FOOTNOTES:
[2] The force of diamagnetism is vastly feebler than that of ordinary magnetism. According to Weber, the magnetism of a thin bar of iron exceeds the diamagnetism of an equal mass of bismuth about two and a-half million times.
CHAPTER III.
“Every secret which is disclosed, every discovery which is made, every new effort which is brought to view, serves to convince us of numberless more which remain concealed, and which we had before no suspicion of.... Knowledge is not our proper happiness. Whoever will in the least attend to the thing will see that it is the gaining, not the having of it, which is the entertainment of the mind.”—Bishop Butler.
Next, probably, to magnetism and electricity, the scientific investigation of the laws of heat has yielded the most fruitful and the most curious results. The science of heat made the greatest progress about the middle of the present century, and Professor Tyndall was one of its most successful investigators. Being a force co-related to electricity, it is scarcely remarkable that the same natural philosopher should reveal to us not a few of these silent operations of magnetism and heat that previously were unobserved or were regarded as mysteries.
When, in 1859, he turned his attention to the absorption of radiant heat by gases and vapours, there was considerable diversity of opinion as to the effect of the atmosphere on radiant heat; and great skill and patience were required in devising experiments, and in detecting and eliminating the various sources of error. Till then it was thought that the subject was outside the realm of experiment, but Professor Tyndall soon demonstrated that heat in gases and vapours was subject to various laws which had most important effects in every part of the world. In his first memoir he established not only the existence of absorption and radiation in gases, but that the differences of absorption and radiation were as great among gases as among liquids and solids. He showed that the elementary gases, hydrogen, oxygen, nitrogen, as well as air freed from moisture and carbonic acid, examined in a length of four feet, absorb about 3½ per cent. of heat radiated from lamp-black at 212°, the slightest impurity in the gas, however, altering the rate of absorption. With compound gases and vapours very different results were obtained. About twenty gases and vapours were examined, and it was found that while the elementary gases already named gave the feeblest action, olephiant gas showed the most energetic action, absorbing 81 per cent. He also made the important discovery that by arranging the various gases in order according to their power, first of radiating heat and then of absorbing radiant heat, the order was the same in both cases; in short, the order of radiation was exactly that of absorption. In his second memoir he introduced a new and remarkable method of determining absorption and radiation. This method he called “dynamic radiation.” Dispensing with the use of any extraneous source of heat, he obtained his results by the heat or cold produced by the condensation or rarefication of the gases. Just as a ball striking a target is heated by collision, so he heated gas contained in one part of a tube by the collision of its particles against the surface of another part into which they rushed to fill a vacuum. He found, he said, by strict experiments that the dynamic radiation of an amount of boracic ether vapour, possessing a tension of only one 1,012,500,000th of an atmosphere, was easily measurable.
His researches on the relation of radiant heat to aqueous vapour, published in 1863, were the most interesting and useful. Such were the difficulties connected with the investigation of this part of the subject that Professor Tyndall and his old friend Professor Magnus, of Berlin, arrived at and long maintained opposite conclusions as to the absorption of radiant heat by the air and the influence of aqueous vapour. Early in his researches Professor Tyndall regarded the action of the atmosphere as a particular part of his inquiry, and, accordingly, his third memoir was specially devoted to the radiation of aqueous vapour. The conclusion he came to was that the aqueous vapour in our atmosphere intercepted or absorbed eighty times more heat than the air, and as there was only one atom of aqueous vapour for every 200 of oxygen and nitrogen composing the air, it appeared that one atom of the former absorbed 16,000 times more than one atom of oxygen or nitrogen. This startling conclusion he verified by a system of checks and counter-checks which were considered as decisive. The applications of this discovery were manifold and important. The aqueous vapour which absorbed so much heat he likened to a blanket which is more necessary to the vegetable life of England than clothing is to man. “Remove for a single summer night,” he said, “the aqueous vapour from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost.” The aqueous vapour constitutes a local dam, which deepens the temperature at the earth’s surface, but which finally overflows and gives to space all that we receive from the sun. This discovery presented an explanation of some phenomena, which hitherto had been imperfectly understood. It was evidently the absence of this aqueous screen which made the winters in Central Asia almost unendurable; and it showed how the burning heat of the Sahara during the day was followed by intense cold at night.
