250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



mained substantially the same as at present, and he attacked this 

 problem from two different points of view. In the first place he 

 attempted to set a limit of time to the duration of the sun's heat; 

 and secondly, from consideration of the earth's internal heat, he 

 aro-ued back to the time when the surface was too hot for the pres- 

 ence of living beings. I have heard a suggestion that there is some 

 mutual- inconsistency in these two lines of argument— consideration 

 of the sun's heat makes the past temperature too low ; consideration 

 of the earth's heat makes it too high— but I do not think that this 

 criticism is more than superficially plausible. The point was 

 rather that from either of these arguments a condition widely differ- 

 ent from the present would be reached, and therefore that, even if 

 there were some unrecognized flaw in one of the arguments, the 

 other would stand. Possibly, looking back into the remote past, a 

 condition of the earth's surface is imaginable where the mean tem- 

 perature was much the same as at present, heat coming from the 

 earth's interior in compensation for a diminished radiation from the 

 sun, but I feel sure you Avill all agree with me that we can not get 

 more time by special pleading of this kind. The fossiliferous rocks 

 have, without doubt, been accumulated under conditions of solar 

 radiation not essentially different from the present. One simple 

 consideration is that the plants in the coal measures obviously had 

 green leaves, and that these could not function without a full allow- 

 ance of solar radiation. 



We have then to consider whether Lord Kelvin's arguments can 

 stand in the light of present knowledge. I think that we must admit 

 that they can not. 



First as regards the earth's heat, it is now generally known that the 

 premises of Lord Kelvin's calculation, carefully particularized by 

 him, are upset by the discovery of radio-active substances in the 

 earth. In 1906 I made a determination of the amount of radium 

 in the superficial parts of the earth which are alone accessible. 

 From radium analysis we can calculate the amount of uranium 

 and other associated substances and the thermal output from them, 

 and the result is to show that if we suppose the same radium con- 

 tent to extend to a depth of some 20 miles, the whole output of 

 heat would be accounted for without assuming that any of it comes 

 from the store of primeval heat as postulated by Lord Kelvin. It is 

 without doubt difficult to understand why the output of heat is 

 not greater, for it would certainly be expected that the rocky crust of 

 earth would be more than 20 miles thick, to say nothing of any 

 radium there might be in the unknown interior. 



Can we at present infer anything definite from the earth's internal 

 heat as to the possible duration of geological time? I think practi- 



