278 1'rof. H. L. Callendar. On the Tl "////'<,// 



depends on the rate of variation of the total heat, so that all the errors 

 in the formula for the total heat are enormously exaggerated in the 

 calculation of the specific heat. 



The last column in the table contains the values of the so-called 

 " Specific Heat of Saturated >team," i.e., the value of the specific lu-at 

 when dpjdO is determined by the condition that the steam is to remain 

 saturated. The value in this case is given by the formula 



S(sat.) = dE/dO-L/0 (28), 



which is most readily obtained by differentiating the expression for 

 the entropy of saturated steam, namely, <f> g = <}> w 4- L/0, where <, P is 

 the entropy of water reckoned from C. The values of S (sat.) also 

 differ considerably from those which have been previously calculated 

 on the usual assumption that (fH/dO is constant, and equal to O305, as 

 in formula (1). 



It will be observed that the mean value of the specific heat of steam 

 at a constant pressure of one atmosphere between the limits 120 

 and 200 C., the range of Regnault's experiments, is 0'512 according to 

 the values given in Table II. This value is not so greatly in excess of 

 the value 0'48 given by Regnault as to be beyond the limits of experi- 

 mental error, especially if we consider that the method which he 

 adopted must necessarily have given rise to constant errors in defect, 

 and that the superheat was only one-sixteenth of the total heat to be 

 measured. 



It is probable that the variation of the specific heats of all other 

 gases and vapours, to which the Joule-Thomson equation can be 

 applied, is of the same type as that exhibited above in the case of 

 steam. I have succeeded in reconciling a good many of the appa- 

 rently discordant experimental data on the subject by means of 

 this hypothesis, but the experiments themselves are difficult, and 

 the question of the variation of the specific heats of gases is obscured 

 by unavoidable errors. Among the most remarkable and accurate of 

 recent results are those of Joly on the specific heats of Air and CO* at 

 constant volume, determined by means of his differential steam- 

 calorimeter. The values which he obtained are much larger than those 

 deduced from Regnault, and cannot be reconciled with them on the 

 common assumption (Van der Waals) that the specific heat at constant 

 volume is constant, but they agree remarkably well, considering the 

 difficulty of the experiments, with the theory proposed in this paper. 

 A fuller discussion of these and similar relations will be reserved for a 

 future communication. 