Before Professor Tyndall had published all his observations on the relations between radiant heat and aqueous vapour, his friend, Professor Frankland, regarded them as sufficient to account for the glacial era, and the action of glaciers over the entire globe. During a visit to Norway in 1863 Frankland considered the subject afresh, and came to the conclusion that the chief cause of the phenomena of the glacial epoch was a higher temperature of the ocean than prevails at present. The critics of the day pointed out that such a view depended upon the accuracy of the assumption that our earth had gradually cooled down from an originally incandescent state; and it is now generally admitted by natural philosophers that the earth has cooled down from a state of liquid heat. In that case the waters of the ocean, when cooling down from the boiling point, would be at a higher temperature than the present; and Professor Frankland maintained that it was in the later stages of the cooling process that the glacial epoch occurred. The great natural glacial apparatus he divided into three parts—the evaporator, the condenser, and the receiver. The cooling ocean was the evaporator; the mountains were the icebearers or receivers; while the dry air which permitted the heat from the vapour to radiate into space, acted as the condenser. He made numerous experiments to show that under these conditions the land would cool more rapidly than the sea; and he maintained that in the glacial epoch the “rays of heat streamed into space from the ice-bearing surfaces with comparatively little interruption, whilst the radiation from the sea was as effectually retarded as if the latter had been protected with a thick envelope of non-conducting material. Thus, whilst the ocean retained a temperature considerably higher than at present, the icebearers had undergone a considerably greater refrigeration.” He calculated that an increase of 20° in the temperature of the coast of Norway would double the evaporation from a given surface, and such an increased evaporation, accompanied of course by a corresponding precipitation, “would suffice to supply the higher portions of the land with that gigantic ice-burden which ground down the mountain slopes during the glacial epoch.” Such a view did not require the assumption of any natural convulsion or catastrophe; on the contrary it accounted for the glacial epoch by the evolution of thermal conditions, the existence of which is now generally admitted.[3]
In his fourth memoir, published in 1864, “On the Radiation and Absorption of Heat by Gaseous and Liquid Matter,” Professor Tyndall showed that generally the absorption of non-luminous radiant heat by vapours was the same as that of the liquids from which the vapours were produced.
His fifth memoir, entitled “Contributions to Molecular Physics,” was made the Bakerian lecture for that year. In it he deduced from numerous experiments the remarkable law that the opacity of a substance with respect to radiant heat from a source of comparatively low temperature increases with the chemical complexity of its molecule. He examined the effects of temperature on the transmission of radiant heat, the radiation from flames of various kinds, and the influence of vibrating periods on the absorption of radiant heat.
In November, 1864, the Royal Society presented him with the Rumford medal for his researches on the absorption and radiation of heat by gases and vapours; and General Sabine, in making the presentation, said such had been the fate of Professor Tyndall that each last achievement might almost be said to have dimmed the lustre of those which preceded it. Curiously enough his very next achievements thereafter did dim the lustre of those published prior to the presentation of the Rumford Medal. It was the discovery of a means of separating light from heat. Melloni had previously discovered a combination of screens by which radiant heat could be arrested or separated from light, an operation which is effected on a vast scale by the moon when it reflects the light of the sun. Professor Tyndall effected the converse operation. He discovered that a solution of iodine in bisulphide of carbon entirely intercepted the light of the most brilliant flames. A hollow prism filled with that opaque liquid and placed in the path of the beam from an electric lamp, completely intercepted the light, but transmitted the heat unimpaired. In this way he succeeded in separating with marvellous sharpness the invisible from the visible radiations of the lime light, the electric light, and the sun. He not only produced combustion, fusion, and incandescence by invisible radiation, but he proved that in the case of the electric light the invisible rays are no less than eight times as powerful as the visible radiations. He obtained all the colours of the solar spectrum from a platinum foil raised to incandescence at the invisible focus; and this rendering of a refractory body incandescent by invisible rays he called calorescence. In connection with these investigations he performed a daring experiment. Knowing that a layer of iodine placed before the eye intercepted the light, he determined to place his own eye in the focus of strong invisible rays. He knew that if in doing so the dark rays were absorbed in a high degree by the humours of the eye, the albumen of the humours might coagulate; and on the other hand, if there was no high absorption, the rays might strike upon the retina with a force sufficient to destroy it. When he first brought his eye, undefended, near the dark focus, the heat on the parts surrounding the pupil was too intense to be endured. He therefore made an aperture in a plate of metal, and placing his eye behind this aperture, he gradually approached the point of convergence of the invisible rays. First the pupil and next the retina were placed in the focus without any sensible damage. Immediately afterwards a sheet of platinum foil placed in the position which the retina had occupied became red-hot.
