AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 345 



I liad considered the cooliug- of the earth due to this loss of heat, aud 

 by traoing backward tlie 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 statement: ^ 



"We are very ignorant as to the effects of high temperatures in 

 altering the conductivities aud specific heats and melting temperatures 

 ot 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 say that the cousolidation 

 can not 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 jjrobably between those 

 dates." 



17. During the thirty-five 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 con- 

 solidation of the earth, and we have now good reason forjudging 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 connection 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 i)ossible for me to do in 1802 with the very meager 

 information then available as to the specific heats, thermal conductivi- 

 ties, and temperatures of fusion of rocks. I had taken 7,000° F. 

 (3,871° C.) as a high estimate of the temperature of melting rock. 

 Even then I might have taken something between 1,000° C. and 2,000° 

 C. as more probable, but I was most anxious not to underestimate the 

 age of the earth, and so I founded my primary calculation on the 7,000° 

 F. for the temperature of melting rock. We know now from the experi- 

 ments of Carl Barus ^ that diabase — a typical basalt of very primitive 

 character — melts between 1,100° and 1,170° C. and is thoroughly liquid 

 at 1,200°. The correction from 3,871° to 1,200° C, or 1/3.22 of that 

 value, for the temperature of solidification would, with no other change 

 of assumptions, reduce my estimate of 100,000,000 to 1/(3.22)2 of its 



1 Oa the Secular Cooling of the Earth, Math, and Phys. Papers, Vol. Ill, § 11 of 

 Art.XCIV. 

 -On the Age of the Earth, Vol. XLV, January, 1893. 

 3 Phil. Mag., 1893, first half-year, pages 186, 187, 301-305. 



