034 Sir J. A. Ewing on Specific Heat of Saturated Vapour 



adiabatic expansion and is supersaturated or partially con- 

 densed by adiabatic compression. The fact that K s is 

 negative in steam was pointed out independently and almost 

 simultaneously by Rankine (in the paper cited) and by 

 Clausius *, who also showed that in any fluid 



-t^-S -^-«t'~r ; rii •) 



JL_L 

 dT T 



w 



here L is the latent heat and K w is the specific heat of" the 

 liquid (at the same temperature) when heated under saturation 

 pressure. Later, when Regnault's data for various fluids 

 became available, Clausius applied this relation to them, 

 finding K s to be negative in most cases. In ether, however, 

 he found that Regnault's data made K s positive "at least at 

 ordinary temperatures," and in other fluids the value of K s 

 calculated from them increased as the temperature rose 

 (that is to say, its negative value decreased). He continues: 

 "In the only case, that of ether, in which it is positive at 

 ordinary temperatures its absolute value increases as the 

 temperature rises. In the other cases, in which it is negative, 

 its absolute value diminishes ; it thus approaches zero, and it 

 would appear that at some Jiigher temperature it would attain 

 the value zero and at still higher temperatures would become 

 positive" f. I have italicised this passage because, although 

 the statement is true of certain fluids, there are many for 

 which it is not true. Other well-known writers have made 

 the same suggestion, that in water and in fluids generally, 

 the specific heat of the saturated vapour changes sign from 

 negative to positive when the temperature is sufficiently 

 raised J. From the form of the entropy- temperature 

 diagram, however, it is clear that this does not happen in 

 steam, nor in any of the fluids commonly used as working sub- 

 stances in refrigerating machines (carbon dioxide, ammonia, 

 sulphurous acid), nor in bisulphide of carbon, nor (as will 

 presently be seen) in alcohol. On the other hand, it does 

 happen in a considerable number of other fluids, such as 

 ether, chloroform, benzene, and many esters of the fatty 

 acids, for which the researches of Ramsay and Young and 



* In a paper communicated to the Academy of Berlin, Feb. 1850, 

 Po(/g. Ann. vol. lxxix. p. 368 and p. 500; Phil. Mag. July 1851. See his 

 'Mechanical Theory of Heat' (Tr. Browne), p. 135. 



t Clausius, ' Mechanical Theory of Heat' (Tr. Browne), p. 139. 



;| Cf. Preston, < Theory of Heat,' 3nd ed. p. 731 ; Peabody, * Thermo- 

 dynamics of the Steam-Engine,' 5th ed. p. 94 ; Lewis, ' Physical 

 Chemistry,' 2nd ed. vol. ii. pp. 83-85 ; L. Natanson, Phil. Mag. Oct. 

 1895, p. 277 ; C. Raveau, Jour, de Phys. (3) vol. i. p. 461 (1892). 