In a subsequent memoir he dealt with the influence of colour and mechanical condition upon radiant heat, demonstrating that white bodies are far more potent absorbers of radiant heat than black ones.
During the first thirteen years of his researches in the laboratory of the Royal Institution he produced thirteen papers, which were published in the Philosophical Transactions. Conspicuous among these were his papers on the radiation and absorption of heat, and his researches on that subject have generally been admitted to be of the most thorough and original character. A lucid epitome of the chief results he obtained was given in the Rede lecture which he delivered before the University of Cambridge in 1865, when the University conferred on him the honorary degree of LL.D.
In 1863 he published the first edition of one of his most popular books, Heat Considered as a Mode of Motion—a book which an eminent electrician has recommended students of electricity to master; in 1867 he published a volume of lectures on “Sound”; and in 1869-74 he published his lectures on “Light.” These works have gone through several editions. As an illustration of the interest with which he can invest such impalpable subjects, it is worth remarking that a Chinese official, named Hsii-chung-hu, was so pleased with the book on Sound that he had it translated into the Chinese language and printed at Shanghai, in order that his countrymen might participate in the pleasure and instruction which he had derived from it. It was published at the expense of the Chinese Government, and sold at 1s. 6d. a copy.
During the ten years from 1859 to 1869, says Professor Tyndall, “researches on radiant heat in its relations to the gaseous form of matter occupied my continual attention.” But towards the close of that period his main inquiry, as it extended into space, began to spread out into various branches. In 1866 he entered upon an examination of the chemical action of light upon vapours, and the action of heat of high refrangibility as an explorer of the molecular condition of matter. “In this investigation one obstacle to be overcome was the presence of the floating matter in the air. The processes for the removal of these particles became the occasion of an independent research, branching out into various channels: on the one hand, it dealt with the practical problem of the preservation of life among firemen exposed to heated smoke; and, on the other, it approached the recondite question of spontaneous generation. He subjected the compound vapours of various substances to the action of a concentrated beam of light. The vapours were decomposed, and non-volatile products were formed. The decompositions always began with a blue cloud, which discharged perfectly polarised light at right angles to the beam. This suggested to him the origin of the blue colour of the sky; and as it showed the extraordinary amount of light that may be scattered by cloudy matter of extreme tenuity, he considered that it might be regarded as a suggestion towards explaining the nature of a comet’s tail.”
Regions of cloud and smoke are proverbial as symbols of the negation of human interest; but Professor Tyndall imparted new beauties to the one and deprived the other of its terrors. He said to the chaotic vapours “Light,” and that which was without form and void instantly assumed the loveliest forms that Nature knows. Incredible as this language may appear to some, it is no mere Oriental hyperbole. He made the light from an electric lamp to pass through a great glass tube containing transparent, invisible vapours, and the action of the light at once commencing chemical decomposition, various cloud forms resembling organic structures were seen in the tube. The following is the beautiful description he gave to the Royal Society of the phenomena presented by hydriodic acid:—
“The cloud extended for about eighteen inches along the tube, and gradually shifted its position from the end nearest the lamp to the most distant end. The portion quitted by the cloud proper was filled by an amorphous haze, the decomposition, which was progressing lower down, being here apparently complete. A spectral cone turned its apex towards the distant end of the tube, and from its circular base filmy drapery seemed to fall. Placed on the base of the cone was an exquisite vase, from the interior of which sprang another vase of similar shape; over the edges of these vases fell the faintest clouds, resembling spectral sheets of liquid. From the centre of the upper vase a straight cord of cloud passed for some distance along the axis of the experimental tube, and at each end of this cord two involved and highly iridescent vortices were generated. The frontal portion of the cloud which the cord penetrated assumed in succession the form of roses, tulips, and sunflowers. It also passed through the appearance of a series of beautifully-shaped bottles placed one within the other. Once it presented the shape of a fish, with eyes, gills, and feelers.”
