Manchester Memoirs, Vol. xli. (1896), No. 3. 11 



Q being a quantity depending only on the initial state, 

 and a being the specific heat at constant pressure of the 

 steam gas, determined by Regnault to be 



0*48. 



Taking the initial state to be at 32 0 , Rankine obtained, as 

 the most probable value, 



Ci = iog2. 



It is to be noticed, however, that although this value 

 0*48, as obtained by Regnault, has been universally 

 accepted, the experiments by which he obtained it 

 were independent of the method by which he determined 

 the total heat of evaporation of saturated steam, and 

 that, as Regnault observes,* the smallness of the scale 

 as compared with that by which the total heats were 

 determined rendered it necessarily less accurate, as 

 regards the measurement of the total quantities of heat 

 observed, although the extreme care with which the 

 numerous experiments in the four cases were made, seems 

 to assure their relative accuracy. The experiments con- 

 sisted in determining the total heat necessary to raise 

 water from 32°F. or o°C. to temperatures of about I20°C. 

 and 220°C. under the pressure of the atmosphere, then 

 taking the differences as being the heat necessary to 

 raise water from I20°C. to 22o°C. It thus involves the 

 assumption that steam at 20°C. (or 36°F.) above the 

 boiling point is in the condition of steam gas. This is 

 probably very near the truth. Had, however, the experi- 

 ments been as absolutely accurate as those for the total 

 heat of saturated steam, they would have afforded the 

 means of comparing the two methods of Regnault by 

 Rankine's thermodynamical formulae. As it is, such a 

 comparison can be made. Thus, substituting the total 

 heats as obtained in the experiments for specific heat 



* Mem. Acad. Sci., Vol. XXVI., pp. 170, 909. 



