents established in the decomposing substance, were evolved, or, as he prefers to say, ejected at its extremities.[67] He afterwards states that the influence which is present in the electric current may be described[68] as an axis of power, having [at each point] contrary forces exactly equal in amount in contrary directions.
[67] 493.
[68] 517.
Having arrived at this point, Faraday rightly wished to reject the term poles, and other words which could hardly be used without suggesting doctrines now proved to be erroneous. He considered, in the case of bodies electrically decomposed, or, as he termed them, electrolytes, the elements as travelling in two opposite directions; which, with reference to the direction of terrestrial magnetism, might be considered as naturally east and west; and he conceived elements as, in this way, arriving at the doors or outlets at which they finally made their separate appearance. The doors he called electrodes, and, separately, the anode and the cathode;[69] and the elements which thus travel he termed the anïon and the catïon (or cathïon).[70] By means of this nomenclature he was able to express his general results with much more distinctness and facility.
[69] 663.
[70] The analogy of the Greek derivation requires catïon; but to make the relation to cathode obvious to the English reader, and to avoid a violation of the habits of English pronunciation, I should prefer cathïon.
But this general view of the electrolytical process required to be pursued further, in order to explain the nature of the action. The identity of electrical and chemical forces, which had been hazarded as [299] a conjecture by Davy, and adopted as the basis of chemistry by Berzelius, could only be established by exact measures and rigorous proofs. Faraday had, in his proof of the identity of voltaic and electric agency, attempted also to devise such a measure as should give him a comparison of their quantity; and in this way he proved that[71] a voltaic group of two small wires of platinum and zinc, placed near each other, and immersed in dilute acid for three seconds, yields as much electricity as the electrical battery, charged by ten turns of a large machine; and this was established both by its momentary electro-magnetic effect, and by the amount of its chemical action.[72]
[71] Researches, Art. 371.
[72] 537.
It was in his “Seventh Series,” that he finally established a principle of definite measurement of the amount of electrolytical action, and described an instrument which he termed[73] a volta-electrometer. In this instrument the amount of action was measured by the quantity of water decomposed: and it was necessary, in order to give validity to the mensuration, to show (as Faraday did show) that neither the size of the electrodes, nor the intensity of the current, nor the strength of the acid solution which acted on the plates of the pile, disturbed the accuracy of this measure. He proved, by experiments upon a great variety of substances, of the most different kinds, that the electro-chemical action is definite in amount according to the measurement of the new instrument.[74] He had already, at an earlier period,[75] asserted, that the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes; but the volta-electrometer enabled him to fix with more precision the meaning of this general proposition, as well as to place it beyond doubt.
[73] 739.
[74] Arts. 758, 814.
[75] 377.
The vast importance of this step in chemistry soon came into view. By the use of the volta-electrometer, Faraday obtained, for each elementary substance, a number which represented the relative amount of its decomposition, and which might properly[76] be called its “electro-chemical equivalent.” And the question naturally occurs, whether these numbers bore any relation to any previously established chemical measures. The answer is remarkable. They were no other than the atomic weights of the Daltonian theory, which formed the climax of the previous ascent of chemistry; and thus here, as everywhere in [300] the progress of science, the generalizations of one generation are absorbed in the wider generalizations of the next.
[76] 792.
But in order to reach securely this wider generalization, Faraday combined the two branches of the subject which we have already noticed;—the theory of electrical decomposition with the theory of the pile. For his researches on the origin of activity of the voltaic circuit (his Eighth Series), led him to see more clearly than any one before him, what, as we have said, the most sagacious of preceding philosophers had maintained, that the current in the pile was due to the mutual chemical action of its elements. He was led to consider the processes which go on in the exciting-cell and in the decomposing place as of the same kind, but opposite in direction. The chemical composition of the fluid with the zinc, in the common apparatus, produces, when the circuit is completed, a current of electric influence in the wire; and this current, if it pass through an electrolyte, manifests itself by decomposition, overcoming the chemical affinity which there resists it. An electrolyte cannot conduct without being decomposed. The forces at the point of composition and the point of decomposition are of the same kind, and are opposed to each other by means of the conducting-wire; the wire may properly be spoken of[77] as conducting chemical affinity: it allows two forces of the same kind to oppose one another;[78] electricity is only another mode of the exertion of chemical forces;[79] and we might express all the circumstances of the voltaic pile without using any other term than chemical affinity, though that of electricity may be very convenient.[80] Bodies are held together by a definite power, which, when it ceases to discharge that office, may be thrown into the condition of an electric current.[81]
[77] Researches Art. 918.