In 1869 it was stated before the British Association that M. Morren, while living in the South of France, had succeeded in producing similar results by the use of sunlight instead of the electric light.
For a long time during his researches on the decomposition of vapours he was troubled by the presence of floating matter revealed by a powerful condensed beam of light, and he tried numerous expedients for the purpose of intercepting this matter. At last he succeeded. By causing the air intended for experimental purposes to pass over the tip of a spirit-lamp flame, the floating matter disappeared. He therefore concluded that it was organic matter, which had been burned out by the flame. This discovery took place on October 5th, 1868. Till then he regarded the dust of our air as for the most part inorganic and noncombustible. This led him on to the investigation of the germ theory. On the one hand he added proof to proof, and experiment to experiment, to show that when a consuming heat was applied to air its organic matter disappeared; and on the other hand he maintained that as surely as a fig comes from a fig, a grape from a grape, and a thorn from a thorn, so surely does the typhoid virus or seed, when planted or scattered about among people, increase and multiply into typhoid fever, scarlatina virus into scarlatina, and small-pox virus into small-pox. These conclusions formed the subject of a famous lecture on “Dust and Disease,” delivered at the Royal Institution on January 21st, 1870. Among his audience were some of the foremost men of the day, such as Mr. W. E. Gladstone, then Prime Minister, Earl Granville, Dean Stanley, Sir Edwin Landseer, Sir Henry Holland, and Professor Huxley. The views which Professor Tyndall then put forth were received with marked disfavour among the medical profession. Even scientific men did not hesitate to pour ridicule upon the germ theory. For example, Professor Bloxam, Lecturer on Chemistry to the Department of Artillery Studies, suggested in one of his lectures that the Committee on Explosives should abandon gun cotton, and collecting the germs of small-pox and similar malignant diseases in cotton or other dust-collecting substances, should load shells with them, and we should then hear of the enemy being dislodged from his position by a volley of typhus or a few rounds of Asiatic cholera. Like most truths, the germ theory survived the ridicule of its opponents.
The labours of Pasteur in relation to the germ theory always appeared to command Professor Tyndall’s admiration. A large part of his lecture on “Dust and Disease” consisted of an account of the successful way in which Pasteur dealt with the epidemic among silkworms in France. Writing in April, 1870, the Professor said: “There is more solid science in one paper of Pasteur than in all the volumes and essays that have been written against him. Schroeder and Pasteur have demonstrated that air filtered through cotton-wool is deprived wholly, or in part, of its power to produce animalcular life. Why? An experiment with a beam of light answers the question; for while it proves our ordinary air to be charged with floating matter, the beam pronounces air, which has been carefully filtered through cotton-wool, to be visibly pure; there are no germs afloat in it; hence it is impossible as a generator of life. Again, Pasteur prepared twenty-one flasks, each containing a decoction of yeast, which he boiled in order to destroy whatever germs it might contain. While the space above the liquid was filled with pure steam he sealed the necks of his flasks with a blow-pipe. He opened ten of them in the damp, still caves of the Paris Observatory, and eleven of them in the courtyard of the same establishment. Of the former only one showed signs of life subsequently. In nine out of the ten flasks no organisms of any kind were developed. In all the others organisms speedily appeared. Pasteur ascribed this unexpected result to the subsidence of the germs in the motionless air of the caves. Is this surmise correct? The beam of light enables us to answer this question. I have had a chamber constructed, the lower half of which is of wood, and the upper half of glass. On the 6th February this chamber was closed, and every crevice that could admit dust or cause a disturbance of the air was carefully stopped. The electric beam when sent through the glass showed the air at the outside to be loaded with floating matter. The chamber was examined almost daily, and a gradual diminution of the floating matter was observed. At the end of the week the chamber was optically empty. The floating matters, germs included, had wholly subsided, and the air held nothing in suspension. Here again the ocular demonstration furnished by the luminous beam goes hand in hand with the experimental result of Pasteur.”
