Properties of Gases and Vapours, &c. 



R = 4-613 x 10 C.G.S. = 0-4613 joule/deg. C. 

 = 0-11037 cal./deg. C. 



The unit of heat adopted in this paper is the thermal capacity of 

 1 gramme of water at 20 C., which is taken as being equivalent to 

 4-180 joules, from the mean of the results of Rowland and of Reynolds 

 and Moorby, compared and reduced by the work of Callendar and 

 Barnes on the variation of the specific heat of water over the whole 

 range to 100 C. 



The limiting values of the specific heats of steam, and of their ratio, 

 in terms of this unit are as follows, 



S = 0-4966 cal./deg. C. s = 0-3862 cal./deg. C. g" = 9/7 = 1-2857. 



In the following table, the values of S, s, and g are given for saturated 

 steam at the point of saturation, in order to illustrate the increase of 

 specific heat with pressure. The values of the specific heat S' at a 

 pressure of one atmosphere at various temperatures are also given, to 

 show the diminution of the specific heat with rise of temperature. The 

 values enclosed in brackets are of course imaginary, but are included to 

 show more clearly the nature of the change. The value of the specific 

 heat at constant pressure has been calculated by Zeuner from Regnault's 

 observations to be 0'568, on the assumption that the specific volume of 

 steam is a linear function of the temperature at constant pressure, in 

 which case the specific heat at constant pressure is independent of the 

 pressure. Another common assumption is that the pressure at constant 

 volume is a linear function of the temperature (Van der Waals). 

 Neither of these assumptions can be reconciled with the most accurate 

 thermometric work at moderate pressures, or with the present experi- 

 ments on steam by the method of the differential throttling calorimeter. 

 The advantage gained by these assumptions is very slight and one-sided. 

 The partial constancy of one specific heat is a small matter, if at the 

 same time the other thermodynamical relations are rendered so com- 

 plicated as to make the equations useless. The values of the specific 

 heat of steam at constant pressure have been recently calculated by 

 Grindley from his throttling experiments,* assuming the linear formula 

 (1) of Regnault for the Total Heat of steam. These values are included 

 in Table II for comparison. The numbers given in brackets are not 

 given by Grindley, but are calculated by an extension of his method to 

 show the effect of his hypothesis. The extraordinary differences 

 between his values and mine, as shown in the adjacent columns (6) 

 and (7) of Table II, are not due to any discrepancy in our experiments, 

 but simply to his assumption of Regnault's formula for the total heat. 

 The method of deducing the specific heat from the total heat, though it 

 has often been applied, is unsound in principle, because the specific heat 



* ' Phil. Trans.,' A, vol. 194, 1900, pp. 1-36. 