[78] 910.
[79] 915.
[80] 917.
[81] 855.
Thus the great principle of the identity of electrical and chemical action was completely established. It was, as Faraday with great candor says,[82] a confirmation of the general views put forth by Davy, in 1806, and might be expressed in his terms, that “chemical and electrical attractions are produced by the same cause;” but it is easy to see that neither was the full import of these expressions understood nor were the quantities to which they refer conceived as measurable quantities, nor was the assertion anything but a sagacious conjecture, till Faraday gave the interpretation, measure, and proof, of which we have spoken. The evidence of the incompleteness of the views of his predecessor we have [already] adduced, in speaking of his vague and [301] inconsistent theoretical account of decomposition. The confirmation of Davy’s discoveries by Faraday is of the nature of Newton’s confirmation of the views of Borelli and Hooke respecting gravity, or like Young’s confirmation of the undulatory theory of Huyghens.
[82] 965.
We must not omit to repeat here the moral which we wish to draw from all great discoveries, that they depend upon the combination of exact facts with clear ideas. The former of these conditions is easily illustrated in the case of Davy and Faraday, both admirable and delicate experimenters. Davy’s rapidity and resource in experimenting were extraordinary,[83] and extreme elegance and ingenuity distinguish almost every process of Faraday. He had published, in 1829, a work on Chemical Manipulation, in which directions are given for performing in the neatest manner all chemical processes. Manipulation, as he there truly says, is to the chemist like the external senses to the mind;[84] and without the supply of fit materials which such senses only can give, the mind can acquire no real knowledge.
[83] Paris, i. 145.
[84] Pref. p. ii.
But still the operations of the mind as well as the information of the senses, ideas as well as facts, are requisite for the attainment of any knowledge; and all great steps in science require a peculiar distinctness and vividness of thought in the discoverer. This it is difficult to exemplify in any better way than by the discoveries themselves. Both Davy and Faraday possessed this vividness of mind; and it was a consequence of this endowment, that Davy’s lectures upon chemistry, and Faraday’s upon almost any subject of physical philosophy, were of the most brilliant and captivating character. In discovering the nature of voltaic action, the essential intellectual requisite was to have a distinct conception of that which Faraday expressed by the remarkable phrase,[85] “an axis of power having equal and opposite forces;” and the distinctness of this idea in Faraday’s mind shines forth in every part of his writings. Thus he says, the force which determines the decomposition of a body is in the body, not in the poles.[86] But for the most part he can of course only convey this fundamental idea by illustrations. Thus[87] he represents the voltaic circuit by a double circle, studded with the elements of the circuit, and shows how the anïons travel round it in one direction, and the cathïons in the opposite. He considers[88] the powers at the two places of action as balancing against each other through the medium of the conductors, in a manner [302] analogous to that in which mechanical forces are balanced against each other by the intervention of the lever. It is impossible to him[89] to resist the idea, that the voltaic current must be preceded by a state of tension in its interrupted condition, which is relieved when the circuit is completed. He appears to possess the idea of this kind of force with the same eminent distinctness with which Archimedes in the ancient, and Stevinus in the modern history of science, possessed the idea of pressure, and were thus able to found the science of mechanics.[90] And when he cannot obtain these distinct modes of conception, he is dissatisfied, and conscious of defect. Thus in the relation between magnetism and electricity,[91] “there appears to be a link in the chain of effects, a wheel in the physical mechanism of the action, as yet unrecognized.” All this variety of expression shows how deeply seated is the thought. This conception of Chemical Affinity as a peculiar influence of force, which, acting in opposite directions, combines and resolves bodies;—which may be liberated and thrown into the form of a voltaic current, and thus be transferred to remote points, and applied in various ways; is essential to the understanding, as it was to the making, of these discoveries.
