162 Prof. Potter on the Relations of the Elastic Force, 



Poisson's assumption was, that in the sudden condensation or 

 rarefaction of a gas the temperature was increased or diminished 

 respectively by 1° Centigrade for every yy^th part of condensa- 

 tion or rarefaction. In the method of Laplace, the ratio of the 

 specific heat of air at a constant pressure to the specific heat with 

 a constant volume was required to be known. The ingenuity of 

 MM. Desormes and Clement devised an experimental method* 

 of determining this ratio which has received the general assent 

 of the scientific world, and which will not be forgotten in the 

 history of science. A receiver furnished with a vertical pipe 

 and stopcock had also a horizontal pipe connecting it with an 

 air-pump. To the horizontal pipe were attached a mercurial 

 barometer-gauge and a water-gauge, which each indicated the 

 elastic force of the air inside the receiver, within their respective 

 limits ; but the water-gauge being the most sensitive, was the 

 one observed in the experiments, and its indications afterwards 

 converted to mercurial pressures. A portion of air being with- 

 drawn by the air-pump, and the reading of the water-gauge 

 noted, the stopcock was then opened, and in two-fifths of a 

 second the atmospheric air ceased to rush into the receiver, and 

 the stopcock was then closed. It was seen that the water in 

 the water-gauge descended to a certain point and then rose to a 

 higher level, which were both noted. This latter rise of level 

 was attributed entirely to the heat which was developed in the 

 sudden condensation, and which gradually disappeared. We 

 see here that the effect of the momentum of the moving column 

 of water in the gauge in passing from one position of equilibrium 

 to another has been attributed to the effect of heat developed in 

 the sudden condensation. 



At the request of M. Laplace f, experiments on the same 

 priDciple were made by MM. Gay-Lussac and Welter, only using 

 condensation where MM. Desormes and Clement used rarefaction. 



The method of M. Dulongf to find the value of the above- 

 named ratio, was to admit as demonstrated Laplace's formula for 

 the velocity of sound ; and comparing it with the observed velo- 

 cities, to deduce, by the inverse process, the value of the ratio 

 required. 



The reader will judge how much need there was of a true ex- 

 perimental basis for determining the fourth law of the relations 

 of the elastic force, density, and temperature in gases, for the 

 quantity of caloric constant. 



Since the publication of my papers on Hydrodynamics in this 



* Journal de Physique, vol. lxxxix. p. 330. 



t Micanique Celeste, vol. v. p. 125. 



X Annates de Chimie et de Physique, vol. xli. p. 131. 



