=115.= Besides their power of acting on a photographic plate, and of ionizing gases, the radiations from active bodies are able to produce marked chemical and physical actions in various substances. Most of these effects are due either to the α or β rays. The γ rays produce little effect in comparison. Since the β rays are similar in all respects to high velocity cathode rays, it is to be expected that they will produce effects similar in character to those produced by the cathode rays in a vacuum tube. Phosphorescent action. Becquerel[184] has studied the action of radium rays in producing phosphorescence in various bodies. The substance to be tested was placed above the radium in the form of powder on a very thin mica plate. Examination was made of the sulphides of calcium and strontium, ruby, diamond, varieties of spar, phosphorus and hexagonal blende. Substances like the ruby and spar, which phosphoresce under luminous rays, did not phosphoresce under the radium rays. On the other hand, those which were made luminous by ultra-violet light were also luminous under the action of radium rays. The radium rays show distinct differences from X rays. For example, a diamond which was very luminous with radium rays was unaffected by X rays. The double sulphate of uranium and potassium is more luminous than hexagonal blende under X rays, but the reverse is true for radium rays; under the influence of these rays, sulphide of calcium gave a blue luminosity but was hardly affected by X rays. The following table shows the relative phosphorescence excited in various bodies. Substance Without screen. Across screen Intensity of black paper Hexagonal blende 13·36 ·04 Platino-cyanide of 1·99 ·05 barium Diamond 1·14 ·01 Double sulphate of 1·00 ·31 Uranium and Potassium Calcium Fluoride ·30 ·02 In the last column the intensity without the screen is in each case taken as unity. The great diminution of intensity after the rays have passed through black paper shows that most of the phosphorescence developed without the screen is, in the majority of cases, due to the α rays. Bary[185] has made a very complete examination of the class of substances which become luminous under radium rays. He found that the great majority of substances belong to the alkali metals and alkaline earths. All these substances were also phosphorescent under the action of X rays.