of the Ordnance Survey. In 1847 he accepted a teaching appointment at Queenwood College, Hampshire. Here he stayed till the following year, and applied himself chiefly to the study of chemistry, along with Dr. Frankland, one of his colleagues. In 1848, Tyndall and Frankland went together to Marburg, in Hesse-Cassel. In those days Germany was ahead of England in the teaching of science, so the two determined to add a German training to their English education. Tyndall's attention was called to the new property of magnetism, which Faraday had lately announced, and it was suggested to him by Dr. Knoblauch that the two should repeat Faraday's experiments, and inquire more closely into the true nature of diamagnetism. Professor Plücker, of Bonn, found that some crystals, made of diamagnetic substances, did not exhibit diamagnetic properties. To account for this he attributed to crystals an optical axis, which he supposed to be influenced in a peculiar manner when placed in a magnetic field.

After long and careful investigation Tyndall and Knoblauch came to the conclusion that the action of magnetism on bodies depended upon their molecular structure, or as Tyndall expressed it, upon their peculiarities of material aggregation. In 1851 Tyndall announced that the same laws govern both magnetic and diamagnetic phenomena. In 1853 he was appointed to the chair of Natural Philosophy at the Royal Institution.

The subject which owes most to Tyndall is that of radiant heat. Before he commenced his investigations but little was known about it. The quantities to be measured were so small, and existing apparatus so crude, that physicists had never attempted any accurate measurements of radiant heat. Tyndall, however, overcame the experimental difficulties, and, in 1864, published a paper " On the radiation and absorption of heat by gases and liquid matter," in which he showed that the absorption of non-luminous heat by vapours is the same as that of the liquids from which they are produced.

On January 21st, 1870, Tyndall delivered a lecture, at the Royal Institution, on "Dust and Disease," and gave the results of some investigations of his own on floating organic matter in the air. Examination of air before and after being subjected to a very high temperature showed that a large proportion of the dust it contained was organic matter, since it disappeared on

being burnt. In the course of the lecture Tyndall propounded a germ theory of diseases, saying that as surely as a pig comes from a pig, or a grape from a grape, so surely does the typhoid virus, or seed, when scattered about among people, give rise to typhoid fever, scarlatina virus to scarlatina, and small-pox virus to small-pox; and that the virus was carried about by the

floating organic matter in the air. Many eminent men were present at the lecture, and those of the medical profession received his views with disfavour, going so far as to ridicule the germ theory. The accuracy of that theory has since, however, been proved over and over again, and the discovery of the way in which diseases are spread has been of incalculable benefit to mankind.

It was as a popular lecturer that Tyndall excelled. The reason of his success in lecturing to the "unscientific may

Faraday's "Tubes of Force."

Clerk
Maxwell.

be found in his aptitude for imparting his knowledge in the simplest language, and in exciting the interest of an audience by homely illustrations. He probably did more than any other man of science to raise the standard of education amongst the uneducated classes.

When Faraday propounded his theory of electromagnetism, in which he explained the various phenomena by means of hypothetical tubes of force" in a hypothetical medium, his views met with distrust on all sides. His methods were not evident to the mathematical physicists, who had been accustomed to base their calculations simply on the laws of force, without concerning themselves with any medium to act as a vehicle of energy. They believed in the idea of action at a distance. Energy disappeared from one place and reappeared at another. They could form their equations without assuming the existence of any transmitting medium, and the solutions were in accord with observed phenomena. Fortunately Faraday had no such preconceived mathematical notions. He was not a mathematician, and was driven to invent a logic of his own; his tube of force took the place of the mathematicians' differential equation. These tubes of force were mathematical quantities, and his whole theory admitted of mathematical representation; but to bring his theory completely within the comprehension of the mathematical physicist needed an interpreter, who should express his ideas in their own familiar language.

In 1831, when Faraday was just beginning his work on electromagnetic induction, James Clerk Maxwell was born in Edinburgh. Educated at Edinburgh, and afterwards at Cambridge, Maxwell graduated in 1854, taking the position of Second Wrangler. His original investigations began whilst he was still in his teens, when he contributed papers to the Royal Society of Edinburgh on "Rolling Curves," and on "The Equilibrium of Elastic Solids." Whilst an undergraduate at Cambridge he devoted himself more to research than to working for the tripos. It was during this period that he carefully studied Faraday's original papers. His inclination was always to study mathematics as a means whereby to express his ideas on physical subjects rather than as an end in itself. His private tutor, William Hopkins, said of him: “It is not possible for that man to think incorrectly on physical subjects."

In 1855 Maxwell wrote a mathematical paper on "Lines of Force," expressing the Faraday line of force in mathematical language, and still further developing the idea. In 1856 he was appointed to the chair of Natural Philosophy at the Marischal

College, Aberdeen. The same year he gained the Adams Essay Prize with a thesis on "The Rings of Saturn," in which he showed that Saturn's rings could not, consistently with stability of structure, be either solid or liquid, but must be of the nature of streams of meteorites revolving round the planet. About the same time he invented the "dynamical top" to illustrate certain problems in dynamics. In 1860 he read a paper at a meeting of the British Association on "The Kinetic Theory of Gases," which supposes a gas to consist of myriads of particles jostling