102 On the QManlity of Vapour in the Almospherey S^c, [Aug. 



" ^ ' («. - So) i ^^ ' 



Hence supposing* the vapour could exist as an air under the 

 same pressure as air, vol. gas : vol. vapour :: (8) : (9) ; and vol. 

 gas + vol. vapour : vol. vapour :: (8) + (9) : (9;. Therefore 

 what we have constantly in the course of our inquiries called 

 the absolute volume of vapour is equal to 



(«) _ 1 _ s,s,(p^-p, ) 

 («) + (9) ps is, - s,) ^^^^ 



which if we had chosen might have been more concisely deduced 

 from (8). 



; Again, putting 5^ for the space occupied by the mixture at the 

 incipient state of condensation 5^ : s :: (9) : r, and 



, ^ (9). I ^Pjj!^^'>±zJihJEtZli> (11) 



The whole pressure at this time will consequently be 

 ■ yx'-= (P.'.-P.'.)l' £_^ (12) 



Dividing P t by this, P being the barometric pressure at the 

 time of the experiment, gives the force of the vapour in the 

 atmosphere equal to 



^ ' ps(^r=r7;y (1'^) 



and consequently, by what we have already shown, 



p s{s, - Sr,) - .V, Sa (pa - p.) . . 



Tsi^r^j ^'^> 



is the absolute volume of the vapour in a unity of space suppos- 

 ing it could exist as an air under the pressure P, at the temper- 

 ature of the atmosphere. Again dividing (13) by (6), and we 

 have 



p p x (v, — sa) - ^t s^ (pa ~p^) ,-^ 



P * (Pi "fi - P2 «•.) 



the humidity of the air, absolute moisture being unity. And if 

 /' be the temperature of the mixture, and we substitute (13) for 

 p in (3) and take five-eighths of the result from the specific gra- 

 vity of the mixture previously obtained, it will leave the specific 

 gravity of the enclosed gas of whatever kind it may be, when 

 the temperature is yi and pressure P— (13). 



This method of making the experiment is independent of any 

 knowledge of the tension of vapour, which indeed it brings out, 

 and may oe applied to determine. The only things necessary to 

 attend to are, the accuracy of the capacity of that part of the 

 glass tube in which the mixed airs under examination is con- 

 tained, and the temperature of this same part of the tube, which it 

 is most essential should be exactly determined and not afterwards 

 affected by handling or otherwise, A little change will also be 



