September 24, 1896] 



NA TURE 



50: 



in making this suggestion he apparently feels neither interest nor 

 responsibility in establishing the data of the calculations which 

 he IxMTdwecl to obtain therefrom a very different result from that 

 olHaincd by their author. 



Prof. Perry's object was not to substitute a more correct age 

 for that obtained by Lord Kelvin, but rather to show that the 

 data from which the true age could be calculated are not really 

 available. We obtain different results by making different 

 assumptions, and there is no sufficient evidence for accepting 

 one assumption rather than another. Nevertheless, there is 

 some evidence which indicates tliat the interior of the earth in 

 all probilMlity conducts better than the surface. Its far higher 

 density is consistent with the belief that it is rich in metals, free 

 or combined. Prof Schuster concludes that the internal electric 

 conductivity must be consideral>ly greater than the external. 

 Geologists have argued from the amount of folding to which the 

 crust has been suljjected that cooling must have taken place to 

 a greater depth than 120 miles, as assumed in Lord Kelvin's 

 argument. Prof Perry's assumption would in\olve cooling to a 

 much greater depth. 



Prof. Perry's conclusion that the age of the habitable earth is 

 lengthened by increased conductivity is the very reverse of that 

 to which we should be led by a superficial examination of the 

 case. Prof. Tait, indeed, in the letter to which I have already 

 alluded, has .said: "Why, then, drag in mathematics at all, 

 since it is absolutely obvious that the better conductor the interior 

 in comparison with the skin, the longer ago must it have been 

 when the whole was at 7000° F., the state of the skin being as at 

 present?" Prof. Perry, in reply, pointed out that one mathe- 

 matician who had refuted the tidal retardation argument (Rev. 

 M. H. Close, in R. Dublin Soc, February 1878), had assumed 

 that the conditions described by Prof Tait would have involved 

 a .shorter period of time. And it is prob.ible that Lord Kelvin 

 thought the same ; for he had assumed conditions which would 

 give the result — so he believed at the time — most acceptable to 

 the geologist and biologist. Prof. Perry's conclusion is very 

 far from obvious, and without the mathematical reasoning would 

 not be arrived at by the vast majority of thinking men. 



The '• natural man " without mathematics would say, so far 

 from this being " absolutely obvious," it is quite clear that 

 increased conductivity, favouring escape of heat, would lead to 

 more rapid cooling, and would make Lord Kelvin's age even 

 shorter. 



The argument can, however, be put clearly without mathe- 

 matics, and, with Prof Perry's help, I am able to state it in a few 

 words. Lord Kelvin's assumption of an earth resembling the 

 surface rock in its relations to heat leads to the present condition 

 of things, namel}^ a surface gradient of i' F. for every 50 feet, 

 in 100,000,000 years, more or less. Deeper than 150 miles he 

 imagines that there has been almost no cooling. If, however, 

 we take one of the cases put by Prof. Perry, and assume that 

 below a depth of four miles there is ten times the conductivity, 

 we find tliat after a period of 10,000,000,000 years the gradient 

 at the surface is still i^ F. for every 50 feet ; but that we have 

 to descend to a depth of 1500 miles before we find the initial 

 temperature of 7000' F. undiminished by cooling. In fact the 

 earth, as a whole, has cooled far more quickly than under Lord 

 Kelvin's conditions, the greater conductivity enabling a far 

 larger amount of the internal heat to escape ; but in escaping it 

 has kept up the temperature gradient at the surface. 



Lord Kelvin, replying to Prof Perry's criticisms, quite admits 

 that the age at which he had arrived by the use of this argu- 

 ment may be insufficient. Thus, he says, in his letter (N.^ture, 

 January 3, 1895): "I thought my range from 20 millions to 

 400 millions was probably wide enough, but it is quite possible 

 that I should have put the superior limit a good deal higher, 

 perhaps 4000 instead of 400." 



