UKANIUM AND GEOLOGY — JOLY. 367 



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, that the central 

 interior 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 

 eifects 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 

 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 premises of this argument.'' 



We naturally ask. Whither does the conclusion lead us? We can 

 take comfort in a possible innocuous outcome. The uranium itself, 

 however slowly its energy is given up, is not everlasting. The decay 

 of the parent substance is continually reducing the amount of heat 

 which each year may be added to the earth's central materials. And 

 the result may be that the accumulated heat will ultimately pass out 

 at the surface by conductivity, during remote future times, and no 

 physical disturbance result. 



The second limitation to our hypotheses arises from this trans- 

 formation 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 escap- 

 ing at the surface attendant on the minute decrement would be enor- 

 mous. An analog}^ may help to make this clear. Consider the 

 familiar case of a boiler maintained at a particular temperature by a 

 furnace within. Let the combustion diminish and the furnace tem- 

 perature fall a little. The whole mass of the boiler and its contents 

 follow the downward movement of temperature, heat of capacity 

 escaping at the surface. An observer, only noting the outflow of 

 radiated heat and unable to observe the minute drop of temperature, 

 would probably ascribe to the continued action of the furnace, heat 

 which, although derived from it in the past, should no longer be 

 regarded as indicating the heating value of the combustion. Magnify 

 the boiler to terrestrial dimensions; the minutest fall in temperature 

 of the entire mass involves immense quantities of heat passing out at 



"■ Prof. H. A. Wilson has made a suggestive estimate of the thermal effects of 

 radium inclosed in the central parts of the earth. (Nature, Feb. 20, 1908.) 



