1826.] in the Atmosphere, and the Specific Gravity of Gases. 101 



specific gravity of the mixture at the temperature and pressure 

 of the atmosphere, leaves the specific gravity of the gas at the 

 temperature /, and pressure (5). 



These two methods being independent in principle and prac- 

 tice, will, if made at the same time, afford a mutual check and 

 corroboration. As they both, however, rest in a great mea- 

 sure on a certain delicacy of ocular observation, I shall now 

 explain a 



Third Method 



ffee from those defects. 



Having introduced any quantity of the atmosphere in D C of 

 our rectangular tube, as well as some mercury in the other leg 

 B A, close the cock D and open C, until the surface of the mer- 

 cury in B A sinks as much as it can below the other surface in 

 C A, so as however not to descend into the horizontal part B C. 

 Then having shut C, measure the pressure on the enclosed 

 mixture, which will be less than the barometric pressure by the 

 difference of the altitudes of the mercury in C D and B A, and 

 call it p. Let 5 also be the space occupied by the mixture, 

 which, supposing C D perfectly cylindrical, may be represented 

 by the depression of the mercury in C D, below the interior 

 upper part of it D. At this time it is clear that all the vapour 

 is in the airiform state, its elasticity being less than in the open 

 air. 



This done, pour in some more mercury in A, until, from the 

 dulness of the glass D 5, we are certain that some of the vapour 

 is condensed ; and let the measured pressure and space occu- 

 pied be |?i and Sj. 



Repeat these admeasurements after having poured in as much 

 more mercury as you conveniently can ; and suppose the neW 

 pressure and space are p„ and s^. Let r be the tension of the 

 vapour corresponding to the temperature of the air f) which, 

 from the w- ell known laws of vapour, must be the same as the 

 elastic force of the vapour in both of the last experiments. 

 Then;?i — t : p^ — r :: elast. of gas in s^ : elast. of gas in ^i 

 :: Sj : s,. Therefore 



Si - *2 " \ y 



Consequently the elastic force of the gas in s, is 



p. --=7^^. (7) 



and in any other space s, it is 



/7\ X - = ^P- ~^'^^' ^^ (8) 



But in s the whole pressure of all the vapour and air is p, and 

 therefore the pressure due to the vapour alone is 



