THE AGE OP THE EAETH. 21 



§ 18. Twelve yearw ago, iu 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 higli 

 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 7.000° F. (3,871° C.) as 

 a high estimate of the temperature of melting roclc. 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 

 under-estimate 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 experiments of Cavl 

 Barust 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, witli no other change of assumptions, reduce 

 my estimate of 100 million to l/(3"22)^' of its amount, or a 

 little less than 10 nnllion years ; but the effect of pressure on 

 the temperature of solidification must also be taken into acr 

 count, and Mr. Clarence King, after a careful scrutiny of all 

 Ihe data given him for this purpose b}^ Dr. Barus^ concludes 

 that without further expei'imental data " we have no warrant 

 for extending the earth's age beyond 24 millions of years." 



§ 19. By an elaborate piece of mathematical book-keeping 

 I have worked out the problem of the, conduction of heat 

 outwards fi'om the earth, with specific heat increasing up to the 

 melting point as found by Riicker and Roberts-Austen and by 

 Barus, but with the conductivity assumed constant; and, by 

 taking into account the augmentation of melting tempera- 

 ture with pressure in a somewhat more complete manner 

 than that adopted by Mr. Clarence King, I am not led to 

 difi'er much from his estimate of 24 million years. But, 

 until we know something more than we know at present as 

 to the probable diminution of thermal conductivity with 



* On the Age of the Eourth, vol. xlv, January, 1893. 



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