[85] Art. 517.
[86] Art. 661.
[87] 963.
[88] 917.
[89] Art. 950.
[90] 990.
[91] 1114.
By those to whom this conception has been conveyed, I venture to trust that I shall be held to have given a faithful account of this important event in the history of science. We may, before we quit the subject, notice one or two of the remarkable subordinate features of Faraday’s discoveries.
Sect. 3.—Consequences of Faraday’s Discoveries.
Faraday’s volta-electrometer, in conjunction with the method he had already employed, as we have seen, for the comparison of voltaic and common electricity, enabled him to measure the actual quantity of electricity which is exhibited, in given cases, in the form of chemical affinity. His results appeared in numbers of that enormous amount which so often comes before us in the expression of natural laws. One grain of water[92] will require for its decomposition as much electricity as would make a powerful flash of lightning. By further calculation, he finds this quantity to be not less than 800,000 charges of his Leyden battery;[93] and this is, by his theory of the identity of the combining with the decomposing force, the quantity of electricity [303] which is naturally associated with the elements of the grain of water, endowing them with their mutual affinity.
[92] 853.
[93] 861.
Many of the subordinate facts and laws which were brought to light by these researches, clearly point to generalizations, not included in that which we have had to consider, and not yet discovered: such laws do not properly belong to our main plan, which is to make our way up to the generalizations. But there is one which so evidently promises to have an important bearing on future chemical theories, that I will briefly mention it. The class of bodies which are capable of electrical decomposition is limited by a very remarkable law: they are such binary compounds only as consist of single proportionals of their elementary principles. It does not belong to us here to speculate on the possible import of this curious law; which, if not fully established, Faraday has rendered, at least, highly probable:[94] but it is impossible not to see how closely it connects the Atomic with the Electro-chemical Theory; and in the connexion of these two great members of Chemistry, is involved the prospect of its reaching wider generalizations, and principles more profound than we have yet caught sight of.
[94] Art. 697.
As another example of this connexion, I will, finally, notice that Faraday has employed his discoveries in order to decide, in some doubtful cases, what is the true chemical equivalent;[95] “I have such conviction,” he says, “that the power which governs electro-decomposition and ordinary chemical attractions is the same; and such confidence in the overruling influence of those natural laws which render the former definite, as to feel no hesitation in believing that the latter must submit to them too. Such being the case, I can have no doubt that, assuming hydrogen as 1, and dismissing small fractions for the simplicity of expression, the equivalent number or atomic weight of oxygen is 8, of chlorine 36, of bromine 78·4, of lead 103·5, of tin 59, &c.; notwithstanding that a very high authority doubles several of these numbers.”
[95] 851.
Sect. 4.—Reception of the Electro-chemical Theory.
The epoch of establishment of the electro-chemical theory, like other great scientific epochs, must have its sequel, the period of its reception and confirmation, application and extension. In that period we [304] are living, and it must be the task of future historians to trace its course.
We may, however, say a word on the reception which the theory met with, in the forms which it assumed, anterior to the labors of Faraday. Even before the great discovery of Davy, Grotthuss, in 1805, had written upon the theory of electro-chemical decomposition; but he and, as we have seen, Davy, and afterwards other writers, as Riffault and Chompré, in 1807, referred the effects to the poles.[96] But the most important attempt to appropriate and employ the generalization which these discoveries suggested, was that of Berzelius; who adopted at once the view of the identity, or at least the universal connexion, of electrical relations with chemical affinity. He considered,[97] that in all chemical combinations the elements may be considered as electro-positive and electro-negative; and made this opposition the basis of his chemical doctrines; in which he was followed by a large body of the chemists of Germany. He held too that the heat and light, evolved during cases of powerful combination, are the consequence of the electric discharge which is at that moment taking place: a conjecture which Faraday at first spoke of with praise.[98] But at a later period he more sagely says,[99] that the flame which is produced in such cases exhibits but a small portion of the electric power which really acts. “These therefore may not, cannot, be taken as evidences of the nature of the action; but are merely incidental results, incomparably small in relation to the forces concerned, and supplying no information of the way in which the particles are active on each other, or in which their forces are finally arranged.” And comparing the evidence which he himself had given of the principle on which Berzelius’s speculations rested, with the speculations themselves, Faraday justly conceived, that he had transferred the doctrine from the domain of what he calls doubtful knowledge, to that of inductive certainty.
