366 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 1908. 



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 an- 

 swered (if it can be answered) by the facts of geological science. 



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

 heated interior of the earth. I assume this heated interior the obvious 

 and natural interpretation of a large class of geological phenomena, 

 and pass on to consider certain limitations to our knowledge which 

 have to be recognized before we are in a position to enter on the some- 

 what treacherous ground of hjqDotheses. 



In the first place, we appear debarred from assuming that the sur- 

 face and central interior of the earth are in thermal connection, for it 

 seems certain that, since the remote period when (probable) con- 

 vective 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 con- 

 ductivity and specific heat may be inferred from the qualities of the 

 surface materials — we must remain in thermal isolation from the great 

 bulk of the interior for hundreds of millions of years, and perhaps 

 even for more than a thousand million of years. Assuming a dif- 

 fusivity similar to that of surface rocks, and starting with a tem- 

 perature of 7000° F., Kelvin found that after one thousand million 

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

 surface greater than 5G8 miles. In short, even if this great period — 

 far beyond our estimates of geological time — has elapsed since the 

 consistentior 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 

 inward, as Professor Perry has contended; and if the central parts 

 are more largely metallic this increase maj 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 Ruther- 

 ford, 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 2.0 X 10'° calories, which, according 

 to the surface gradients, the earth is losing per annum, we can not be 

 taken as advancing a theory of radio-active heating, but only a sig- 

 nificant 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 knowl- 

 edge of the earth's interior, a possibility has to be faced. That 



