Earth as an Abode fitted for Life. 75 



" We are very ignorant as to the effects of high, temperatures 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 ; but I think 

 we may with much probability sa} r 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 

 consistentior 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 wide-ranged estimate, experimental investigation has 

 supplied much of the knowledge then wanting regarding the 

 thermal 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 for judging that it was more 

 than 20 and less than 40 million years ago ; and probably 

 much nearer 20 than 40, 



§ 18. Twelve years ago, in a laboratory established by 

 Mr. Clarence King in connexion with the United States 

 Geological Survey, a very important series of experimental 

 researches on the physical properties of rocks at high 

 temperatures was commenced by Dr. Carl Barus, for the 

 purpose of supplying trustworthy data for geological theory. 

 Mr, Clarence King, in an article published in the 'American 

 Journal of Science ; *, used data thus supplied, to estimate 

 the age of the earth more definitely than was possible for me 

 to do in 1862, with the very meagre information then available 

 as to the specific heats, thermal conductivities, and tempera- 

 tures of fusion, of rocks. I had taken 7000° F. (3781° C.) 

 as a high estimate of the temperature of melting rock. Even 

 then I might have taken something between 1000° C. and 

 2000° C. as more probable, but I was most anxious not to 

 under-estimate the age of the earth, and so I founded my 

 primary calculation on the 7000° F. for the temperature of 

 melting rock. We know now from the experiments of Carl 

 Barus f that diabase, a typical basalt of very primitive 

 character, melts between 1100° C. and 1170°, and is tho- 

 roughly liquid at 1200°. The correction from 3871° C. to 

 1200° or 1/3*22 of that value, for the temperature of solidifi- 

 cation, would, with no other change of assumptions, reduce 

 my estimate of 100 million to 1/(3*22) 2 of its amount, or a 

 little less than 10 million years ; but the effect of pressure 

 on the temperature of solidification must also be taken into 



* * On the Age of the Earth,' vol. xlv. January 1893. 

 t Phil. Mag. 1893, first half-year, pp. 186, 187, 301-305. 



