THE TERRESTRIAL DISTRIBUTION OF 



RADIUM 



By ARTHUR HOLMES, B.Sc, A.R.C.S., F.G.S. 



Imperial College, London 



Discussing the origin and duration of the sun's heat in 1862, 

 Lord Kelvin concluded that " the inhabitants of the earth cannot 

 continue to enjoy the light and heat essential to their life for 

 many million years longer," but he prudently added, " unless 

 sources now unknown to us are prepared in the great store- 

 houses of creation." To the geologist the extreme importance 

 of the radioactive elements lies in their fulfilment of Kelvin's 

 cautious qualification. Atomic disintegration is, in all known 

 cases, accompanied by a spontaneous evolution of energy which 

 ultimately appears in the form of heat. This unexpected discovery 

 was first announced in 1903 by Curie and Laborde, who showed 

 that radium was capable of maintaining a temperature slightly 

 above that of its immediate environment. Already Elster and 

 Geitel had begun to investigate the radioactivity of the 

 atmosphere. It was found that the air in caverns and cellars 

 was unusually high in its content of active matter, and this 

 led naturally to the view that the latter had escaped as radium 

 emanation from the soil. Impressed with the significance of 

 these observations, implying as they did a widespread distri- 

 bution of radioactive matter in the surface materials of the 

 earth, Professor (now Sir Ernest) Rutherford suggested in 1905 

 that the heat constantly evolved in virtue of the disintegration 

 of the earth's supply of radium might be sufficient to maintain 

 the observed temperature gradient. On the assumption that 

 100 calories per hour represented the heat output of one gram 

 of radium, he calculated that if each gram of the substance of 



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