r88 SCIENCE PROGRESS 



the heat-producing capacity of that substance. If it was 

 originally scattered through the earth-stuff, not in a uniform 

 distribution, but to some extent concentrated fortuitously in a 

 manner depending on the origin of terrestrial ingredients, then 

 these radio-active nuclei, heating and expanding beyond the 

 capacity of surrounding materials, would rise to the surface of 

 a world in which convective actions were still possible, and, 

 very conceivably, even after such conditions had ceased to be 

 general ; and in this way the surface materials would become 

 richer than the interior. 



Taken all together, the case stands thus as regards the earth. 

 We are assured of radium as a widely distributed surface 

 material, and to such depths as we can penetrate. By inference 

 from the presence of radium in meteoric substances, and its very 

 probable presence in the sun, from which the whole of terrestrial 

 stuff probably originated, as well as by the inherent likelihood 

 that every element at the surface is in some measure distributed 

 throughout the entire mass, we arrive at the conclusion that 

 radium is indeed a universal terrestrial constituent. 



The dependent question then confronts us — Are we living 

 on a world heated throughout by radio-thermal actions ? This 

 question — one of the most interesting which has originated in 

 the discovery that internal atomic changes may prove a source of 

 heat — can only be answered (if it can be answered) by the facts 

 of geological science. 



It is very unlikely that uranium is entirely absent from the 

 centre. If it is present it follows that the central parts of the 

 earth, which are, so to speak, blanketed off from the surface, are 

 growing hotter. From this conclusion Prof. Joly saw no escape, 



although we can adduce the evidence of certain surface- 

 phenomena to show that the rise in temperature during 

 geological time must be small or its effects in some manner 

 kept under control. In a word, whether we assume that the 

 whole heat-loss of the earth is now being made good by the 

 radio-active heating or not, we find, on any probable value of 

 the conductivity, a central core almost protected from loss by 

 the immense mass of heated material interposed between it and 

 the surface, and within this core very probably a continuous 

 source of heat. It is hard to set aside any of the premisses of 

 this argument. 



That is one point. Another is the effect of this heat, which 

 will be greater in certain areas and strains of detrited matter, 

 on the surface of the earth. It is granted that the deposition of 

 sediments somehow or other involves their ultimate upheaval. 

 Of this phenomenon we have had no satisfactory explanation. 



