M. Poisson on the Caloric of Gases and Vajwurs. 335 



tity of heat necessary to form this quantity of vapour, the 

 water being first at zero temperature, V will be the product 

 of this number of grammes and the quantity Q, given by ( ] 0) ; 

 so that we shall have 



■y' * ^ 187-33 |-v 



The unity to which V has respect is the quantity of heat 

 necessary to elevate the temperature of a gramme of water one 

 degree, which, as we know, is 75 times that requisite to liquefy 

 a gramme of ice at zero. Consequently, if we assume this last 

 quantity to be the unity of heat, we must multiply the above 

 value for V by 75. 



In steam engines, in which this fluid is employed in a state 

 of satm-ation, Q does not sensibly vary : the ratio of V to h, 

 or of the quantity of heat usefully employed in pressure on the 

 piston, is then, all other things being alike, reciprocally as 

 266'67+fl. The higher the temperature 5, therefore, of the 

 vapour, the less will be this ratio ; and consequently the ex- 

 panse of heat will increase less rapidly than the force pi'oduced. 

 But the economy which thus results in favour of high pressure 

 engines is far infei"ior to that which experience seems to indi- 

 cate ; and it is in a less waste of heat, or in other circumstances 

 relative to their construction, that we must look for an expli- 

 cation of the advantage which they present. 



§ III. Let us suppose that we have two different gases of 

 the same temperature 6 and elasticity p ; and whose volumes 

 are v and v. Were they now put one on the other in a closed 

 vessel of the capacity v+v, it is plain they could preserve an 

 equilibrium, because the temperature is the same and the mu- 

 tual pressures are equal ; but this equilibrium would not be 

 stable. Experience proves that these gases would gradually 

 penetrate each other until they are completely intermixed. It 

 further shows that during this operation heat is neither evolved 

 nor absorbed ; so that after a certain time the mixture is per- 

 fectly homogeneous ; the two gases holding the same propor- 

 tion in every part, and the temperature and pressure being 9 

 and p. From these facts, established by observation, we may 

 deduce another equally well verified by experience. 



If two gases mixed together at the temperature S fill a vo- 

 lume V ; and if p, p denote the pressures they would sepa- 

 rately exert, separately occupying the same volume w, at the 

 same temperature 6, the pressure of the mixture will be p-¥p'. 

 In effect, let us suppose that the two gases at first are distinct, 

 and Xittp'yp; then dilating the gas under the pressure 7/ until 

 p' changes to p, its volume will become 



vp 

 ~' provided 



