1826.] in the Atmosphere, and the Specific Gravitj/ of' Gases, 99 



is plainly the absolute force of the vapour at the temperature^' 

 when the whole pressure is p and 



/+448 



(2) 



/,+448 

 is the elastic force of the enclosed gas. 



If we put unity for the specific gravity of dry atmosphere at 

 theFahr. temperature 32'^ and barometric pressure 30, its specific 

 gravity, it may be easily shown at any other temperature Ji and 

 pressure ^ is 



/Hi48 •• (^^ 



Putting therefore for p the absolute force of the vapour (1) 

 just found, the specific gravity of the air becomes 



five-eighths of which, or 



/. + 448' 



10 T, 



/xH-448' 



is, by Lussac*s observations, the specific gravity of the vapour 

 confined with the gas. And if this be subtracted from the 

 specific gravity of the mixture supposed to be already determined, 

 it will leave the specific gravity of the gas at the temperature 

 y'and pressure (2). 



Second Method, 



Let A B C D be any rectangular glass tube A 

 open at A, and having stop-cocks at C, D, 

 whose orifices are ^, a ; and let the glass be 

 attached to a board or any other thing in a 

 vertical position. Having closed the cocks 

 G, D, pour into A mercury of the same tem- 

 perature as the air and tube, until the air 

 enclosed in CD just begins to cloud the glass 

 with the deposition of its vapour ; and sup- 

 pose at this moment r, s, are the respective 

 surfaces of the mercury in the legs B A, CD; 

 and let the difference of their altitudes be m, 

 the barometric pressure being as before p. 

 Then the elastic force of the air in D 5 is to 

 the elastic force of the atmosphere, or, which 

 is in the same proportion, the elastic force of 

 the vapour in D s is to the elastic force of the 

 vapour in the air, as /> -}- m : p. But the 

 Vapour in D 6- just beginning to deposit itself, and being of the 

 same temperature as the external air, must manifestly have ail 

 elastic force equal to the tension of vapour corresponding to the 

 temperature of this external air. That is, t being the tension 



h2 





1^ 



