by showing that when a sheet of metal is negatively electrified and exposed to ultra-violet light, the adjacent air is thrown into a state which permits the charge to leak rapidly away.
Interest was now thoroughly aroused in the problem of conductivity in gases; and it was generally felt that the best hope of divining the nature of the process lay in studying the discharge at high rarefactions. "If a first step towards understanding the relations between aether and ponderable matter is to be made," said Lord Kelvin in 1893,[1] "it seems to me that the most hopeful foundation for it is knowledge derived from experiments on electricity in high vacuum."
Within the two following years considerable progress was effected in this direction. J.J. Thomson,[2] by a rotating-mirror method, succeeded in measuring the velocity of the cathode rays, finding it to be[3] 1·9 x 107 cm./sec.; a value so much smaller than that of the velocity of light that it was scarcely possible to conceive of the rays as vibrations of the aether. A further blow was dealt at the latter hypothesis when Jean Perrin,[4] having received the rays in a metallic cylinder, found that the cylinder became charged with resinous electricity. When the rays were deviated by a magnet in such a way that they could no longer enter the cylinder, it no longer acquired a charge. This appeared to demonstrate that the rays transport negative electricity.
With cathode rays is closely connected another type of radiation, which was discovered in December, 1895, by W. C. Röntgen.[5] The discovery seems to have originated in an accident: a photographic plate which, protected in the usual way, had been kept in a room in which vacuum-tube experiments were carried on, was found on development to show distinct markings. Experiments suggested by this showed
- ↑ Proc. Roy. Soc. liv (1893), p. 389.
- ↑ Phil. Mag. xxxviii (1894), p. 358.
- ↑ The value found by the same investigator in 1897 was much larger than this.
- ↑ Comptes Rendus, cxxi (1895), p. 1130.
- ↑ Sitzungeber, der Würzburger Physikal.-Medie. Gesellschaft, 1895; reprinted, Ann. d. Phys. lxiv (1898), pp. 1, 12; translated, Nature, liii (1896), p. 274.
