278 SEE— THE CAUSE OF EARTHQUAKES. . [October 19 



temperature of an equal mass of water 9954° C. ; and as decidedly 

 more than half of it is still retained in the globe, we may conclude 

 that the internal heat of the earth is ample to raise the whole globe 

 to a temperature of something like 20,000° or 25,000° C, accord- 

 ing to the average specific heat of the earth's matter. If radium and 

 other related elements exist within the earth in appreciable quan- 

 tities, the amount of heat stored up, as Sir G. H. Darwin and 

 others^ have remarked, may be vastly greater yet. Now it is recog- 

 nized that the crust or cooled layer on the outside of our planet is 

 extremely thin, and we know that the temperature increases down- 

 ward at an average rate of something like 1° C. for each 30 metres 

 of descent. This accords also with Lord Kelvin's calculations on 

 the cooling of a molten globe, carried out in conformity with 

 Fourier's Analytical Theory of the propagation of heat in solid 

 bodies. - 



From this we may infer, as geologists have long since remarked, 

 that, even without the penetration of steam, molten rock would be 

 encountered at a depth of decidedly less than 30 kilometres. As the 

 percolation of hot water and steam appreciably lowers the melting 

 point of silicious and perhaps other rocks (the lavas are mainly 

 silicates), and itself develops at the very low temperature of only 

 100° C. under atmospheric pressure, we may infer that it would 

 form in the earth at a depth much smaller than 30 kms. At no more 

 than 10 or 15 kms. under the ocean beds large quantities of it might 

 be produced and give rise to imprisoned forces of tremendous power. 

 Besides it would rapidly absorb and spread in the hotter layers of 

 rock beneath, just as in the case of gases absorbed in hot steel, cited 

 by Tait and quoted in § 5. That this absorption actually takes place 

 is proved by the vast clouds of steam given off by melted lava after 

 it pours from a volcano, such as Vesuvius. 



We are thus confronted with the following situation : 



The internal temperature of the earth is extremely high, with 



^ Presidential address to the British Association for the Advancement of 

 Science, Capetown, 1905 ; also a very recent paper presented to the Royal 

 Society, April 5, 1906, by the Hon. R. J. Strutt, F.R.S., reported in Nature 

 of May 17, 1906. 



^ " The Secular Cooling of the Earth," Appendix D, Thomson and Tait's 

 "Nat. Phil." 



