20 THE RIGHT HON. LORD KELVIN, G.C.Y.O., ON 



Metazoa from the Protozoa, and the branching of the former into its 

 numerous PhyLa, witli all their Classes, Ordeis, Families, Genera, and 

 Species. But we shall find that this is quite enough to necessitate a very 

 large increase in the time estimated hy the geologist." 



§ 15. In my own short paper from Avbicli I have read you 

 a sentence, the rate at which heat is at the present time lost 

 from the earth by conduction outwards through the upper 

 crust, as proved by observations of underground temperature 

 in different parts of the world, and by measurement of the 

 thermal condnctivity of surface rocks and strata, sufficed to 

 utterly refute the Doctrine of Uniformity as taught by Hutton, 

 Lyell, and their followers ; Avhich was the sole object of that 

 paper. 



§ 16. In an earlier communication to the Royal Society 

 of Edinburgh,* I had considered the cooling of the earth 

 due to this loss of heat; and by tracing backwards the 

 process of cooling had formed a definite estimate of the 

 greatest and least number of million years which can 

 possibly have passed since the surface of the earth was 

 everywhere red hot. I expressed my conclusion in the 

 following statemontt: — 



"We are very ignorant as to the effects of high temperatiu^es in altering 

 the conductivities and specific heats and melting temperatures of rocks, 

 and as to their latent heat of fusion. We must, therefore allow very 

 wide limits in such an estimate as I have attempted to make ; biit I 

 think we may with much piobability say that the consolidation cannot 

 have taken place less than 20 million years ago, or we should now have 

 more underground heat than we actually have ; nor more than 400 million 

 years ago, or we should now have less underground heat than we actually 

 have. That is to say, I conclude that Leibnitz's epoch of emergence of 

 the con.v'stentior status [the consolidation of the earth from red hot or 

 white hot molten matter] was probably between those dates." 



§ 17. During the 35 years which have passed since I 

 gave this Avide-ranged estimate, experimental investigation 

 has supplied much of the knowledge then Avanting regard- 

 ing the therjnal properties of rocks to form a closer estimate 

 of the time which has passed since the consolidation of the 

 earth, and we have now good reason forjudging that it was 

 more than 20 and less than 40 million years ago ; and pro- 

 bably much nearer 20 than 40. 



* On the Secular Cooling of thi Earth, Trans. Roy. Soc, Edinburgh, 

 vol. xxiii, April 28th, 1662, reprinted in Thomson and Tait, vol. iii, 

 pp. 468-485, and Math, and Fhys. Papers, Art. XCIV, ])\). 295-311. 



+ On the iSecvlar Cooling of the Earth, Math, and I'hijs. Papers, vol. iii, 

 i? 11 of Art. XCIV. 



