Page:Dictionary of National Biography volume 57.djvu/438

 of their assumption they showed that, by pressure, the magnetic axis of a bismuth crystal could be shifted 90° in azimuth, the line of pressure always setting itself parallel with, or at right angles to, the line joining the two magnetic poles, according as the crystal was magnetic or diamagnetic. This explanation differed essentially from that of Faraday and Plücker. In June 1850 Tyndall went to England, and at the meeting of the British Association of that year in Edinburgh he read an account of his investigation which excited considerable interest. He afterwards returned to Marburg for six months, and carried out a lengthy inquiry into electro-magnetic attractions at short distances (Phil. Mag. April 1851).

At Easter 1851 Tyndall finally left Marburg and went to Berlin, where he became acquainted with many eminent men of science. In the laboratory of Professor Magnus he conducted a second investigation on ‘Diamagnetism and Magne-crystallic Action’ (ib. September 1851), which formed a sequel to that previously undertaken with Knoblauch. A paper describing his results was read at the Ipswich meeting of the British Association. He showed that the antithesis of the two forces was absolute: diamagnetism resembling magnetism as to polarity and all other characteristics, differing from it only by the substitution of repulsion for attraction and vice versa.

The question of diamagnetic polarity was much discussed. Its existence, originally asserted by Faraday and reaffirmed by Weber in 1848, had been subsequently denied by Faraday, who still continued doubtful. To meet all objections, Tyndall, at a later date, again took up the subject, and in three conclusive investigations, the second of which formed the subject of the Bakerian lecture delivered before the Royal Society in 1855, he put the polarity of bismuth and other diamagnetic bodies beyond question (ib. November 1851; Phil. Trans. 1855; ib. 1856, pt. i.) Five years were devoted by him to the investigation of diamagnetism and the influence of crystalline structure and mechanical pressure upon the manifestations of magnetic force. The original papers (with a few omissions in the last edition) are collected in his book on ‘Diamagnetism’ (see below).

Before leaving Marburg in 1851, Tyndall had agreed to return to Queenwood; this time as lecturer on mathematics and natural philosophy. Here he remained two years. The first of the three investigations just alluded to was carried out at Queenwood, as was also a series of experiments on the ‘Conduction of Heat through Wood’ (see ‘Molecular Influences,’ Phil. Trans. January 1853). On 3 June 1852 Tyndall was elected fellow of the Royal Society.

While at Queenwood he applied for several positions which offered a wider scope for his abilities. On his way to Ipswich in 1851 he had made the acquaintance of T. H. Huxley, and a warm and enduring friendship resulted. They made joint applications for the chairs respectively of natural history and physics then vacant at Toronto, but, in spite of high testimonials, they were unsuccessful. They also failed in candidatures for chairs in the newly founded university of Sydney, New South Wales. Meanwhile, soon after Tyndall's departure from Berlin, Dr. Henry Bence Jones [q. v.] visited that city, and, hearing much of Tyndall's labours and personality, caused him to be invited to give a Friday evening lecture at the Royal Institution. The lecture, ‘On the Influence of Material Aggregation upon the Manifestations of Force’ (Roy. Inst. Proc. i. 185), was delivered on 11 Feb. 1853. It produced an extraordinary impression, and Tyndall, hitherto known only among physicists, became famous beyond the limits of scientific society. In May 1853 he was unanimously chosen as professor of natural philosophy in the Royal Institution. The appointment had the special charm of making him the colleague of Faraday. Seldom have two men worked together so harmoniously as did Faraday and Tyndall during the years that followed. Their relationship from first to last resembled that of father and son. Tyndall's ‘Faraday as a Discoverer’ bears striking testimony to their attachment. Other sketches of Faraday by Tyndall are in his ‘Fragments of Science,’ and in the life of Faraday in this dictionary.

Tyndall's career was now definitely marked out. To the end of his active life his best energies were devoted to the service of the Royal Institution. In 1867, when Faraday died, Tyndall succeeded him in his position as superintendent of the Institution. On his own retirement in the autumn of 1887 he was elected honorary professor.

In 1854, after attending the British Association meeting at Liverpool, Tyndall visited the slate quarries of Penrhyn. His familiarity with the effects of pressure upon the structure of crystals led him to give special attention to the problem of slaty cleavage. By careful observation and experiments with white wax and many other substances which develop cleavage in planes perpendicular to pressure, he satisfied himself that pressure alone was sufficient to produce the cleavage