280 Mr. Ivory on the Constitution of the Atmosphere. 

 moist air, will be reduced to p . -w ; so that the whole elasti- 

 city of the moist air will be p . -^ + w. But the pressure of 



the external air upon the volume V of moist air is evidently 

 equal to /) + ot, which is greater than the elasticity of the 

 mixture : wherefore, in an atmosphere at rest, the volume of 

 moist air would not be permanent in its figure but would con- 

 tract its dimensions till the external and internal elasticities were 

 reduced to an equality. If we next suppose that v is greater 

 than V, it is obvious that the elasticity of the moist air would 

 be greater than jy + ct ; and in consequence the volume V 

 will expand in order to produce an equilibrium with the ex- 

 ternal air. The only remaining supposition is, that v is equal 

 to V ; or that every infinitesimal volume in the moist atmo- 

 sphere contains equal volumes of dry air and vapour; which 

 supposition evidently satisfies the conditions of equilibrium, 

 the elasticity of the air within the partial volume and the 

 pressure of the external air being alike equal to p-{-'UT. 



The same conclusion might have been deduced from the 

 theory of the diffusion of gases and vapours. For if vapour 

 be added to a quiescent atmosphere of dry air, it will diffuse 

 itself through the dry air, till every partial volume of the mix- 

 ture is at rest by the mutual action of the elasticities of the 

 two fluids. 



It now remains to determine the relation between the den- 

 sities of the same volume in the dry and moist atmospheres ; 

 which v/ill enable us to ascertain the proportion between the 

 weights of a volume of the two atmospheres, when the pressure 

 p of the dry air and the tension ct of the vapour, are given. 

 Let p be the density of the dry air ; S that of the vapour, and 

 A that of the mixture : then, the volume being V, the sum 

 of the masses of the dry air and vapour will be equal to the 

 mass of the mixture, and we shall have 



p V + S V = A V, and A = p + S. 

 Now, the temperature being 6, the density is p when the 

 pressure is p; it will therefore be p when the pressure 



is ot: so that — p and S are the densities of dry air and va- 



P 

 pour, under the pressure p and the temperature 6. Where- 

 fore, V being any volume, the mass or weight — p V of dry 

 air, and the mass or weight S V of vapour, both under the 