[96] Faraday (Researches, Art. 481, 492).
[97] Ann. Chim. lxxxvi. 146, for 1813.
[98] Researches, Art. 870
[99] 960.
Now that we are arrived at the starting-place, from which this well-proved truth, the identity of electric and chemical forces, must make its future advances, it would be trifling to dwell longer on the details of the diffusion of that doubtful knowledge which preceded this more certain science. Our history of chemistry is, therefore, here at an end. I have, as far as I could, executed my task; which was, to mark all the [305] great steps of its advance, from the most unconnected facts and the most imperfect speculations, to the highest generalization at which chemical philosophers have yet arrived.
Yet it will appear to our purpose to say a few words on the connexion of this science with those of which we are next to treat; and that I now proceed to do.
CHAPTER X.
Transition from the Chemical to the Classificatory Sciences.
IT is the object and the boast of chemistry to acquire a knowledge of bodies which is more exact and constant than any knowledge borrowed from their sensible qualities can be; since it penetrates into their intimate constitution, and discloses to us the invariable laws of their composition. But yet it will be seen, on a little reflection, that such knowledge could not have any existence, if we were not also attentive to their sensible qualities.
The whole fabric of chemistry rests, even at the present day, upon the opposition of acids and bases: an acid was certainly at first known by its sensible qualities, and how otherwise, even now, do we perceive its quality? It was a great discovery of modern times that earths and alkalies have for their bases metals: but what are metals? or how, except from lustre, hardness, weight, and the like, do we recognize a body as a metal? And how, except by such characters, even before its analysis, was it known to be an earth or an alkali? We must suppose some classification established, before we can make any advance by experiment or observation.
It is easy to see that all attempts to avoid this difficulty by referring to processes and analogies, as well as to substances, bring us back to the same point in a circle of fallacies. If we say that an acid and alkali are known by combining with each other, we still must ask, What is the criterion that they have combined? If we say that the distinctive qualities of metals and earths are, that metals become earths by oxidation, we must still inquire how we recognize the process of oxidation? We have seen how important a part combustion plays in the history of chemical speculation; and we may usefully form such classes of [306] bodies as combustibles and supporters of combustion. But even combustion is not capable of being infallibly known, for it passes by insensible shades into oxidation. We can find no basis for our reasonings, which does not assume a classification of obvious facts and qualities.
But any classification of substances on such grounds, appears, at first sight, to involve us in vagueness, ambiguity, and contradiction. Do we really take the sensible qualities of an acid as the criterion of its being an acid?—for instance, its sourness? Prussic acid, arsenious acid, are not sour. “I remember,” says Dr. Paris,[100] “a chemist having been exposed to much ridicule from speaking of a sweet acid,—why not?” When Davy had discovered potassium, it was disputed whether it was a metal; for though its lustre and texture are metallic, it is so light as to swim on water. And if potassium be allowed to be a metal, is silicium one, a body which wants the metallic lustre, and is a non-conductor of electricity? It is clear that, at least, the obvious application of a classification by physical characters, is attended with endless perplexity.
[100] Life of Davy, i. 263.
But since we cannot even begin our researches without assuming a classification, and since the forms of such a classification which first occur, end in apparent confusion, it is clear that we must look to our philosophy for a solution of this difficulty; and must avoid the embarrassments and contradictions of casual and unreflective classification, by obtaining a consistent and philosophical arrangement. We must employ external characters and analogies in a connected and systematic manner; we must have Classificatory Sciences, and these must have a bearing even on Chemistry.