Professor Tyndall did not, however, adopt the germ theory on the authority of Pasteur. He not only discovered it for himself, but demonstrated its accuracy by innumerable experiments, in the course of which he made use of 10,000 vessels. To him, too, science owes the use of the electric beam as an explorer of germ particles which could not otherwise be made visible by the best optical aids. The most exquisitely minute particles, which could not be detected by the most powerful glasses, have been revealed in the air by the electric beam.
For some time he carried on a controversy with some doughty champions of the old theory of spontaneous generation; but as the evidences in favour of the germ theory increased, the antagonism to it diminished. One practical evidence, not only of the reality, but of the utility of the germ theory, was Pasteur’s discovery of the nature of the organisms in yeast that produced “beer disease;” and when Pasteur visited England, after that discovery, and explained the cause of beer turning sour, Professor Tyndall afterwards visited some of the most prominent breweries in London to make inquiries on the subject. He was extremely surprised at the paucity of knowledge possessed by the brewers, although they had over and over again incurred disastrous losses in consequence of their lack of knowledge. He said that when the brewers found their beer becoming bad they used to exchange their yeast among themselves, and thus get on with their losses, when five minutes’ examination with the microscope would have prevented this waste and loss; for it would have shown them the minute organisms which spoiled the beer.
In connection with his researches on the germ theory, he produced a useful invention which had a philanthropic rather than a commercial object. To the title of inventor he never made any claim; on the contrary, he repeatedly expressed his view of the difference between a scientific discoverer and a mechanical inventor; contending that while the practical man is not usually the man to make the necessary antecedent discoveries, the cases are rare in which the discoverer in science knows how to turn his labours to practical account.
Nevertheless scientific reflection enabled him to devise a form of respirator which protects firemen from the stifling effects of dense smoke. His attention had repeatedly been directed to the risks that firemen encountered when in conflict with smoke and flame, and he had been told that smoke was a greater enemy to them than flame. He therefore endeavoured to find a means of protecting them from suffocation. First he tried a respirator made of cotton-wool, but that was insufficient; so to the cotton-wool he added glycerine; and though this was an improvement, still it only enabled them to remain in dense smoke for three or four minutes. He next added charcoal and this greatly increased the utility of the respirator, which when complete was composed of a layer of cotton-wool moistened with glycerine, next a thin layer of dry wool, then a layer of charcoal fragments, succeeded by another thin layer of dry cotton-wool and a layer of fragments of caustic lime. These were inclosed in a wire gauze cover. The first experiments with this respirator were made in a small cellar-like chamber with stone flooring and stone walls in the basement of the Royal Institution. A fire of resinous pine-wood was lighted, and was so covered over as to generate dense smoke instead of flames. Professor Tyndall and his assistant, having each put on one of the new respirators, and suitable glasses to protect their eyes, were able to remain for half an hour or longer in that apartment full of smoke so dense and pungent that he believed a single inhalation through the undefended mouth would have been perfectly unendurable. Captain Shaw, the chief officer of the Metropolitan Fire Brigade, on being asked whether such a respirator would be of use to him, replied that it would be most valuable; but he had made himself acquainted with every contrivance of the kind in this and other countries, and had found none of them of any practical use. However, at the request of Professor Tyndall, the Captain and some of his men went to the Royal Institution to test the new invention. The small room was again filled with dense smoke, three men went successively into it, and remained there as long as their Captain desired. On coming out they declared that with the respirators they had not felt the least discomfort, and that they could have remained all day in the smoke. Captain Shaw himself then tested it with the same result, and he afterwards stated that Professor Tyndall, in the kindest possible manner, at once placed his invention at the service of the Fire Brigade.
In 1870 he accompanied the eclipse expedition to Oran, and having been disappointed in the special object of his journey, he determined in returning to investigate the causes of the varying tints presented by sea-water. On board H.M.S. Urgent, between Gibraltar and Spithead, he filled nineteen bottles with sea-water, and afterwards examined them by the electric light. This examination showed that the yellowish water of the coast and harbours contained a large quantity of particles, that in the green water the particles were finer and less abundant, and that the blue water of the deep was comparatively clear of them. The explanation he gave of the colours of the ocean, in a lecture at the Royal Institution, was that when a beam of light entered the sea the heat-rays were absorbed at the surface, the red rays by a very superficial layer of water, the green rays next, and ultimately the blue rays; but when the light encountered particles in the water the green rays would be reflected by them. If there were no particles, the green rays would continue their course till they were wholly quenched, and thus water of more than ordinary depth and purity would appear as black as ink.
