Page:Dictionary of National Biography volume 18.djvu/204

Faraday figured the particles of the di-electric as polarised, and concluded that electric induction was carried on from particle to particle from the inner sphere to the outer one. To this power of propagation he gave the name 'specific inductive capacity.' He then glanced at conduction in its relation to induction, and generalised thus: 'Can we not, by a gradual chain of association, carry up the discharge from its occurrence in air through spermaceti and water to solutions, and then on to chlorides, oxides, and metals, without any essential change in its character?' The action of the particles of the best conductor differs, according to Faraday, only in degree from that of the particles of the insulator. Particles of copper, for example, are first charged in succession by induction; but they rapidly discharge themselves, and this quick molecular discharge is what we call conduction. It may be stated here that Faraday, in 1838, foresaw that retardation must occur in wires circumstanced like those of submarine cables.

In 1841 his health broke down, and for three years he did nothing, not even 'reading on science.' Memoranda written by Faraday at this time prove that his mind was seriously shaken. He went to Switzerland accompanied by his wife and brother-in-law. His nerves had been shattered, but his muscles were strong. At the table d'hôte he was quite unable to enter into conversation; but outside he was capable of great physical exertion. A journal entry of his made at Interlaaken has been already quoted. Another, which strikingly reveals the religious tone of his mind, may be given here. On 12 Aug. 1841 he stood before the falls of the Giessbach. 'The sun shone brightly, and the rainbows seen from various points were very beautiful. One, at the bottom of a fine but furious fall, was very pleasant—there it remained motionless while the gusts of cloud and spray swept furiously across its place, and were dashed against the rock. It looked like a spirit strong in faith and steadfast in the midst of the storm of passions sweeping across it; and, though it might fade and revive, still it held on to the rock, as in hope, and giving hope.'

As soon as his health permitted, he resumed his work, and in November 1845 announced a discovery which he called 'the magnetisation of light, and the illumination of the lines of electric force.' The title provoked comment at the time, and caused misapprehension. It was soon, however, translated into 'the rotation of the plane of polarisation by magnets and by electric currents.' However it may have been described, this is one of Faraday's most pregnant and beautiful discoveries. He always thought that more lay concealed in it than was admitted by the scientific men of his time, and this thought is even now in process of verification. The discovery was made by means of that heavy glass which had failed to produce the optical effects expected from it. ‘A piece of this glass, about 2 inches square, and 0.5 of an inch thick, having flat and polished edges, was placed between the poles (not as yet magnetised by the electric current), so that the polarised ray should pass through its length. The glass acted as air, water, or any other transparent substance would do; and if the eye-piece were previously turned into such a position that the polarised ray was extinguished, then the introduction of the glass made no alteration in this respect. In this state of circumstances the force of the electro-magnet was developed by sending an electric current through its coils, and immediately the image of the lamp flame became visible, and continued so as long as the arrangement continued magnetic. On stopping the electric current, and so causing the magnetic force to cease, the light instantly disappeared. These phenomena could be renewed at pleasure at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.’ Many substances, oil of turpentine and quartz for example, cause the plane of polarisation to rotate without the intervention of magnetism. The difference, however, between Faraday's rotation and the rotation known before his time is profound. If, for example, a polarised beam, after having been caused to rotate by oil of turpentine, could by any means be reflected back through the liquid, the rotation impressed on the direct beam would be exactly neutralised by that impressed on the reflected one. Not so with Faraday's rotation, which was doubled by the act of reflection. With exquisite skill he augmented his effect by multiplying his reflections. When, for example, the rotation impressed on the direct beam was 12°, that acquired by three passages through the glass was 36°, while that derived from five passages was 60°.

Faraday's next great step was the discovery of diamagnetism. Brugmanns, Becquerel, Le Baillif, Saigy, and Seebeck had previously indicated the existence of a repulsive force exerted by a magnet on two or three substances. It is surprising that the observation was not pushed further. Every indication of this kind, however small, roused Faraday's ardour, causing him to expand and multiply it. It was a fragment of his famous heavy glass that revealed to him the fact of