20 SCIENCE PROGRESS 



the granites which constitute so large a part of the outermost 

 layers of the earth's crust, it is as yet impossible to say. If, how- 

 ever, the hypothesis is true that the atmosphere, the oceans, and 

 the carbon dioxide represented by carbonaceous deposits and 

 limestones were all originally "juvenile" magmatic gases, then 

 it seems justifiable to assume that their escape did not take place 

 without profoundly affecting the nature and distribution of the 

 magmas which they helped to carry upwards and through which 

 they passed. It may be that the evolution of the earth's crust 

 and its peculiar chemical composition is to be correlated with 

 that of the oceans and atmosphere, the latter representing part 

 of the " mineralising agents " which helped to enrich the outer- 

 most, lighter, and more mobile magmas in silica and alkalies, and 

 their associates, at the expense of the deeper-seated, heavier, and 

 more viscous magmas. 



Leaving these far-reaching speculations in petrogenesis, let 

 us return to the thermal significance of the radio-elements. In 

 order to calculate the total heating effect of the radio-elements 

 it is necessary to take into consideration the complete families 

 of uranium and thorium. Expressing the former in terms of 

 the equilibrium amount of radium, we have for the respective 

 heat outputs of the two families : 



Radium per gram . . . 226 calories per hour. 

 Thorium 270 x io~ 7 „ „ 



If we take 2*5 x io~ 12 grams as the average radium content in a 

 gram of the crystal rocks, and 2'o x io -5 grams as the corre- 

 sponding average for thorium, then each gram of the earth's 

 crust is a source of heat emitting 565 x io~ 12 calories per hour 

 on account of its radium content, and 540 x 10 -12 calories per 

 hour on account of its thorium content. The total heat emission 

 per gram of the known crustal rocks is therefore of the order 

 1,105 x IO_12 calories per hour. It is important to notice here 

 that radium and its congeners are responsible only for approxi- 

 mately half of the earth's radiothermal energy, for thorium, in 

 virtue of its greater abundance, is equally potent as a generator 

 of heat. 



The total amount of heat, Q, which escapes from the earth 

 by conduction to its surface and radiation into space, is given 

 by the formula : 



