as deduced hy himself and Dr. Suerman. 267 



specific gravities, the general expression for the specific heats 

 of equal volumes will be 



^ X ^ (C.) 



Now, in this expression d = t—1),f' the force of vapour 

 at t\ and^ the existing pressure, are given by observation; 

 e is also a known quantity, being, according to the most ac- 

 curate experiments, and upon the hypothesis that the sum of 

 the sensible and latent heat of aqueous vapour at all tempera- 

 tures is a constant quantity, equal to 1 179 — ^. By aid, there- 

 fore, of a hygrometric observation with a wet and dry ther- 

 mometer in any gas, the barometer being also observed, the 

 specific heat of a given volume of such gas may be calculated. 

 Such is the method which, as far as my knowledge extends, 

 I have been the first to propose and practise. To this general 

 explanation it is only necessary to add that, at the close of 

 each experiment the gas operated with was subjected to ana- 

 lysis, and a correction made for the per centage of atmo- 

 spheric air which it was found to include, by means of a for- 

 mula which may be thus investigated. 



Let a be the calculated specific heat of the mixture of air 

 and gas, a' the true specific heat of the gas, n the number of 

 volumes of air in 100 of the gas, and c the specific heat of air 

 as deduced by means of formula (c) from a distinct experi- 

 ment with air alone. Then, on the principle that the specific 

 heat of the mixture of air and gas multiplied by its volume is 

 equal to the sum of the products of the respective volumes of 

 air and gas multiplied by their respective specific heats, 



a' (100 — w) + we = 100 « 



which gives 



, {a—e\n 



a = a + — . 



^ 100— n 



As examples of the course to be pursued, I shall adduce 

 the following observations made successively on air and hy- 

 drogen, on the Sth of August 1835. 



t "^ t' d p 



Hydrogen 68 4-8 20 30*1 U 



Air 68 .43 25 SO'll* 



By applying to these results the equation a = -^-^ x — 



we get 



Specific heat of air = '2767 = c 



Specific heat of hydrogen = '4090 = a. 



But the hydrogen upon analysis was found to contain 5 per 



