Page:Proceedings of the Royal Society of London Vol 60.djvu/163

148 Of the substances used, the above showed the effect best, but with wood, paraffin, and water, although small, it could always be detected. We conclude from the above results that the Rontgen rays are of different kinds, and that the substances given in the table differ very much from tinfoil in their selective absorption. After the rays have been screened by passing through some tinfoil layers additional layers are much less absorbent, while the absorption produced by other substances is not so much diminished.

Of the substances tried, those which are fluorescent gave the most marked difference as compared with tinfoil.

The above results were all obtained with one vacuum tube, which was working extremely well. It produced a very rapid leak from the charged disk, and the pressure of its residual air was very small. In fact, after working for a time it became too strong for the coil that was being used to work it. Another vacuum tube, in which the pressure of the residual air was greater and which was not so efficient in producing leakages, was then used, and several of the substances used before were again tested, but in no case was any evidence of selective absorption obtained. As far as the test was efficient, the radiation from this bulb was homogeneous.

A third tube was then used, more efficient than the last in producing leakage, but not so good as the first used. With this tube experiments made in the same way as before gave evidence of selective absorption, but not so marked as with the first tube.

It seems therefore that as a tube becomes more efficient the character of the rays given off becomes less homogeneous.

The authors begin their paper by stating that it has been shown by Herbert Tomlinson that the atomic volume of metals is intimately connected with their thermal capacity* and with Young’s modulus.f He considers, in view of the work of Wertheim,^ of Maxwell,§ and of Heen,|| and as the result of his own experiments, that the value of

f 4 Phil. Trans./ Part I, 1883, p. 32. J 1 Ann. de Chim. efc de Phys./ vol. 12? 1844. § 4 Phil. Trans./ vol. 156, 1866, p. 249.
 * ‘ Roy. Soc. Proc.,’ vol. 88 (1884-85), p. 488.
 * ‘Bull. de TAcad. Roy. de Belgique/ vol. 4 (1882).