M. V. Regnault on the Expansion of Gases. 129 



plains these differences : it shows that it is not sufficient to con- 

 sider the initial and final densities of the gas, but that account 

 must be taken of the changes produced in the motion of trans- 

 lation of the gas, and even in its molecular motions. 



It is easy to define theoretically a certain number of conditions 

 in which a gas may undergo the same change of density and ex- 

 perience different variations of temperature. But it is far more 

 difficult, and often even impossible, to realize these conditions 

 by experiment, retaining the simplicity of the theoretical enuncia- 

 tion. Most frequently secondary phenomena are produced which 

 cannot even now be clearly defined, and of which it is still more 

 difficult to measure or calculate the effects. 



I shall distinguish in this memoir two kinds of expansion; 

 for I have had to employ for each of them a special method : — 



1. Simple expansion, which I shall call statical expansion ; it 

 is that which a gas at rest undergoes when by the aid of external 

 work the space it occupies is increased, the gas resuming the 

 state of rest after the expansion. There is the same mass of gas 

 in a state of rest in the calorimetric reservoir at the commence- 

 ment and at the end of the experiment. 



2. The expansion which I shall call dynamical; this is when 

 a gas either at rest or in motion expands in traversing an orifice, 

 and then enters or escapes from the calorimetrical apparatus with 

 all its acquired velocity. The mass of the gas contained in the ca- 

 lorimetrical reservoir is not constant during the experiment : one 

 portion enters or escapes, with a variable velocity determined by 

 the varying excess of pressure. 



I shall commence with the dynamical expansion, although it is 

 the most complex ; but it is that which I first studied, as it fre- 

 quently occurred in all my researches on gases. The numerous 

 experiments which refer to it may be divided into two parts. 



The first comprises those in which a gas compressed and in 

 motion reaches the calorimeter and emerges from it with its ori- 

 ginal motion modified by the effect of the expansion. The calo- 

 rimeter contains then at the commencement and at the end of 

 the experiment the same very small quantity ; its temperature is 

 modified solely by the calorific effects that the gas undergoes 

 which traverses it. 



The second part refers to experiments in which the total quan- 

 tity of gas is contained in a calorimetrical reservoir either at the 

 commencement or at the end of the experiment. I shall here 

 distinguish two cases. 



First case. — The compressed gas is in a state of rest in the 

 calorimetrical reservoir. It is allowed to escape by opening a 

 more or less capillary orifice ; it escapes into the atmosphere 

 with the varying velocity produced by its successive expansion. 



Phil. Mag. S. 4. Vol. 39. No. 259. Feb. 1870. K 



