THE AGE OF THE EAIiTH. 23 



sixty years' Jubilee of Queen Victoria's reign hj legalising 

 the French metrical system for the United Kingdom. 



§ 23. To prepare for considering consolidation at the surface 

 let us go back to a time (probably not more than twenty 

 years earlier as we shall presently see — § 24) when the solid 

 nucleus was covered with liquid lava to a depth of several 

 kilometres ; to fix our ideas let us say 40 kilometres (or 4 

 million centimetres). At this depth in lava, if of specific 

 gravity 2*5, the hydrostatic pressure is 10 tons weight (10 

 million grammes) per square centimetre, or ten thousand 

 atmospheres approximately. According to the laboratory 

 experiments of Clarence King and Carl Barns* on Diabase, 

 and the thermodynamic theory! of my brother, the late 

 Professor James Thomson, the melting temperature of 

 diabase is 1170° C. at ordinary atmospheric pressure, and 

 would be 1420° under the pressure of ten thousand atmos- 

 pheres, if the rise of temperature with pressure followed the 

 law of simple proportion up to so high a pressure. 



§ 24. The temperature of our 40 kilometres deep lava 

 ocean of melted diabase may therefore be taken as but 

 little less than 1420° from surface to bottom. Its surface 

 would radiate heat out into space at some such rate as 2 

 (gramme-water) thermal units Centigrade per square centi- 

 metre per second.^ Thus, in a year (31^ million seconds) 

 63 million thermal units would be lost per square centimetre 

 from the surface. This is, according to Carl Barns, very 

 nearly equal to the latent heat of fusion abandoned by a 

 million cubic centimetres of melted diabase in solidifying into 

 the glassy condition (pitch-stone) which is assumed when 

 the freezing takes place in the course of a few minutes. 

 But, as found by Sir James Hall in his Edinburgh experi- 

 ments§ of 100 years ago, when more than a few minutes is 

 taken for the freezing, the solid formed is n(.)t a glass but a 

 heterogeneous crystalline solid of rough fracture ; and if a 



* P/iil. Mag., 1893, first half-year, p. 306. 



t Trans. Roy. Soc, Edinburgh, Jan. 2, 1849 ; Cambridge and Dublin 

 Mathematical Journal, Nov. 1850. Reprinted in Math, and Phys. Papers 

 (Kelvin), vol. i, p. 156. 



\ This is a very rough estimate which I have formed from consideration 

 of J. T. Bottomley's accurate determinations in absolute measure of 

 thermal radiation at temperatures up to 920° C. from platinum wire and 

 from polished and blackened surfaces of various kinds in receivers of air- 

 pumps exhausted down to one ten-millionth of the atmospheric pressure. 

 Phil. Trans. Roy. Soc, 1887 and 1893. 



!^ Trans. Roy. Soc, Edinburgh 