Accordingly, the most philosophical chemists now proceed upon this principle. “The method which I have followed,” says M. Thenard, in his Traité de Chimie, published in 1824, “is, to unite in one group all analogous bodies; and the advantage of this method, which is that employed by naturalists, is very great, especially in the study of the metals and their compounds.”[101] In this, as in all good systems of chemistry, which have appeared since the establishment of the phlogistic theory, combustion, and the analogous processes, are one great element in the arrangement, while the difference of metallic and non-metallic, is another element. Thus Thenard, in the first place, speaks of Oxygen; in the next place, of the Non-metallic Combustibles, as Hydrogen, Carbon, Sulphur, Chlorine; and in the next place, of Metals. But the Metals are again divided into six Sections, with reference, [307] principally, to their facility of combination with oxygen. Thus, the First Section is the Metals of the Earths; the Second, the Metals of the Alkalies; the Third, the Easily Oxidable Metals, as Iron; the Fourth, Metals Less Oxidable, as Copper and Lead; the Fifth Section contains only Mercury and Osmium; and the Sixth, what were at an earlier period termed the Noble Metals, Gold, Silver, Platinum, and others.
[101] Pref., p. viii.
How such principles are to be applied, so as to produce a definite and consistent arrangement, will be explained in speaking of the philosophy of the Classificatory Sciences; but there are one or two peculiarities in the classes of bodies thus recognized by modern chemistry, which it may be useful to notice.
1. The distinction of Metallic and Non-metallic is still employed, as of fundamental importance. The discovery of new metals is so much connected with the inquiries concerning chemical elements, that we may notice the general progress of such discoveries. Gold, Silver, Iron, Copper, Quicksilver, Lead, Tin, were known from the earliest antiquity. In the beginning of the sixteenth century, mine-directors, like George Agricola, had advanced so far in practical metallurgy, that they had discovered the means of extracting three additional metals, Zinc, Bismuth, Antimony. After this, there was no new metal discovered for a century, and then such discoveries were made by the theoretical chemists, a race of men who had not existed before Beccher and Stahl. Thus Arsenic and Cobalt were made known by Brandt, in the middle of the eighteenth century, and we have a long list of similar discoveries belonging to the same period; Nickel, Manganese, and Tungsten, which were detected by Cronstedt, Gahn, and Scheele, and Delhuyart, respectively; metals of a very different kind, Tellurium and Molybdenum, which were brought to light by Müller, Scheele, Bergman, and Hielm; Platinum, which was known as early as 1741, but with the ore of which, in 1802 and 1803, the English chemists, Wollaston and Tennant, found that no less than four other new metals (Palladium, Rhodium, Iridium and Osmium) were associated. Finally, (omitting some other new metals,) we have another period of discovery, opened in 1807, by Davy’s discovery of Potassium, and including the resolution of all, or almost all, the alkalies and earths into metallic bases.
[2nd Ed.] [The next few years made some, at least some conjectural, additions to the list of simple substances, detected by a more minute scrutiny of known substances. Thorium was discovered by Berzelius in 1828; and Vanadium by Professor Sefström in 1830. A [308] metal named Cerium, was discovered in 1803, by Hisinger and Berzelius, in a rare Swedish mineral known by the name of Cerit. Mosander more recently has found combined with Cerium, other new metals which he has called Lanthanium, Didymium, Erbium, and Terbium: M. Klaus has found a new metal, Ruthenium, in the ore of Platinum; and Rose has discovered in Tantalite two other new metals, which he has announced under the names of Pelopium and Niobium. Svanberg is said to have discovered a new earth in Eudialyt, which is supposed to have, like the rest, a new radical. If these last discoveries be confirmed, the number of simple substances will be raised to sixty-two.]
