268 I'M-:'. II. L. Callfiular. On the TV/-/-/ >>-al 



li>tril>uting tap intx) the idle calorimeter. It is clear that he regards 

 the observations ;ilove this point with greater confidence. In any case 

 it is unlikely that the order of accuracy attained in his experiments 

 was greater than one-half of 1 per cent, at any point, because his 

 thermometers were not sxifficiently perfect, and because it is practically 

 certain from the recent determinations of Reynolds and Moorby of 

 the mean specific heat of water between and 100 C., and from the 

 work of Callendar and Barnes on the variation of the specific heat 

 over the whole range to 100 C., that his value for the specific heat 

 of water at 100 C. is at least 1 per cent, too large. He was also 

 ignorant of the considerable changes which occur in the specific heat 

 of water at low temperatures, and it is evident that his work, though 

 far in advance of his time, requires revision when considered in the 

 light of the great advances which have been made in the last fifty years. 

 It is obvious from the nature of the problem that the most appropri- 

 ate method of determining, either the variation of the total heat of 

 steam, or the specific heat of steam, is by the application of some 

 differential method, which shall be independent of the determination of 

 the latent heat. In the papers which follow, I have described the 

 application of two such methods to the case of steam. By means of 

 the " Differential Throttling Calorimeter " it is possible, following the 

 method of Joule and Thomson, to determine accurately the variation 

 of the total heat of steam, and the deviations of the specific volume 

 from the ideal gaseous state, in terms of the specific heat at constant 

 pressure. By the "Electrical Method of Measuring the Specific Heat," 

 which is exactly similar to the method already applied* in the case of 

 water, it is possible to determine the specific heat without reference to 

 the latent heat. The details of these experiments are reserved for 

 subsequent communications, the object of the present paper is to 

 explain the thermodyriamical relations involved, and to exhibit the 

 calculation of the variations of the specific volume, the specific heats, 

 the total heat, the latent heat, and the pressure of saturated steam in 

 terms of the quantities which are directly observed. The theory of 

 the method is applicable, and has been already applied, to some 

 problems connected with gases, but in dealing with vapours some 

 additions are required, and it is clear that the original equation of 

 Joule and Thomson requires some important modifications. 



Modification of tlie Jwle-Thotruon A'y""//<///. 



In order to represent the observations of Kegnault on the deviations 

 of CO;, from Boyle's law, Rankine in 1854 proposed the equation, 



pv = R0-/0r (4). 



* Callendar and Barnes, 'Brit. Assoc. Report,' 1897 and 1899. 



