Experiments on the Latent Heat of Steam. 5 1 



XXXVII., p. 494, 1889)] ; but on this point no agreement 

 as yet exists. 



It occurred to us that our results might be controlled by 

 the experiments made by Joly with his ingenious steam 

 calorimeter. {Proc. Roy. Soc, Vol. XLI., p. 352 (1886), and 

 Vol. XLVII, p. 219 (1889). By the use of this instrument we 

 can calculate the specific heat of a body if we suppose the 

 latent heat of steam to be known, and its author used it for 

 this purpose ; and, inversely, we can also use it to calculate 

 the latent heat of steam if we assume the specific heat of 

 the bodies experimented on to be known. Unfortunately 

 the specific heat even of bodies like silver, which are easily 

 obtained in the pure state, is not known with the requisite 

 accuracy. Thus Regnault gives for the sp. heat of silver 

 0'0570i, Kopp gives 0*056, and Bunsen gives 0*055 9. (The 

 numbers are quoted from Joly, Proc. Roy. Soc, XLI., p. 358.) 

 In order to use the Joly calorimeter for our purpose it 

 is evidently necessary that we should make use of a par- 

 ticular body of which the specific heat has been determined 

 immediately beforehand with the water calorimeter. This 

 we propose to do shortly. 



The other means at our disposal for controlling our 

 numbers is less direct. 



A well-known equation in thermodynamics gives us a 

 relation between 



L, the latent heat of steam at the absolute tempera- 

 ture T°, 

 (s—s), the difference between the specific volumes of 

 saturated steam and water at T°, 

 dp the differential coefficient of the vapour tension 

 dT of water with regard to the temperature, at T°, 

 and 

 J, the mechanical equivalent of heat, namely : — 



