[ 211 ] 



XXII. On the Speci/lc Heat of Saturated Vapours and 

 Entropy- Temperature Diagrams of Fluids, By Alfred 

 W. Porter, D.Se., F.R.S* 



IN a paper with the above title in the June number o£ the 

 Philosophical Magazine (p. 633) Sir J. A. Ewing 

 extends our detailed knowledge of the specific heats of 

 saturated vapours, and at the same time corrects certain 

 loose statements which have appeared at various times. 



I desire to point out that the question (as might be 

 expected) is very thoroughly discussed by Duhem in his 

 TraitS Flementaire de Mechanique Chimique, t. ii. p. 211, &c. 

 Duhem shows that if the specific heat of the saturated vapour 

 is plotted against temperature, its form is that of an inverted, 

 unsymmetrical U ; that sometimes this lies wholly in the 

 negative region ; but that it may lie higher up so as to cross 

 into the positive region, and that if it does this it crosses 

 twice. 



These are the two possibilities of which Sir J. A. Ewing 

 gives detailed examples. 



He asserts, however, that positive values do not occur for 

 sulphurous acid. This statement itself appears to be in 

 error. The case of sulphurous acid (S0 2 ) has been directly 

 studied by Mathias {Comptes Rendus, t. cxix. p. 849 ; A?in. 

 de la Facidte des Sciences de Toulouse, t. x. (1896), quoted by 

 Duhem, loc. cit.). The importance of this substance in con- 

 nexion with mechanical refrigeration warrants one in quoting 

 his results in the following table :■ — 



t° C. 







-0-410 



100 



4-0-027 



10 



•390 



110 



4-0062 



20 



•357 



120 



-0-078 



30 



•330 



125 



•176 



40 



•300 



130 



•306 



50 



•270 



135 



452 



60 



•235 



140 



•620 



70 



•205 



145 



•848 



80 



•165 



150 



1-253 



90 



-0095 



155 



-3-850 



t° c. 



The temperatures of inversion are 97°*5 C. and 114° C. 

 In order to free the subject from all possible misunder- 

 standing, I wish to add that the entropy of the liquid is not 



M!/f 



Jo T ' 



where C p is the specific heat at constant pressure, even with 

 * Co t niniinicated by the Author. 



