6 M. V. Regnault on the Elastic Forces of Vapours 



I have detailed (at page 134 of my memoir) * the experiments 

 which I made in order to determine directly the elastic force of 

 aqueous vapour at saturation in air and nitrogen gas at different 

 temperatures. The ninety-one determinations which I made 

 between 0° and 40° C. gave, without exception, the elastic force 

 of aqueous vapour as weaker in the air than in vacuo. The dif- 

 ferences are of the same kind as those previously found between 

 the weight of water which saturates a given volume of air, and 

 the weight deduced from calculation. Nevertheless, in these 

 experiments I endeavoured to vary the circumstances as far as 

 possible, and especially to realize those conditions in which it 

 might be expected that saturation would be obtained. 



Aqueous vapour having but a feeble tension at low tempera- 

 tures, it became important to extend these experiments to more 

 volatile fluids. I accordingly made use of sether, sulphuret of 

 carbon, and benzine. The apparatus employed was that described 

 in the Annates de Chimie et de Physique, 3 ser. vol. xv. p. 131 f- 

 Into a balloon, of the capacity of 600 to 700 cubic centims., 

 a small hermetically-closed capsule was introduced, containing 

 the liquid to be operated upon. This balloon, to the neck of 

 which a tube of considerable diameter had been previously 

 attached, communicated with a mercurial manometer. The 

 entire apparatus was placed in a large tub filled with water, 

 which was kept at a constant temperature. A piece of glass was 

 let into one part of the side of the tub, through which the ma- 

 nometer might be observed. The apparatus being arranged, a 

 vacuum was repeatedly produced in the balloon, the air admitted 

 after each exhaustion being perfectly dry. Lastly, the apparatus 

 was hermetically sealed, containing air under the pressure of the 

 atmosphere. 



The elastic force acquired by this dry air, kept constantly at 

 the same volume, at different temperatures, having been deter- 

 mined, the capsule was ruptured by the action of heat, and the 

 series of observations was recommenced on the air saturated with 

 vapour. The liquid enclosed in the capsule had previously been 

 kept from contact with the air, in order that it should not have 

 a tendency to absorb the air of the balloon. The volume of the 

 capsule in proportion to that of the balloon may also be varied 

 in different experiments, so as to make sure that the absorption 

 or disengagement of air by the liquid has not caused any sensible 

 perturbation. The following are some of the results that I have 

 obtained : — 



* Scientific Memoirs, vol. iv. p. 609. 

 t Ibid. p. 706. 