In later years he made some practical additions to our knowledge of sound. His advice had repeatedly been asked as to the laws which affected the distribution of sound variously in different buildings—a subject upon which volumes had been written, but which was still imperfectly understood. As an illustration of the unexpected circumstances that affected the transmission of sound, he sometimes related what occurred to himself in the Senate House of Cambridge University when he delivered the Rede lecture in 1865. On going to the Senate House to test its acoustic qualities, he was astonished to find that from the usual place of speaking his words could not be heard at all by a friend whom he had placed at the extreme end of the hall as his auditory. He found that the reverberation from the floor and walls followed the direct sound of his voice in such a way as to destroy the clearness of the words as they were uttered. Dismayed at this effect, he made up his mind that in respect of audibleness his lecture was doomed to be a failure. But the reverse was the case. The lecture was in every respect a great success. An overflowing audience filled the hall, and listened to him with rapt attention. During the hour and a half that he spoke every syllable was heard by the most distant hearer; and he attributed this unexpected result to the presence of the audience, which, he said, quenched the prejudicial effect of the reverberation of his voice produced by the sides and bottom of the room. After that experience, he advocated the making of different experiments with the view of extending the practical knowledge of acoustics.
To that knowledge he himself became a valuable contributor. In 1873 he conducted a series of experiments with a view to determine the properties of the atmosphere as a vehicle of sound. Navigators had often been at a loss to understand how it was that the most powerful fog-signals—such as gongs, whistles, and guns—were sometimes easily heard at a great distance on rainy days, and were inaudible at comparatively short distances on fine days. Even within a few minutes the acoustic properties of the atmosphere sometimes underwent remarkable variations. Professor Tyndall’s experiments led him to the conclusion that the aqueous vapour raised by the sun, though often invisible, produced a cloud which formed as impervious a barrier to the waves of sound as a dense black cloud does to the waves of light. The presence of water in a vaporous form being the real enemy to the transmission of sound through the atmosphere, it was easy to understand its frequent occurrence on days apparently clear and bright. This was previously unknown.
He also furnished an interesting illustration of the corelation of heat and sound.
Notwithstanding the elaborate data upon which he had founded his conclusions as to the interaction of radiant heat on vapours, some Continental physicists questioned their accuracy, and accordingly Professor Tyndall in later years resumed the inquiry and obtained some remarkable results. He had previously shown that heat will pass without any loss through a long glass tube filled with nitrogen or air, and closed up at the ends by lenses of crystal; but if the same tube is filled with carbonic acid or the vapour of ether the heat, instead of being transmitted through it, is almost entirely intercepted. In 1880 Mr. Graham Bell showed him that musical sounds were produced by a beam of light striking upon thin discs of matter; and Professor Tyndall at once discovered the secret of this surprising effect. He said that before making an experiment he pictured in his mind a highly-absorbent vapour exposed to the shocks of an intermittent beam suddenly expanding at the moment of exposure, and as suddenly contracting when the beam was intercepted; and thus pulses of an amplitude probably far greater than those obtainable with solids would be produced, and would be sufficient to give forth musical sounds. He soon proved this surmise to be correct. He filled a glass tube or bulb with absorbent gas or vapour, and between it and the limelight he placed a round piece of cardboard with equi-distant holes in it; then by placing the bulb in such a position that when the light passed through the holes it impinged upon the glass bulb, and by causing the cardboard to revolve, the action of the beam became intermittent, as it only reached the vapour when one of the holes in the revolving cardboard came in front of the bulb. By this contrivance a series of calorific shocks were produced that gave sound vibrations of surprising intensity. When, however, the bulbs were filled with gases or vapours, such as nitrogen or air, that transmitted the heat, no sounds were produced. He tried the sounding power of ten gases and eighty vapours, and found that the sounds produced by chloride of methyl were the loudest; and that, conveyed to the ear by a tube of indiarubber, they seemed as loud as the peal of an organ. He also found that in respect of intensity the order of the sound in gases was the same as the order of their absorption of radiant heat. These marvellous results he described in his Bakerian lecture for 1881, “On the Action of Free Molecules on Radiant Heat and its Conversion thereby into Sound.”