As in the case of uranium or thorium, the photographic action is mainly due to the penetrating or cathodic rays. The radiographs obtained with radium are very similar to those obtained with X rays, but lack the sharpness and detail of the latter. The rays are unequally absorbed by different kinds of matter, the absorption varying approximately as the density. In photographs of the hand the bones do not stand out as in X ray photographs. Curie and Laborde have shown that the compounds of radium possess the remarkable property of always keeping their temperature several degrees above the temperature of the surrounding air. Each gram of radium radiates an amount of energy corresponding to 100 gram-calories per hour. This and other properties of radium are discussed in detail in chapters V and XII. =16. Compounds of radium.= When first prepared in the solid state, all the salts of radium—the chloride, bromide, acetate, sulphate, and carbonate—are very similar in appearance to the corresponding salts of barium, but in time they gradually become coloured. In chemical properties the salts of radium are practically the same as those of barium, with the exception that the chloride and bromide of radium are less soluble in water than the corresponding salts of barium. All the salts of radium are naturally phosphorescent. The phosphorescence of impure radium preparations is in some cases very marked. All the radium salts possess the property of causing rapid colorations of the glass vessel which contains them. For feebly active material the colour is usually violet, for more active material a yellowish-brown, and finally black. =17. Actinium.= The discovery of radium in pitchblende gave a great impetus to the chemical examination of uranium residues, and a systematic search early led to the detection of several new radio-active bodies. Although these show distinctive radio-active properties, so far none of them have been purified sufficiently to give a definite spectrum as in the case of radium. One of the most interesting and important of these substances was discovered by Debierne[28] while working up the uranium residues, obtained by M. and Mme Curie from the Austrian government, and was called by him actinium. This active substance is precipitated with the iron group, and appears to be very closely allied in chemical properties to thorium, though it is many thousand times more active. It is very difficult to separate from thorium and the rare earths. Debierne has made use of the following methods for partial separation: (1) Precipitation in hot solutions, slightly acidulated with hydrochloric acid, by excess of hyposulphite of soda. The active matter is present almost entirely in the precipitate. (2) Action of hydrofluoric acid upon the hydrates freshly precipitated, and held in suspension in water. The portion dissolved is only slightly active. By this method titanium may be separated.