706 



8CIENCB 



[N. S. Vol. XXVIIl. No. 725 



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 Pro- 

 fessor Perry 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 un- 

 known. 



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 quan- 

 titative 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 2.6 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 significant 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 tem- 

 perature. This view, that the central in- 

 terior is rising in temperature, is difficult 

 to dispose of, 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 radio-active heating or not, we 

 find, on any probable value of the con- 

 ductivity, a central core almost protected 

 from loss by the immense mass of heated 

 material interposed between it and the sur- 



face, and within this core very probably a 

 continuous source of heat. It is hard to 

 set aside any of the premises of this argu- 

 ment.^^ 



We naturally ask. Whither does the con- 

 clusion lead us? We can take comfort in 

 a possible innocuous outcome. The uran- 

 ium itself, however slowly its energy is 

 given up, is not everlasting. The decay 

 of the parent substance is continually re- 

 ducing the amount of heat which each year 

 may be added to the earth's central ma- 

 terials. And the result may be that the 

 accumulated heat will ultimately pass out 

 at the surface by conductivity, during re- 

 mote future times, and no physical dis* 

 turbance result. 



The second limitation to our hypotheses 

 arises from this transformation and 

 gradual disappearance of the uranium. 

 And this limitation seems as destructive of 

 definite geothermal theories as the first. 

 To understand its significance requires a 

 little consideration. The fraction of 

 uranium decaying each year is vanishingly 

 small, about the ten thousand-millionth 

 part ; but if the temperature of the earth is 

 maintained by uranium and consequently 

 its decay involves the fall in temperature 

 of the whole earth, the quantity of heat 

 escaping at the surface attendant on the 

 minute decrement would be enormous. An 

 analogy may help to make this clear. Con- 

 sider the case of a boiler maintained at a 

 particular temperature by a furnace with- 

 in. Let the combustion diminish and the 

 furnace temperature fall a little. The 

 whole mass of the boiler and its contents 

 follow the downward movement of tem- 

 perature, heat of capacity escaping at the 

 surface. An observer, only noting the out- 

 flow of radiated heat and unable to observe 



•^Professor H. A. Wilson has made a suggestive 

 estimate of the thermal effects of radium enclosed 

 in the central parts of the earth {Nature, Feb- 

 ruary 20, 1908). 



