Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/422

The rapidity of the vibrations explains the failure of all attempts to refract the X-rays. For in the formula

$\mu ^{2}=1+{\frac {\sigma p^{2}}{\rho (p^{2}-n^{2})}}$

of the Maxwell-Sellmeier theory,^[1] n denotes the frequency, and so is in this case extremely large; whence we have

$\mu ^{2}=1$,

i.e., the refractive index of all substances for the X-rays is unity. In fact, the vibrations alternate too rapidly to have an effect on the sluggish systems which are concerned in refraction.

Some years afterwards H. Haga and C. H. Wind,^[2] having measured the diffraction-patterns produced by X-rays, concluded that the wave-length of the vibrations concerned was of the order of one Ångstrom unit, that is about 1/6000 of the wavelength of the yellow light of sodium.

One of the most important properties of X-rays was discovered, shortly after the rays themselves had become known, by J. J. Thomson,^[3] who announced that when they pass through any substance, whether solid, liquid, or gaseous, they render it conducting. This he attributed, in accordance with the ionic theory of conduction, to " a kind of electrolysis, the molecule of the non-conductor being split up, or nearly split up, by the Röntgen rays."

The conductivity produced in gases by this means was at once investigated^[4] more closely. It was found that a gas which had acquired conducting power by exposure to X-rays lost this quality when forced through a plug of glass-wool; whence it was inferred that the structure in virtue of which the gas conducts is of so coarse a character that it is unable to survive the passage through the fine pores of the plug. The

1. Cf. p. 293.
2. Proceedings of the Amsterdam Acad., March 25th, 1899 (English edition, i, p. 420), and September 27th, 1902 (English edition, v, p. 247).
3. Nature, February 27, 1896, p. 391.
4. J.J. Thomson and E. Rutherford, Phil. Mag. xlii (1896), p. 392.