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



But this vapour comes in contact with the walls of the vessel, 

 which by their hygroscopic affinity condense a portion of it on 

 their surface. The space is consequently below saturation as 

 long as its walls have not condensed the quantity of vapour 

 necessary to satisfy their attractive action upon the vapour. 

 This quantity may be valued by the thickness of the liquid stra- 

 tum. On the vertical portions of the wall the liquid stratum 

 cannot increase indefinitely in thickness, as its weight tends to 

 make it run towards the bottom. It will be understood, there- 

 fore, that if the stratum which remains adherent to the wall be 

 too thin to neutralize the attractive action of the wall upon the 

 vapour, the space must lose vapour; and if the lost vapour be 

 not supplied with the same rapidity by the excess of liquid, the 

 elastic force must always appear less than that observed in 

 vacuo. 



In this manner is explained why the greatest elastic forces 

 corresponding with a given temperature are always observed 

 immediately after a diminution of volume, or after refrigeration, 

 because then the liquid which is just condensed has saturated 

 the wall, and has not had time to run down by its own weight. 

 This drainage gradually takes place, and the elastic force of the 

 vapour is diminished in consequence. A similar effect cannot 

 be produced in vacuo, because in this case the vaporization of the 

 liquid is very rapid, and immediately replaces the vapour which 

 has condensed upon the walls. 



Many facts observed by physicists may also be explained in 

 this manner. The air saturated with vapour by an abundant 

 rain, descends below saturation soon after the cessation of the 

 rain, because the bodies which are immersed in it deprive it of 

 vapour by a hygroscopic action. If air be drawn by an aspirator 

 from a space where it is saturated with vapour, and passed through 

 a dry glass tube kept at the same temperature as the air, the 

 walls of the tube will become covered with an abundance of 

 drops, &c. 



To sum up, I believe we may conclude from my observations 

 in general, that Dalton's law regarding mixtures of gases and 

 vapours may be regarded as a theoretical law, which would pro- 

 bably be verified most strictly if we could enclose the gas in a 

 vessel, of which the walls might be formed of a certain thickness 

 of the volatile fluid itself. But this law can only be very imper- 

 fectly realized in our apparatus ; the hygroscopic affinity of their 

 walls reducing the vapour to a variable tension, which is always 

 lower than that corresponding with saturation. 