2. Attempts have been made to indicate the classification of chemical substances by some peculiarity in the Name; and the Metals, for example, have been designated generally by names in um, like the Latin names of the ancient metals, aurum, ferrum. This artifice is a convenient nomenclature for the purpose of marking a recognized difference; and it would be worth the while of chemists to agree to make it universal, by writing molybdenum and platinum; which is sometimes done, but not always.
3. I am not now to attempt to determine how far this class,—Metals,—extends; but where the analogies of the class cease to hold there the nomenclature must also change. Thus, some chemists, as Dr. Thomson, have conceived that the base of Silica is more analogous to Carbon and Boron, which form acids with oxygen, than it is to the metals: and he has accordingly associated this base with these substances, and has given it the same termination, Silicon. But on the validity of this analogy chemists appear not to be generally agreed.
4. There is another class of bodies which have attracted much notice among modern chemists, and which have also been assimilated to each other in the form of their names; the English writers calling them Chlorine, Fluorine, Iodine, Bromine, while the French use the terms Chlore, Phtore, Iode, Brome. We have already noticed the establishment of the doctrine—that muriatic acid is formed of a base, chlorine, and of hydrogen,—as a great reform in the oxygen theory; with regard to which rival claims were advanced by Davy, and by MM. Gay-Lussac and Thenard in 1800. Iodine, a remarkable body which, from a dark powder, is converted into a violet-colored gas by the application of heat, was also, in 1813, the subject of a similar rivalry between the same English and French chemists. Bromine [309] was only discovered as late as 1826; and Fluorine, or Phtore, as, from its destructive nature, it has been proposed to term it, has not been obtained as a separate substance, and is inferred to exist by analogy only. The analogies of these bodies (Chlore, Phtore, &c.) are very peculiar; for instance, by combination with metals they form salts; by combination with hydrogen they form very strong acids; and all, at the common temperature of the atmosphere, operate on other bodies in the most energetic manner. Berzelius[102] proposes to call them halogenous bodies, or halogenes.
[102] Chem. i. 262.
5. The number of Elementary Substances which are at present presented in our treatises of chemistry[103] is fifty-three, [or rather, as we have said above, sixty-two.] It is naturally often asked what evidence we have, that all these are elementary, and what evidence that they are all the elementary bodies;—how we know that new elements may not hereafter be discovered, or these supposed simple bodies resolved into simpler still? To these questions we can only answer, by referring to the history of chemistry;—by pointing out what chemists have understood by analysis, according to the preceding narrative. They have considered, as the analysis of a substance, that elementary constitution of it which gives the only intelligible explanation of the results of chemical manipulation, and which is proved to be complete as to quantity, by the balance, since the whole can only be equal to all its parts. It is impossible to maintain that new substances may not hereafter be discovered; for they may lurk, even in familiar substances, in doses so minute that they have not yet been missed amid the inevitable slight inaccuracies of all analysis; in the way in which iodine and bromine remained so long undetected in sea-water; and new minerals, or old ones not yet sufficiently examined, can hardly fail to add something to our list. As to the possibility of a further analysis of our supposed simple bodies, we may venture to say that, in regard to such supposed simple bodies as compose a numerous and well-characterized class, no such step can be made, except through some great change in chemical theory, which gives us a new view of all the general relations which chemistry has yet discovered. The proper evidence of the reality of any supposed new analysis is, that it is more consistent with the known analogies of chemistry, to suppose the process analytical than synthetical. Thus, as has already been said, chemists admit the existence of fluorine, from the analogy of chlorine; and Davy, when it was found [310] that ammonia formed an amalgam with mercury, was tempted to assign to it a metallic basis. But then he again hesitates,[104] and doubts whether the analogies of our knowledge are not better preserved by supposing that ammonia, as a compound of hydrogen and another principle, is “a type of the composition of the metals.”
[103] Turner, p. 971.
[104] Elem. Chem. Phil. 1812, p. 481.
Our history, which is the history of what we know, has little to do with such conjectures. There are, however, some not unimportant principles which bear upon them, and which, as they are usually employed, belong to the science which next comes under our review, Mineralogy.
~Additional material in the [3rd edition].~