4d Explanation of the Theory of the [July, 



become the same as though it had been formed in that diminished 

 temperature. 



(28.) The chamber V being saturated with vapour of any 

 elasticity, cut off the communication with the reservoir W, by 

 means of the stopper S. The piston having a pressure equal, or 

 superior to the force of the vapour, will, in the former case, 

 remain stationary on being liberated : on the latter supposition, 

 it will descend,Mas before, to Xy liquefying the whole of the 

 vapour. 1 M'^ )\v\vvu\ .y-Tpv (^ 



(29.) The pressure being inferior in any ratio to the force of 

 the vapour, the volume of the vapour will be augmented, and 

 its elasticity and density diminished in the same proportion. 



(30,) The vapour not being in contact with the generating 

 liquid, and supporting any pressure whatever, will have its 

 volume augmented on being exposed to superior temperatures, 

 at the uniform rate of -^^-^ per degree from 32° F. ; whence its 

 diminished density and increased elasticity may be easily calcu- 

 lated. In this, and in the preceding case, the space containing 

 tlie vapour is said to be but partially saturated. 



(31.) It is inferred from tne experiments of Gay-Lussac that 

 aqueous vapour is specifically lighter than dry air of equal elas- 

 ticity and existing in the same temperature, very nearly in the 

 ratio of 5 to 8. Ihe temperature of the vapour of the experi- 

 ments being 100° C. (some little inferior to 212° F.) in order to 

 prevent the possibility of any portion of the Hquid, of which the 

 weight had been previously ascertained, remaining unevaporated, 

 which might have occasioned the grossest errors, the pressure 

 was slightly reduced, so as not to be fully equal to the tension 

 of the vapour, or to 30 inches. Saussure, as well as Watt, had 

 before estimated the difference to be as that of 6 to 7, but as the 

 experiments of Gay-Lussac were made under greater advantages, 

 and subsequent to the discoveries of Dalton, they are undoubt- 

 edly entitled to the preference. We should, however, remark, 

 that Laplace and Biot have made use of the ratio of 5 to 7 in 

 their barometrical investigations. 



Of Air containing aqueous Vapour* 



The chamber V containing perfectly dry air of an elasticity 

 equal to the pressure of the gravitatmg weight P, secure the 

 latter by means of the clamp T, and effect a communication 

 between the dry air and water of the reservoir W, by unscrew- 

 ing the stopper S. 'f Oil i i^nij')!. -m\ tiff 10 a ;du ^f 5J 



\Z2.) Vapour of a force detenMifi^'d'^dld^bytH^'^etiipferattire 

 of the air (supposed to be uniform), rising frOtn th'e aquebus 

 fluid, will gradually, but not instantaneously, diffuse itself 

 uniformly within the space occupied by the air, precisely the 

 same as in a vacuum. Arrived at the point of extreme satura- 

 tion, the height of the water, slightly reduced by evaporation. 



