ox THE PHYSICAL VIKW OF XATURK. 



in expanding by freezing is made lu du wuik, it over- 

 comes pressure ; it has to freeze under pressure. The 

 temperature of water freezing under pressure must lie 

 lower than that of water freezing under ordinary con- 

 ditions.^ Knowing the mechanical duty of a degree of 

 temperature and the work of the expansion of ice, he 

 could calculate how much the freezing-point of water 

 must be lowered by pressure. In 1850 his brother 

 William Thomson verified this theoretical prediction by 

 actual experiment." It is well known how Helmholtz 

 in 1865 made use of this theoretically predicted and 

 practically verified phenomenon in liis celebrated glacier 

 theory.^ Both James and AVilliam Thomson, when 

 they drew the conclusions from Carnot's theory, still 

 adhered to the doctrine of the entire conservation of 

 heat.^ But William Thomson, who was equally ac- 



freezing ; and therefore it seemed 

 to follow that if a quantity of it 

 were merely enclosed in a vessel 

 with a movable piston and frozen, 

 the motion of the piston conse- 

 quent on the expansion being re- 

 sisted by pressure, mechanical work 

 would be given out without any 

 corresponding expenditure ; or, in 

 other words, a perpetual source of 

 mechanical work, commonly called 

 a perpetual motion, would be pos- 

 sible. . . . To avoid the absurdity 

 of .supposing that mechanical work 

 could be got out of nothing, it 

 occurred to me that it is necessary 

 further tocoiiclude that the freezing- 

 point becomes lower as the pressure 

 to which the water is subjected is 

 increased." 



^ " The mechanical pressure pro- 

 motes — as is generally the case with 

 the alternate action of different 

 forces in nature — euch a change, 

 viz., melting of ice, as is favourable 



to the effect of its own action " 

 (Helmholtz, ' Vorthige und Reden,' 

 vol. i. p. 217). 



- ' Proceedings of the Roy. Soc. 

 of Edinburgh,' January 1850, re- 

 printed in 'Math, and Phj's. Papers,' 

 vol. i. p. 165. 



■* Helmholtz, loc. cit., p. 215 377., 

 where also the jdienomenon dis- 

 covered and called " regelation of 

 ice," by Faraday, is similarly ex- 

 plained. 



■* It is important to notice this, 

 as the formula with which we are 

 now familiar, that the mechanical 

 work gained meant consumption of 

 heat, wiis not available at tiiat time. 

 This is significantly pointed out by 

 Helm ('Energetik,' p. 69). The 

 reasoning was accordingly more 

 difficult and refined. James Thom- 

 son, however, had at the time some 

 misgivings on the tlion jirevalcnt 

 view, and in a footnote he refers to 

 the " j)ossibility of the absolute for- 



