Lives of the electricians

by William T. Jeans

[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.