1] RADIO-ACTIVE SUBSTANCES 19 
absorption varying approximately as the density. In photographs 
of the hand the bones do not show out as in X ray photographs. 
Curie and Laborde have shown that the compounds of radium 
possess the remarkable property of always keeping their tempe- 
rature several degrees above the temperature of the surrounding 
air. Each gram of radium radiates an amount of energy corre- 
sponding to 100 gram-calories per hour. This and other properties 
of radium are discussed in detail in chapters v and VI. 
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 prac- 
tically the same as those of barium, with the exception that 
the chloride and bromide of radium are less soluble than the 
corresponding salts of barium. All the salts of radium are natu- 
rally phosphorescent. The phosphorescence of impure radium 
preparations 1s 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. Polonium. Polonium was the first of the active sub- 
stances obtained from pitchblende. It has been investigated in 
detail by its discoverer Mme Curie!. The pitchblende was dissolved 
im acid and sulphuretted hydrogen added. The precipitated 
sulphides contained an active substance, which, after separation 
of impurities, was found associated with bismuth. This active 
substance, which has been named polonium, is so closely allied in 
chemical properties to bismuth that it has so far been found 
impossible to effect a complete separation. Partial separation of 
polonium can be made by successive fractionations based on one 
of the following modes of procedure : 
(1) Sublimation in a vacuum. The active sulphide is more 
volatile than that of bismuth. It is deposited as a black substance 
at portions of the tube, where the temperature is between 250° 
1G, R. 127, p. 175, 1898. 
2—2 
