On the Elastic Force of the Vapor of Mercury. 295 



The law of the tension of the vapor of mercury may now be ap- 

 plied to test certain principles and theoretical formulae which have 

 been proposed to represent the elasticity of vapors in general, and 

 which have been applied to the vapors of water and of some other 

 liquids. If the principles of these formulae should prove applicable 

 to mercury, a liquid so different from water, they would receive a 

 striking confirmation, and if not, their conformity with the law of 

 elasticity in certain other liquids may be looked upon as entirely ac- 

 cidental. 



First, it is evident that the elasticity of the vapor of mercury does 

 not conform to the theory advanced by Dalton, that the tensions of 

 the vapor of all hquids, at temperatures equally distant from their re- 

 spective boiling points are equal. If this were true of the vapor of 

 mercury, as compared with that of water, the tension at 500° F. 

 or 180° below the boiling point ought to be about .2 of an inch, 

 whereas according to my experiments it is about 5.3 inches. The 

 inaccuracy of this theory had already been remarked in relation to 

 liquids more volatile than water, and Dalton, himself, seems to have 

 abandoned it. M. August, of Berlin, in Poggendorff's Annals of 

 Philosophy and Chemistry, No. 5, 1828, and Professor Roche, of 

 Toulon, in a memoir presented to the Academy of Sciences of Paris, 

 during the same year, have both proposed formulae to represent the 

 law of the tensions of the vapor of water, based, at least in part, upon 

 theoretical principles, and which, as I have shown in my memoir, 

 although different in form are really identical. These formulae are 



essentially of the following form, log. e=-o , ,, in which e is the 



elasticity of the vapor, t the number of degrees from its boiling point, 

 and A and B two constants to be determined by observation. Messrs, 

 August and Roche propose to determine the constant B by con- 

 sidering that the tension must be nought at — 448° Fahr.,* which 

 they regard as the absolute zero of temperature. Calling n the 

 number of degrees from the boiling point of the liquid to this abso- 



n 

 lute zero, we have e=0 when t= — n, whence log. e= — A • j>_„ = 



— cx), or B — »=0, and B=n. Substituting the value just found for 



t 

 B, the formula becomes log. e=A — — , and there remams only 



the constant A to be found by observation. 



^ ~266|C. 



