24 SODDY, Evolution of Matter by Radio-active Elements. 



Professor Rutherford and myself to suppose that possibly 

 helium was such a product The evidence was of course 

 indirect, but very suggestive. Sir William Ramsay in 1895 

 discovered helium for the first time in terrestrial sources, as 

 a constituent of certain minerals, and drew attention to the 

 fact that it was only found in those minerals which contained 

 uranium and thorium. Many of these we now know 

 contain radium also, so that it may be stated that helium 

 only occurs in minerals which contain the radio-elements. 

 The study of the chemical nature of helium showed that 

 it was a gas, like argon, belonging to a chemically inert 

 family of elements, none of which up to the present time 

 have ever been made to enter into chemical combination. 

 Moreover, helium of all the known gases is the only one 

 that has never been liquefied, and the origin of its 

 occurrence in the uranium and thorium minerals, pent up 

 or 'occluded' in a curious and unexplained way, remained 

 a complete mystery. It may be released from the mineral 

 by heat or solution, but once liberated cannot be re- 

 absorbed. In the work by Professor Rutherford and 

 myself on the radio-active emanations of radium and 

 thorium it has been noticed that these gases, which also 

 resemble argon in their chemical inertness, frequently do 

 not escape from the dry solid radium and thorium com- 

 pounds producing them, but remain stored up or 'occluded,' 

 much in the same way as the helium in minerals, being 

 liberated by heat or solution. It seemed likely that the 

 explanation in both cases was the same. The gases 

 being formed throughout the bulk of the substance, under 

 favourable conditions remain mechanically imprisoned 

 within it. On this view helium represents the accumula- 

 tions during past epochs of one of the final products of 

 change of one of the disintegrating elements present in the 

 mineral. 



