The different actions of the radium rays on these fluorescent substances can be illustrated very simply and beautifully by the following experiment. A small U tube is filled with fragments of the fluorescent substance arranged in layers. The U tube is immersed in liquid air and the emanation from about 30 mgrs. of radium bromide is condensed in the tube. On closing the tube and removing it from the liquid air, the emanation distributes itself uniformly in the tube. The shades of colour produced in the different substances are clearly seen. It is observed that all the crystals increase in luminosity for several hours, on account of the excited activity produced by the emanation. This effect is especially observed in kunzite, which at first hardly responds to the rays, since the β and γ rays, which causes it to fluoresce, are not given out by the emanation itself but by one of its later products. The intensity of the β and γ rays is, in consequence, small at first but rises to a maximum after several hours; the luminosity observed varies in a corresponding manner. Sir William Crookes[188] has made an examination of the effect of continued exposure of a diamond to the radium rays. An “off-colour” diamond, of a pale yellow colour, was placed inside a tube with radium bromide. After 78 days’ exposure, the diamond had darkened and become bluish green in tint; when heated at 50° in a mixture of potassium chlorate for ten days, the diamond lost its dull surface colour and was bright and transparent, and its tint had changed to a pale bluish green. The rays have thus a double action on the diamond; the less penetrating β rays produce a superficial darkening due to the change of the surface into graphite, while the more penetrating β rays and the γ rays produce a change of colour throughout its mass. The diamond phosphoresced brightly during the whole course of its exposure to the rays. Crookes also observed that the diamond still retained enough activity to affect a photographic plate 35 days after removal, although, during the period of 10 days, it was heated in a mixture sufficiently powerful to remove the outer skin of graphite. This residual activity may possibly be due to a slow transformation product of the emanation which is deposited on the surface of bodies (see chapter XI). Marckwald observed that the α rays from radio-tellurium produced marked phosphorescence on some kinds of diamonds. An account of the various luminous effects produced on different gems by exposure to the radium and actinium rays has been given by Kunz and Baskerville[189].