78 Lord Kelvin on the Age of the 



63 million thermal units would be lost per square centimetre 

 from the surface. This is, according to Carl Barus, 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 not a glass but a 

 heterogeneous crystalline solid of rough fracture ; and if a 

 few hours or days, or any longer time, is taken, the solid 

 formed has the well known rough crystalline structure of 

 basaltic rocks found in all parts of the world. Now Carl 

 Barus finds that basaltic diabase is 14 per cent, denser than 

 melted diabase, and 10 per cent, denser than the glass pro- 

 duced by quick freezing of the liquid. He gives no data, 

 nor do Riicker and Roberts-Austen, who have also experi- 

 mented on the thermodynamic properties of melted basalt, 

 give any data, as to the latent heat evolved in the consolida- 

 tion of liquid lava into rock of basaltic quality. Guessing 

 it as three times the latent heat of fusion of the diabase 

 pitch- stone, I estimate a million cubic centimetres of liquid 

 frozen per square centimetre per centimetre per three years. 

 This would diminish the depth of the liquid at the rate of a 

 million centimetres per three years, or 40 kilometres in twelve 

 years. 



§ 25. Let us now consider in what manner this diminution 

 of depth of the lava ocean must have proceeded, by the 

 freezing of portions of it ; all having been at temperatures 

 very little below the assumed 1420° melting temperature of 

 the bottom, when the depth was 40 kilometreSo The loss of 

 heat from the white-hot surface (temperatures from 1420° to 

 perhaps 1380° in different parts) at our assumed rate of two 

 (gramme-water Centigrade) thermal units per sq. cm. per 

 sec. produces very rapid cooling of the liquid within a few 

 centimetres of the surface (thermal capacity *36 per gramme, 

 according to Barus) and in consequence great downward 

 rushes of this cooled liquid, and upwards of hot liquid, 

 spreading out horizontally in all directions when it reaches 

 the surface. When the sinking liquid gets within perhaps 

 20 or 10 or 5 kilometres of the bottom, its temperature f 



* Trans. Roy. Soc. Edinburgh. 



t Tlie temperature of the sinking liquid rock rises in virtue of the 

 increasing pressure : but much less than does the freezing point of the 

 liquid or of some of its ingredients. (See Kelvin, Math. andPhys. Papers, 

 vol. iii. pp. 69, 70.) 



