EXPERIMENTAL KNOWLEDGE OF THE PEOPERTIES OF MATTER. 491 



Specific Seat and Pressure. 



In the cases of atmosplieric air, hydrogen, and carbon dioxide, 

 Regnanlt found by experiments at several atmospheres' pressure, e.g., 

 for air 12 '4 atmos., no observable difference in the specific heat of unit 

 mass due to pressure, but that this is not the case for chlorine, at least 

 at ordinary temperatures, and is probably only true for bodies at tem- 

 peratures at which they conform to Boyle's law. 



Specific Heat of Elements in tlie Gaseotis State. 



He concludes^ that the law which had been announced by several 

 physicists, ' that simple bodies (elements) in the state of gas had identical 

 specific heats (heat-capacities) of equal volumes,' can only be considered 

 as an ' ideal law,' applicable to bodies which had quite uniform laws of 

 compressibility and dilatation (obeying Boyle's and Gay-Lussac's laws 

 exactly), and therefore not applicable to bodies as we actually know 

 them in the state of gas. 



Specific Heats of Liquids — Begnault. 



From experiments on a number of bodies, viz., water, bromine, 

 alcohol, ether, carbon disulphide, methyl alcohol, acetone, ethyl sulphy- 

 dride, ethyl chloride, ethyl bromide, ethyl cyanide, ethyl acetate, 

 chloroform, ethene dibromide, benzene, oil of turpentine, silicon chloride, 

 phosphorous chloride, arsenious chloride, stannic chloride, titanic 

 chloride, of which the specific heats in state of vapour were determined, 

 Regnault determined the specific heats in the liquid state, and found that 

 the mean specific heat of each body in the state of gas is always less than 

 in the liquid state, especially when compared at about the same range of 

 temperature ; and that in the case of several of the bodies the former 

 value is only a small fraction, less than half, of the latter. Hence, he says, 

 there cannot exist a rigorous law connecting specific heats of bodies with 

 their atomic (or molecular) heats alone, for the specific heats vary, as he 

 has shown, with change of temperature and of state. 



It must here be mentioned that Joule ^ pubhshed the results of three 

 experiments on the specific heat of air, the mean of which gave 0*2296 ; 

 these are noticed by Regnault.^ 



Specific Heat at Constant Volume — Begnault. 



The experimental results given by Regnault are obtained in every 

 case by finding the amount of heat given to the calorimeter by a gas or 

 vapour parting with heat either at atmospheric pressure or at some 

 constant, or nearly constant, pressure greater than atmospheric. The 

 problem of finding the specific heat of a gas or vapour, the volume of 

 which is not allowed to vary, is solved by applying the principles of 

 thermodynamics to the results for constant pressure. Through the 

 ranges of temperature within which Regnault worked, the following 

 gases alone were found to have specific heats independent of temperature 

 and of pressure, so far as the effect of pressure was determined : oxygen, 



' Mem. xxvi. p. 309. ^ Phil. Mag. vi. p. 143. * M6m. xxvi. p. 40. 



