686 TRANSACTIONS OF SECTION C. 



aud the future cooling of tlie earth controlled mainly hy decay of the uranium ? 

 I do not think there are any good grounds for rejecting this view. Observe, it is 

 the condition towards which every planetary body and every solar body containing 

 stores of uranium must tend ; and apparently roust attain when the rate of loss 

 of initial stores of heat, dimhiishing as the body grows colder, finally arrives 

 at equilibrium with the radio-thermal supplies. This final state appears inevitable 

 in every case unless the radio-active materials are so subordinate that they entirely 

 perish before the original store of heat is exhausted. 



Now, judging from the surface richness in radium of the earth and the 

 present loss of terrestrial heat, it does not seem reasonable to assign a sub- 

 ordinate influence to radio-thermal actions ; and it appears not improbable that 

 the earth has attained, or nearly attained, this final stage of cooling. 



How, then, may we suppose the existing thermal state maintained ? A 

 uniformly radio-active surface layer possessing a basal temperature in accordance 

 with the requirements of geolog}' is, I believe, not realisable on any probable 

 estimate of the allowable radium, or on any concentration of it which my own 

 experiments on igneous rocks would justify. 



But we may take refuge in a less definite statement, and assume a distribution 

 by means of which the existing thermal state of the crust may be maintained. A 

 specially rich surface layer we must recognise, but this need be no more than a 

 very faw miles deep; after which the balance of the radium maybe supposed 

 distributed to any depth with which we are thermally connected. Below that 

 our knowledge is indeOnite. 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, aud ii state i< maintained which is in equilibrium with 

 the thermal factors involved, but which cafinot be considered steady, using the 

 word in a strictly accurate sense, in view of the decay of the uranium. 



While the existing thermal state may, T thinlf, thus be maintained by radio- 

 active heating and radio-active decay, wc find ourselves in considerable difliculties 

 if we extend this view into the past and assume 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 wiili 

 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 ])eriod of the 

 ronsistmiior statm must be put back by thousands of millions of years. Apart 

 from hopeless contradiction with every geological indication 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 main- 

 tain his heat all that great lapse of time ; and again this uranium would perish at 

 just the same race 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 

 (^omposed of uranium. But there are other troubles to face if we adopt this 

 view. The earth, or rather those parts of it which are sufficiently 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 crustal thickness will be propor- 

 tionately small, and we must put back our epochs of mountain building to suit so 

 slow a rate of cooling and shrinkage and refer the earlier events of the kind into 

 a past of inconceivable remoteness. Otherwise we must abandon the only tenable 

 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 

 accumulated a hundredfold in thickness. 



Nor do the facts as we know them require from us such sacrifices. We are not 

 asked to raise these difficulties on supposititious quantities of uranium for the 

 e.xistence of which there is no evidence. Radium baa occasioned no questioning 



