190 ESSAYS BIOGRAPHICAL AND CHEMICAL 



that they undergo spontaneous change into other forms of 

 matter, themselves radioactive, and themselves unstable ; 

 and that finally elements are produced which, on account 

 of their non-radioactivity, are, as a rule, impossible to 

 recognise, for their minute amount precludes the applica- 

 tion of any ordinary test with success. The recognition of 

 helium, however, which is comparatively easy of detection, 

 lends great support to this hypothesis. 



The natural question which suggests itself is : Are other 

 elements undergoing similar change? Can it be that 

 their rate of change is so slow that it cannot be detected ? 

 Professor J. J. Thomson has attempted to answer this 

 question, and he has found that many ordinary elements 

 are faintly radioactive ; but the answer is still incomplete, 

 for, first, radium is so enormously radioactive that the 

 merest trace of one of its salts in the salt of another 

 element would produce such radioactivity ; and, second, it 

 is not proved that radioactivity is an invariable accompani- 

 ment of such change ; or, again, it may be evolved so 

 slowly as to escape detection. A lump of coal, for 

 example, is slowly being oxidised by the oxygen of the 

 air; oxidation is attended by a rise of temperature, but 

 the most delicate thermometer would detect no difference 

 between the temperature of a lump of coal and that 

 of the surrounding air, for the rate of oxidation is so 

 slow. 



Another question which arises is : Seeing that an element 

 like radium is changing into other substances, and that its 

 life is a comparatively short one, it must be in course of 

 formation, else its amount would be exhausted in several 

 thousand years. An attempt has been made by Soddy to 

 see if uranium salts, carefully purified from radium, have 

 reproduced radium after an interval of a year; but his 

 result was a negative one. Possibly some other form of 

 matter besides uranium contributes to the synthesis of 



