THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 231 



material and the ri.se of the lighter material. This slow conversion of 

 pottMitial energy into sensible heat is thought to give to the slow- 

 accretion earth a very distinct superiority over the hot-liquid earth 

 when the combined sum of sensible and potential heat is considered. 

 The theoretical difference is capable of approximate computation, and 

 ]Mr. F. R. Moulton has kindly undertaken to make the computation in 

 a simplified hj^pothetical case which may give some impression of the 

 possible order of magnitude of this factor. For the purposes of the 

 computation the earth was assumed to have been composed of 40 per 

 cent of metal with a normal surface specific gravity of 7 and 60 per 

 cent of rock with a normal surface specific gravity of 3. These com- 

 bined would give an earth whose average specific gravity would be 

 only 4.6. The real specific gravity (5.6) is supposed to have been 

 obtained by compression which would amount to about 18 per cent. 

 Very likely the proportion of metal is put too high and the effect of 

 compression too low, but the purpose of the computation is only to 

 show the theoretical possibilities of the case. The metal is supposed 

 to have been originally scattered uniformly through the rock material 

 in meteoric fashion, and to have gathered thence to the center, forcing 

 the rock material outward so far as necessary. The heat produced 

 Mr. ]\Ioulton found to be sufficient to raise the temperature of the 

 whole earth (specific heat taken at 0.2) more than 3,000^^ C. The mag- 

 nitude of this result is sufficient to require the careful consideration of 

 the potential element unless the whole hypothesis can be set aside. It 

 is large enough to cast the gravest doubt on any conclusion based on 

 the rate of a supposed decline of internal temperature. Complete 

 readjustment of the interior matter, however, is not postulated under 

 the slow-accretion hypothesis. It is only assumed that a slow read- 

 justment has been in progress throughout the geological ages and still 

 is in progress, and that this has changed a certain amount of potential 

 energy into sensible heat and that this heat has contributed to the. 

 maintenance of the internal temperature of the earth. 



But there are in addition incidental factors which enter effectively 

 into the case. The gravitative readjustment of the heterogeneous 

 interior material is presumed to have taken place by the descent of the 

 metallic and other heavier materials toward the center and the recipro- 

 cal ascent of lighter materials from the central region toward the 

 surface, this being accomplished in various ways, the most declared 

 of which has its superficial manifestation in volcanic action. Now,* 

 this process of vertical transfer, besides developing heat in proportion 

 to the work done, as above indicated, also incidentally brings the hotter 

 material of the interior toward the surface and thus increases the sub- 

 surficial temperature. It is a species of slow convection. This con- 

 vection is in no radial sense different from that which is supposed to 

 have taken place in the liquid earth, save that it was delayed so that 



