128 M. V. Regnault on the Expansion of Gases. . 



with such an excess of pressure that, when the gas expanded to 

 the surrounding atmospheric pressure, its volume should remain 

 constant notwithstanding the change of temperature. 



With the ideas I then had as to the structure of gases, and 

 which were at that time generally admitted by physicists, I 

 thought I had thus realized the conditions under which the spe- 

 cific heat of a gas under constant volume can be directly obtained. 



I intended to make these new experiments when those which 

 gave the specific heats under constant pressure had been termi- 

 nated. But these latter occupied me during several years, and 

 I decided on the 17th of July, 1848, to deposit in the Academy 

 of Sciences a sealed packet which contains a complete descrip- 

 tion of my method for determining successively, with the same 

 apparatus, the specific heat of a gas under constant pressure 

 and that of this gas under constant volume (Comptes Rendus, 

 vol. xxvii. p. 77). 



Nevertheless the first experiment which I thus made, the 22nd 

 of October, 1849, gave me a far different result from that which 

 I expected : I found that the gas undergoing expansion in the ca- 

 lorimeter indicated the same calorific capacity as that which it gave, 

 me when it traversed the calorimeter without undergoing expansion. 



To render this fact more striking, I greatly increased the 

 pressure under which the heated gas reached the capillary orifice. 

 I then observed that the heat given up by the hot gas when it 

 undergoes a great expansion in the calorimeter, is even a little 

 greater than that which is given by the same gas while traversing 

 this calorimeter and at the same time retaining its elastic force. 



Thus, under the conditions in which my experiments were made, 

 the specific heat of gas under constant volume is virtually equal 

 to the specific heat of this gas under constant pressure. I an- 

 nounced this fact to the Academy on the 18th of April, 1853 

 (compare Comptes Rendus de V Academie des Sciences, vol. xxxvi. 

 p. 680). 



I had also to conclude from my experiments that, if the com- 

 pressed air enters the calorimeter at the same temperature as the 

 calorimeter, the expansion should take place therein without causing 

 it to undergo a sensible change of temperature. 



This in fact is what 1 proved by a great number of experiments. 



In short, my researches have shown me that,when a gas expands 

 under the conditions of my experiments (that is to say, when it 

 leaves the calorimetrical apparatus with the whole of the motion 

 which the expansion has given to it), the calorific phenomenon is far 

 different from that which obtains for the same gas when it is 

 contained in a state of rest in a cylinder and when its volume is 

 made to vary by displacing a piston. 



At the present time the mechanical theory of heat quite ex- 



