708 



SCIENCE 



IN. S. Vol. XXVIII. No. 725 



radium may be supposed distributed to any 

 depth with which we are thermally con- 

 nected. Below that our knowledge is in- 

 definite. The heat outflow at the surface 

 is in part from the surface radium, in part 

 due to the cooling arising from the 

 diminishing amount of uranium, in part 

 from the deep-seated radium. In this 

 manner the isogeotherms are kept in their 

 places, and a state is maintained which is 

 in equilibrium with the thermal factors 

 involved, but which can not be considered 

 steady, using the word in a strictly ac- 

 curate sense, in view of the decay of the 

 uranium. 



While the existing thermal state may, 

 I think, thus be maintained by radio- 

 active heating and radio-active decay, we 

 find ourselves in considerable difficulties if 

 we extend this view into the past and as- 

 sume that the same could be said of any 

 previous stage of the earth's history. If 

 the heat emitted by the earth, when the 

 surface was at melting temperature, was 

 in a state of equilibrium with the radio- 

 active supplies, then, at that date, there 

 must have been many thousands of times 

 the present amount of uranium on the 

 earth, and the period of the consist entior 

 status must be put back by thousands of 

 millions of years. Apart from hopeless 

 contradiction with every geological indica- 

 tion as to the age of the earth, difficulties 

 in solar physics arise. For the sun must 

 be supposed of equal duration, and we are 

 required to assume impossible amounts of 

 uranium to maintain his heat all that great 

 lapse of time; and again this uranium 

 would perish at just the same rate as that 

 upon the earth, so that at the present time 

 the solar mass must be, for by far the 

 greater part, composed of inert materials 

 of high atomic weight: the products of the 

 transformations of the uranium family. 

 The difficulty is best appreciated when we 

 consider that even to maintain his present 



rate of heat-loss by radium supplies, some 

 60 per cent, of his mass must be composed 

 of uranium. But there are other troubles 

 to face if we adopt this view. The earth, 

 or rather those parts of it which are suffi- 

 ciently near the surface to lose heat at the 

 requisite rate, would have cooled but one 

 per cent, in 10' years. Shrinkage of the 

 outer parts and erustal thickness will be 

 proportionately small, and we must put 

 back our epochs of mountain building to 

 suit so slow a rate of cooling and skrink- 

 age and refer the earlier events of the kind 

 into a past of inconceivable remoteness. 

 Otherwise we must abandon the only ten- 

 able theory of mountain formation with 

 which we are acquainted. On such a time- 

 scale the ocean would be supersaturated 

 under the influence of the prolonged 

 denudation like the waters of certain salt 

 lakes, and the sediments would have ac- 

 cumulated a hundredfold in thickness. 



Nor do the facts as we know them re- 

 quire from us such sacrifices. "We are not 

 asked to raise these difficulties on supposi- 

 tious quantities of uranium for the exist- 

 ence of which there is no evidence. 

 Eadium has occasioned no questioning of 

 the older view that the cooling of the earth 

 from a consistentior status has been mainly 

 controlled by radiation. But, on the con- 

 trary, this new revelation of science has 

 come to smooth over what difficulties at- 

 tended the reconciliation of physical and 

 geological evidence on the Kelvin hypoth- 

 esis. It shows us how the advent of the 

 present thermal state might be delayed and 

 geological time lengthened, so that Kelvin's 

 forty or fifty million years might be recon- 

 ciled with the hundred million years which 

 some of us hold to be the reading of the 

 records of denudation. 



On this more pacific view of the mission 

 of radium to geology, what has been the 

 history of the earth? In the earlier days 

 of the earth's cooling the radiation loss was 



