Properties of Gases and Vapours, &c. 267 



to employ two different formulae to cover the range 0" 220 C. of his 

 experiments. 



The specific volume of saturated steam, owing to the effects of 

 surface condensation,* cannot be determined by direct experiment, 

 and is generally deduced from the above empirical formulae, by the 

 application of the well-known thermodynamical relation, 



(3), 



where L is the latent heat, v the specific volume of the saturated 

 vapour, and b that of the liquid, the absolute temperature, and dp/dB 

 the rate of increase of the saturation-pressure with temperature. 



Eegnault also determined the specific heat of superheated steam at 

 atmospheric pressure by condensing highly superheated steam in a 

 calorimeter ; but owing to the small proportion which the superheat 

 of the steam bears to the latent heat, and to the difficulty of calori- 

 metric work at high temperatures, the measurements were not very 

 certain, and many recent experimentalists and writers (e.g., Ewing, 

 Perry, Grindley) have preferred to adopt widely different values 

 deduced by other methods from the formula for the total heat. It 

 was proved by Rankinet that the rate of change of the total heat of 

 steam at low temperatures, at which it very nearly follows the laws of 

 an ideal gas on account of its low pressure and large specific volume, 

 must be very nearly equal to the specific heat of the vapour at con- 

 stant pressure. Therefore either the specific heat of steam at low 

 temperatures must be O305, increasing considerably with the tempera- 

 ture so as to reach the value 0'48 between 100 and 200 C., or else the 

 observations of Regnault must be wrong. In any case it is clear that 

 the variation of the total heat should not be linear, unless we abandon 

 the experimental evidence in favour of the constancy of the specific 

 heat of an ideal gas. It is most likely that the source of the discre- 

 pancy is to be found in the difficult calorimetric measurements of the 

 rate of change of the total heat at low temperatures. The determina- 

 tions of the latent heat by Griffiths, 572'6 calories at 40'2 C., and by 

 Dieterici, 596'7 calories at C.,J are from 6 to 10 calories smaller 

 than Regnault's, and imply a rate of change of total heat about 

 30 per cent, larger, and more nearly equal to the theoretical value. 

 At temperatures above 100 C., the determinations of Regnault are 

 more consistent, but it is very likely, from the method which he 

 employed, that they may be considerably in error. His observations 

 show a sudden increase of six calories above 175 C., which is explained 

 by the discovery and rectification of a leakage of steam through the 



* Eamsay and Young, 'Phil. Trans.,' A, 1892. 



t ' Koy. Soc. Edin. Proc.,' 1850. 



I Griffiths, ' ROT. Soc. Proc.,' December, 1894. 



