The Curies found that radium compounds rapidly produced coloration in glass. For moderately active material the colour is violet, for more active material it is yellow. Long continued action blackens the glass, although the glass may have no lead in its composition. This coloration gradually extends through the glass, and is dependent to some extent on the kind of glass used. Giesel[211] found that he could obtain as much coloration in rock-salt and fluor-spar by radium rays, as by exposure to the action of cathode rays in a vacuum tube. The coloration, however, extended much deeper than that produced by the cathode rays. This is to be expected, since the radium rays have a higher velocity, and consequently greater penetrating power, than the cathode rays produced in an ordinary vacuum tube. Goldstein observed that the coloration is far more intense and rapid when the salts are melted or heated to a red heat. Melted potassium sulphate, under the action of a very active preparation of radium, was rapidly coloured a strong greenish blue which gradually changed into a dark green. Salomonsen and Dreyer[212] found that plates of quartz were coloured by exposure to radium rays. When examined minutely, plates cut perpendicular to the optic axis showed the presence of lines and striae, parallel to the binary axes. Adjacent portions of the striated system differed considerably in intensity of coloration and clearly revealed the heterogeneity of structures of the crystal. The cause of these colorations by cathode and radium rays has been the subject of much discussion. Elster and Geitel[213] observed that a specimen of potassium sulphate, coloured green by radium rays, showed a strong photo-electric action, _i.e._ it rapidly lost a negative charge of electricity when exposed to the action of ultra-violet light. All substances coloured by cathode rays show a strong photo-electric action, and, since the metals sodium and potassium themselves show photo-electric action to a very remarkable degree, Elster and Geitel have suggested that the colorations are caused by a solid solution of the metal in the salt. Although the coloration due to radium rays extends deeper than that due to the cathode rays, when exposed to light the colour fades away at about the same rate in the two cases. Becquerel[214] found that white phosphorus is changed into the red variety by the action of radium rays. This action was shown to be due mainly to the β rays. The secondary radiation set up by the primary rays also produced a marked effect. Radium rays, like ordinary light rays, also caused a precipitate of calomel in the presence of oxalic acid.