68i Transactions of SECtioN c. 



Uranium and live Internal Heal of the Earth. 



Wliile forced to deny of the earth's interior any such richness in fadium tts 

 prevails near the surface, the inference that uranium exists yet in small 

 quantities far down in the materials of the globe is highly probable. This 

 view is supported by the presence of radium in meteoric substances and by its 

 very probable presence in the sun — that greatest of meteorites. True, the 

 radio-thermal theory cannot be supposed to account for any great part of solar heat 

 unless we are prepared to believe that a very large percentage of uranium can 

 be present in the sun, and yet yield 1)ut feeble spectroscopic evidence of its 

 existence. Taken all together, the ease 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 sub- 

 stances and its very probable presence in the sun, from which tlie 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 

 llu'oughout by radio-thermal actions ? This question — one of the most interesting 

 which has originated in the discoverj' 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. 



I will not stop to discuss the evidence for and against a highly lieated interior 

 of the earth. I assume this heated interior the obvious and natural interpretation 

 of a large classof geological phenomena, and pass on to consider certain limitations 

 to our knowledge which have to be recognised before we are in a position to enter 

 on the somewhat treacherous ground of hypotheses. 



In the first place, we appear debarred from assuming that the surface and 

 central interior of the eartli are in thermal connection, for it seems certain that, 

 since the remote period when (probable) convective effects became arrested by 

 reason of increasing viscosity, the thermal relations of the surface and interior 

 have become dependent solely on conductivity. From this it follows if the state 

 of matter in the interior is such as Lord Kelvin assumed — that is, that the 

 conductivity and specific heat may be inferred from the qualities of the surface 

 materials — we have remained in thermal isolation from the great bidk of the in- 

 terior for hundreds of millions of years, and perhaps even for more than a thou- 

 sand million of years. Assuming a diflusivity similar to that of surface rocks, and 

 starting with a temperature of 7000° F., Kelvin found that after 1000 million 

 years of cooling there would be no sensible change at a depth from the surface 

 greater than 568 miles. In short, even if this great period — far beyond our esti- 

 mates of geological time — has elapsed since the consistentiov status, the cooling 

 surface has as yet borrowed heat from only half the bulk of the earth. 



It is possible, on the other hand, that the conductivity increases inwards, as 

 Professor PeiTy has contended ; and if the central parts are more largely metallic, 

 this increase may be considerable. But we find ourselves here in the regions of 

 the unknown. 



With this limitation to our knowledge, the province of geothermal speculation 

 is a somewhat disheartening one. Thus if with Rutherford, who first gave us a 

 quantitative estimate of the kind, we say that such and such a quantity of radium 

 per gram of the earth's mass would serve to account for the 26 x iO-* calories 

 which, according to the surface gradients, the earth is losing per annum, we 

 cannot be taken as advancing a theory of radio-active heating, but only a signi- 

 ficant quantitative estimate. For, in fact, the heat emitted by radium in the 

 interior may never have reached the surface since the convective conditions came 

 to an end. 



And here, depending upon the physical limitations to our knowledge of the 

 earth's interior, a possibility has to be faced. That uranium is entirely absent 

 from the interior is, as I have said, in the highest degree unlikely. If it is 

 present, then the central parts of the earth are rising in temperature This view, 



