NA TURE 



289 



THURSDAY, JULY 26, 1894. 



MA THEM A TICAL GEO LOG V. 

 Popular Lectures and Addresses by Sir William 

 Thomson {Baron Kelvin), P.R.S., LL.D., D.C.L., fi^c. 

 In three volumes. Vol. II. "Geology and General 

 Physics." With illustrations. Nature -Series, pp. 

 X. + 599, with inde.x. (London and New York : Mac- 

 millan and Co., 1894.) 



THIS handy republication of the lighter scientific 

 utterances of Lord Kelvin was begun in 1889, with 

 the volume reviewed in N.A.TURE (vol. .\1. p. 433), was con- 

 tinued in 1 89 1, with a volume on " Navigational Affairs," 

 and is now concluded for the present by a volume, 

 nominally the second of the series, which deals mainly 

 with Geological Dynamics, or the application of the 

 physical sciences to the past history of our planet, and 

 likewise includes such later addresses on general physical 

 topics as were not included in the volumes already 

 issued. 



The preliminary remarks appropriate before reviewing 

 utterances of leaders in science, were made in connection 

 with the first of the series (vol. .xl. p. 433), and need not 

 now be repeated ; we will enter straight upon a sum- 

 mary, and perhaps an occasional slight criticism, of the 

 contents. 



The first paper is a little article on dew, wherein it is 

 pointed out that the protectiveact ion conspicuously exerted 

 on vegetation by invisible aqueous vapour is due not to its 

 "athermancy," as Tyndall imagined, and as text-books 

 teach, but to its infinite heat-capacity. The temperature 

 of bodies which cool only from the surface cannot fall 

 below the point at which dew is being deposited upon 

 them ; and naturally the moister the air the higher is this 

 said point. 



Then comes a brief note, a kind of text or starting- 

 point for many subsequent addresses, in which the 

 extreme doctrine of geological uniformity is briefly 

 refuted. The refutation consists in the simple arith- 

 metical calculation, that if the observed gradient of 

 temperature in the earth's crust had been uniform for, 

 say, twenty thousand million years back, the amount of 

 heat that must have flowed out from it into space in that 

 time would be enough to heat the whole earth ten thou- 

 sand centigrade degrees, unless it were made of material 

 very different from surface rock, or unless fresh quantities 

 of heat had been generated by chemical action. In any 

 case, allowing for these possibilities to the uttermost, the 

 past temperature would have been at some such date 

 so excessively high that ipso facto no approach to uni- 

 formity of other conditions could possibly be maintained 

 or contemplated. 



In this simple argument the "mathematics" is but 

 little more severe than that needed in what the author 

 later on (p. 240) calls "a simple effort of geological cal- 

 culus," whereby it is estimated that 1° per 30 metres is 

 the same as looc per 30,000 metres ; or (a fairer com- 

 parison) that quoted on p. 86, that a deposit at the rate of 

 one inch per century demands ninety-six million years for 

 the deposition of the stratified rocks ; yet the simple 

 argument may well be held as more conclusive and 

 NO. I 29 I, VOL. 50] 



convincing than an appeal to Fourier and the laws of 

 distribution of temperature in a cooling sphere. For this 

 reason : an immediate application of Fourier to the 

 gradient of temperature observed in the earth's outer 

 skin is liable to all the uncertainties attaching to very 

 violent extrapolation both in space and time ; it has to 

 assume that the sole operative cause is conduction of 

 heat, and always has been conduction of heat, up to a 

 certain past date deduced from the data as the era of 

 reckoning. 



Now, it is quite possible to hold that the main mass of 

 the earth consists of metal, chiefly iron, and that the 

 heat, observed in the damp skin or coat of rust on which 

 we live and into which we bore, is being generated de 

 novo by the rusting action still going on. 



The heat generated by the oxidation of a pound of 

 iron is (I estimate) sufficient to warm an equal mass of 

 rocky material something like three thousand centigrade 

 degrees ; so that small confidence can be felt, by those 

 who are impressed with the probability of a meteoric 

 view of the earth's origin, in refined calculations as to 

 successive distributions of temperature in a simply 

 cooling globe started in a molten condition and left to 

 radiate into space undisturbed. 



But it is to be observed that the argument of Lord 

 Kelvin contemplates the possibility of fresh generation 

 of heat, but maintains that nevertheless at some by no 

 means infinitely distant date the obvious physical con- 

 ditions of the earth's surface must have been extremely 

 different from what they are now. 



The doctrine of extreme uniformity, which at one time 

 was undoubtedly held by some leading geologists, is now 

 however abandoned, a result due most likely in large 

 measure to the author's calculations and reiterated argu- 

 ments ; and the only reasonable hesitation which can now 

 be felt is as to how far the numerical data available, 

 from observations hitherto made in the earth's outer 

 skin, are sufficient for fixing an upper limit to the age of 

 the earth : especially since these underground ther- 

 mometer-readings are likely to be disturbed by local 

 and by general chemical action at considerable depths. 



The author suggests borings in the African deserts, 

 where moisture is less prevalent than elsewhere, and it 

 may be that observation of underground temperature 

 there conducted will be productive of valuable infor- 

 mation ; but it is unlikely that these data have already 

 been obtained. 



Whatever hesitation may still rationally be felt as to 

 the acceptance of the author's numerical estimate of the 

 earth's age — and he is careful to allow ample margin 

 when he extends the more strictly estimated ten million 

 into a possible hundred million years — -yet the reception 

 of his calculations by contemporary pateontological and 

 stratigraphical geologists, as summarised in a controver- 

 sial address on Geological Dynamics in this volume, will 

 probably be surprising to a more fully informed posterity. 

 Instead of heartily welcoming fresh light on the subject 

 of the earth's past history, from an unexpected quarter, 

 they seem to resent interference from what they are 

 pleased to consider " outside," and their most able 

 advocate, Prof. Huxley, accepts a brief to repel the 

 intruder. 



The quotations made by Lord Kelvin from Playfair, 



O 



