JO SI AH WILLARD GIBBS 475 



the first statement of the second law of thermodynamics, that as water 

 flows towards the sea-level, but never backwards to its source, so heat 

 can not flow from a colder to a warmer body. But Carnot, like every 

 one else in his day, still thought that heat (calorique) , like the water in 

 the waterfall, was an indestructible, material substance and that the 

 quantity of heat given out by the exhaust chamber of the engine is 

 exactly the same as that taken in at the boiler. Although his post- 

 humous papers indicate that he corrected this view before his death, he 

 assumed that if we could find some way to consume the heat of a given 

 body without the necessity of conveying it to a colder body, we might 

 create motor power without fuel or obtain work from nothing, which 

 would be perpetual motion. As late as 1865 an authority like 

 Eankine^^ still believed that heat is of material essence, and when in 

 1842-7 the labors of Eobert Mayer and of Joule established the 

 mechanical equivalent of heat and Helmholtz^*^ in 1847 showed that 

 the first law of thermodynamics, the principle of conservation of energy, 

 is applicable to all physical phenomena, it was found difficult to recon- 

 cile this principle with Carnot's tacit assumption that heat is un- 

 changeable and indestructible. Even a physicist like William 

 Thomson-^ (the late Lord Kelvin) confessed himself baffled by the 

 problem in 1849 and turned aside to establish his " absolute scale of 

 temperature," without which further progress in the science would have 

 been impossible; but his brother James Thomson, one of the earlier 

 pioneers of physical chemistry, was able, by an implicit denial of 

 Carnot's assumption, to predict and prove that the freezing point of 

 water would be lowered by pressure (1849).^^ The difficulty was, at 

 length, settled in 1850 by Clausius, whose memoir " On the motor power 

 of heat," " marks," says Gibbs, " an epoch in the history of physics," 

 for before its publication, '^' truth and error were in a confusing state 

 of mixture,"^^ and "wrong answers were confidently urged by the 

 highest authorities." 



To Clausius we owe the doctrine, foreshadowed by Bacon, that the 

 heat of a body is the rapid movement, or vis viva, of its molecules ; the 

 kinetic theory of gases and the molecular theory of electrolysis, since 

 extended by Arrhenius into the doctrine of electrolytic or ionic disso- 

 ciation. Clausius showed that part of the heat in a Carnot cycle is 

 converted into available mechanical energy and consumed as work, while 

 the rest of the heat can not be so utilized, because it exists in a com- 

 pletely diffused state. The perpetual motion which might be obtained 

 from utilizing the heat of surrounding objects is impossible because 

 such heat being completely diffused is, in Lord Kelvin's phrase, un- 



"Rankine, Phil. Mag., 1865, 244. 



=»HeImholtz, " Ueber die Erhaltung der Kraft," Berlin, 1847. 



=" Sir W. Thomson, Tr. Roy. Soc. Edinb., 1849, XVI., 543. 



^ J. Thomson, Ibid., 575-80. 



''Gibbs, Proc. Am. Acad. Arts and Sci., 1888-9, n. s., XVI., 459. 



