Lives of the electricians

by William T. Jeans

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.”

FOOTNOTES:

[3] This glacier theory is all the more deserving of prominence since the publication in 1886 of Lieutenant Greely’s discovery of lakes, rivers, and valleys rich in vegetation and animal life in the interior of Grinnell Land at points the farthest north ever reached by explorers.