.M. V. Regnault on the Expansion of Gases. 127 



wherein we agree, and that the controversy, so far from having 

 been about non-essentials, has been for the most part about ra- 

 dical differences of doctrine or of scientific method. What actual 

 scientific importance this diversity of fundamental views may 

 have, the future must decide. In bringing our amicable contro- 

 versy to a close, I must not withhold my acknowledgment of the 

 courtesy and general fairness manifested by my opponent, both 

 in the attacks he has made upon my positions, and in the main- 

 tenance of his own. 



XVIII. Memoir on the Expansion of Gases. 

 By M. V. Regnault*. 



I HAVE collected in this memoir the numerous experiments I 

 have made during the last twenty years, to determine the 

 losses of heat which a gas undergoes when it expands under the 

 very various conditions in which this phenomenon is met with in 

 nature and in our laboratories. I shall briefly indicate the idea 

 which has led me to undertake these researches ; the reader will 

 thus more readily understand the successive phases through 

 which they have passed. 



When in 1842 I commenced my experiments to determine 

 the calorific capacity of gases, I thought I had found a certain 

 and easy method of determining with the same apparatus and 

 without any petitio principii : — 



1. The specific heat of a gas under constant pressure and 

 changing volume ; 



2. Its specific heat under constant volume, the pressure vary- 

 ing so as to leave the volume constant. 



In the Memoires de VAcademie, vol. xxxi. p. 58, I have de- 

 scribed the experiments I have made with the view of determining 

 the specific heat of a great number of vapours and gases under 

 a constant pressure but with variation of volume; I have no 

 need to revert to them. I will merely say that in these experi- 

 ments the heated gas traversed a calorimeter with a constant 

 velocity, retaining virtually the same pressure. The heat which 

 it gave to the calorimeter served for calculating the calorific capa- 

 city of the gas between the limits of temperature which the gas 

 had attained. In this mode of working there is only a single, 

 extremely small correction — that due to the circumstance that 

 the hot gas arrives at the calorimeter with a far greater molecular 

 velocity than that with which it emerges. 



To obtain with the same apparatus the specific heat under a 

 constant volume, I made the heated gas enter the calorimeter 



* Translated from the Comptes Rendus, October 11, 1869. 