The third argument was suggested by Ilelmholtz, and depends 

 on the life of the sun. If the energy of the sun is due only to 

 the mutual gravitation of its parts, and if the sun is now of 

 uniform density, "the amount of heat generated by his con- 

 traction to his present volume would have been sufficient to last 

 18 million years at his present rate of radiation." (Newcomb's 

 "Popular Astronomy," p. 523). Lord Kelvin rejects the 

 assumption of uniform density, and is, in consequence of this 

 change, able to offer a much higher upward limit of 500 million 

 years 



This argument also implies the strictest uniformitarianism as 

 regards the sun. We know that other suns may suddenly gain 



NO. 1404, VOL. 54] 



a great accession of energy, so that their radiation is innnensely 

 increased. We only detect such changes when they are large 

 and sudden, but they prepare us to believe that smaller acces- 

 sions may be much more frequent, and perhaps a normal 

 occurrence in the evolution of a sun. Such accessions may 

 have followed from the convergence of a stream of meteors. 

 Again, it is possible that the radiation of the sun may have 

 been diminished and his energy conserved by a solar atmo- 

 sphere. 



Newcomb has objected to these two possible modes by which 

 the life of the sun may have been greatly lengthened, that a 

 lessening of the sun's heat by under a quarter would cause all 

 the water on the earth to freeze, while an increase of much over 

 half would probably boil it all away. But such changes in the 

 amount of radiation received would follow from a greater 

 distance from the sun of 15^ per cent., and a greater proximity 

 to him of l8'4 per cent., respectively. Venus is inside the 

 latter limit, and Mars outside the fi)rmer, and yet it would be 

 a very large assumption to conclude that all the water in the 

 former is steam, and all in the latter ice. Indeed, the existence 

 of water and the melting of snow on Mars are considered to be 

 thoroughly well authenticated. It is further possible that in a 

 time of lessened solar radiation the earth may have possessed 

 an atmosphere which would retain a larger proportion of the 

 sun's heat ; and the internal heat of the earth itself, great lakes 

 of lava under a canopy of cloud for example, may have played 

 an important part in supplying warmth. 



Again we have a greater age if there was more energy avail- 

 able than in Helmholtz's hypothesis. Lord Kelvin maintains 

 that this is improbable because of the slow rotation of the sun, 

 but Perry has given reasons for an opposite conclusion. 



The collapse of the first argument of tidal retardation, and ot 

 the second of the cooling of the earth, warn us to beware of a 

 conclusion founded on the assumption that the sun's energ)- 

 depends, and has ever depended, on a single source of which 

 we know the beginning and the end. It may be safely main- 

 tained that such a conclusion has not that degree of certainty 

 which justifies the followers of one science in assuming that the 

 cc)nclusion of other sciences must be wrong, and in disregarding 

 the evidence brought forward by workers in other lines of 

 research. 



We must freely admit that this third argument has not yet 

 fully shared the fate of the two other lines of reasoning. Indeed, 

 Prof George Darwin, although attacking these latter, agrees 

 with Lord Kelvin in regarding 500 million years as the maximum 

 life of the sun. [British Association Reports, 1886, pp. 514- 

 518.) 



We may observe, too, that 500 million years is by no means 

 to be despised : a great deal may happen in such a period of 

 time. Although I should be very sorry to say that it is suffi- 

 cient, it IS a very different offer from Prof Tail's 10 million. 



In drawing up this account of the physical arguments, I owe 

 almost everything to Prof. Perry for his articles in Nature 

 (January 3 and April 18, 1895), and his kindness in explaining 

 any difficulties that arose. I have thought it right to enter into 

 these arguments in some detail, and to consume a considerable 

 proportion of our time in their discussion. This was impera- 

 tively necessary, because they claimed to stand as barriers 

 across our path, and, so long as they were admitted to be 

 impassable, any further progress was out of the question. What 

 I hope has been an unbiassed examination has shown that, as 

 barriers, they are more imposing than effective ; and we are 

 free to proceed, and to look for the conclusions warranted by 

 our own evidence. In this matter we are at one with the 

 geologists ; for, as has already been pointed out, we rely on 

 them for an estimate of the time occupied by the deposition of 

 the stratified rocks, while they rely on us for a conclusion as to 

 how far this period is sufficient for the whole of organic 

 evolution. 



First, then, we must briefly consider the geological argument, 

 and I cannot do better than take the case as put by Sir Archi- 

 bald Geikie in his Presidential Address to this Association at 

 Edinburgh in 1892. 



Arguing from the amount of material removed from the land 

 by denuding agencies, and carried down to the sea by rivers, he 

 showed that the time required to reduce the height of the land 

 by one foot, varies, according to the activity of the agencies at 

 work, from 730 years to 6800 years. But this also supplies a 



