140 ESSAYS BIOGRAPHICAL AND CHEMICAL 



compounds may be prepared in the dark and still possess 

 actinic power. And the rays emitted from them have the 

 power of discharging both positively and negatively 

 electrified bodies. Not merely salts of uranium possess 

 this property, but even metallic uranium itself, a dark- 

 coloured, brittle metal, which emits sparks of fire when 

 shaken in a bottle, a phenomenon due probably to 

 oxidation. 



Shortly after this discovery of Becquerel's, Madame 

 Curie, a Polish lady working in Paris, discovered that a 

 certain specimen of pitchblende, the common ore of 

 uranium, possesses the properties of uranium, and in 

 greater measure. Pitchblende, though consisting mainly 

 of an oxide of uranium of the formula U 3 8 , contains 

 small amounts of other elements. On separating these, 

 Monsieur and Madame Curie found that the bismuth 

 obtained from this source is particularly radio-active, 

 while ordinary bismuth shows no trace of that property. 

 Attributing this behaviour to its containing a new element, 

 they patriotically named it ' polonium/ in allusion to 

 Madame Curie's nationality. But it was not long before 

 they discovered that it was not only the bismuth which 

 exhibited radio-activity, but also the barium ; and they 

 inferred the presence of a second element, naming it 

 'radium.' A third substance has also been separated 

 from the same uranium ores by Debierne, who, following 

 precedent, has termed it 'actinium.' It appears to be 

 associated with another element, titanium, contained in 

 pitchblende ; and thorium, an element whose compounds 

 were discovered to possess radio-activity by G. C. Schmidt, 

 must be added to the list. We have therefore at present 

 no fewer than four radio-active substances : polonium, 

 associated with bismuth ; radium, with barium ; actinium, 

 with titanium ; and thorium. Associated with thorium is 

 a much more powerfully radio-active material, to which 



