DISCOVERY AND PROPERTIES OF RADIUM 143 



By the examination of a large number of uranium and thorium 

 minerals it was found that the electrical conductivity of the air 

 induced by the rays from an uranium compound varies directly 

 with the amount of this element present in the mineral. All 

 uranium compounds are active, and the metal itself more so 

 than any of its salts, except pitchblende or uraninite, U 3 8 , and 

 native chalcolite (copper uranyl-phosphate). The latter sub- 

 stance, when prepared artificially, was found to be less active 

 than the metal. Hence it appeared that the natural minerals 

 contained a substance far more active than uranium. Thorium 

 compounds were found to be active, the action of some of them 

 being actually more pronounced than that of uranium. 



A specimen of pitchblende possessing 2J times the activity 

 of uranium was examined chemically with the object of isolating 

 the radio-active substance. The mineral dissolved in acids was 

 brought into contact with sulphuretted hydrogen, and it was 

 found that the uranium and thorium remaining in solution, the 

 active substance was precipitated with the sulphides insoluble 

 in ammonium sulphide. After separating these in the usual 

 manner it was found that the active substance remained with 

 the bismuth. An extremely active substance was also obtained 

 from pitchblende by sublimation, and when the sulphides were 

 heated in a vacuum at 700 a sublimate was obtained possessing 

 an activity 400 times that of uranium. By further treatment a 

 very active substance was obtained to which the name polonium 

 was given in honour of Madame Curie's native country. 



The chemical examination of the uraniferous minerals studied 

 by the Curies proceeded nearly on the lines of the ordinary 

 method of qualitative analysis. In the sulphides obtained 

 polonium had been recognised, but another very active sub- 

 stance was found to be associated with the barium also present. 

 Barium is easily separated from solution by sulphuric acid, 

 which causes it to be thrown down as a white insoluble precipitate 

 consisting of the sulphate. This sulphate was accompanied by 

 the active substance. The sulphate was converted into the 

 chloride, and it was then found that a partial separation of the 

 inactive barium from the attendant active substance could be 

 effected by taking advantage of the greater degree of solubility 

 of the barium chloride in water, alcohol, or hydrochloric acid. 

 Ultimately it was found that separation was more easily effected 

 by fractional crystallisation of the bromides. The new substance 



