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



surement of the rapidity of succession may be made in the fol- 

 lowing way. Move across the line of discharge with a steady 

 hand a strip of writing-paper, it will be punctured with a row of 

 holes, which will be the more closely approximated in proportion 

 as the succession of discharges is more rapid. By a disc of paper 

 attached to an axis moving with a given rate of revolution, this 

 measurement may be made very accurate and useful. 



Those who possess the coil apparatus will find it very conve- 

 nient to have a plate of glass, coated on each side with tinfoil, 

 placed on the base of the machine, and having strips leading 

 from each coating to binding-screws, with which the terminals 

 of the secondary coil can be corrected at will. 



December 7, 1854. 



II. On the Elastic Forces of Vapours in vacuo and in Gases, at 

 different Temperatures ; and on the Tensions of the Vapours 

 furnished by mixed or supeiposed Liquids. ByY. Regnault*. 

 [Concluded from vol. viii. p. 280.] 

 Part III. — On the Elastic Force of Vapours in Gases. 



PHYSICISTS in general admit that vapours behave in the 

 same manner in gases as in vacuo, with this difference only, 

 that in gases the equilibrium of tension is slowly established, 

 whilst in vacuo it is produced almost instantaneously. An ap- 

 paratus invented by Gay-Lussac is usually employed in lectures 

 on physics, to show that the elastic force of vapours is exactly 

 the same in gases as in vacuo. Nevertheless there is no record 

 of any exact experiments having been made for the establish- 

 ment of this law, which is of great importance, especially in 

 meteorology. 



During my hygrometric investigations (published in the fif- 

 teenth volume of the Annates de Chimie et de Physique-^), I very 

 carefully determined the weight of the aqueous vapour existing 

 in the atmosphere at its different states of saturation. I ascer- 

 tained that, even during long-continued rains, which must have 

 maintained the air in a state of saturation, the quantity of water 

 found by experiment was always less than that determined by 

 my calculations, based on the one hand upon the elastic force 

 which I had found for aqueous vapour in vacuo, and on the other 

 upon the theoretical density of this vapour. 



This circumstance might arise from two causes : — 

 1. The elastic force of vapour of water might be exactly the 

 same in the air as in vacuo ; 



* From the Comptes Rendus for August 21 and 28, 1854. 

 t Taylor's Scientific Memoirs, Part XVI. 



