as applied to Gases and Vapours. 533 



Section V. Of the Elasticity of Vapour in contact with the same 

 Substance in the Liquid or Solid State. 



(29.) As the most important phsenomena of evaporation take 

 place from the liquid state, I shall generally use the word liquid 

 alone throughout this section in speaking of the condition op- 

 posed to the gaseous state; but all the reasonings are equally- 

 applicable to those cases in which a substance evaporates from 

 the solid state. 



(30.) In considering the state of a limited space, entirely oc- 

 cupied by a portion of a substance in the liquid form, and by 

 another portion of the same substance in the form of vapour, 

 both being at rest, the most obvious condition of equilibrium is, 

 that the total elasticity of the substance in each of the two states 

 must be the same, that is to say, 



P=i'o+/(I>o)=;'.+/(D,), .... (33) 



where Pq represents the superficial atomic elasticity in the liquid 

 state, p^ that in the gaseous state, and/(DQ), /(l)j) the corre- 

 sponding values of the pressures, positive or negative, due to 

 mutual actions of distinct atoms. 



(31.) A second condition of equilibrium is, that the super- 

 ficial elasticities of two contiguous atoms must be equal at their 

 surface of contact. Hence, although there may be an abrupt 

 change of density at the bounding surface between the liquid 

 and the vapour, there must be no change of superficial atomic 

 elasticity except by inappreciable degrees ; and at that bounding 

 surface, if there is an abrupt change of density (as the reflexion 

 of light renders probable), there must be two densities corre- 

 sponding to the same superficial atomic elasticity. 



(32.) A third condition of equilibrium is to be deduced from 

 the mutual attractions and repulsions of the atoms of liquid and 

 of vapour. In a gas of uniform density, those forces, acting on 

 each individual particle at an appreciable distance from the 

 bounding surface, balance each other, and have accordingly been 

 treated as merely afiecting the total elasticity of the body by an 

 amount denoted by /(D) ; but near the bounding surface of a 

 liquid and its vapour, it is obvious that the action of the liquid 

 upon any atom must be greater than that of the vapour. A force 

 is thus produced which acts on each particle in a line perpendi- 

 cular to that bounding surface, and which is probably attractive 

 towards the liquid, very intense close to the bounding surface, 

 but inappreciable at all distances from it perceptible to our 

 senses. Such a force can be balanced only by a gradual increase 

 of superficial atomic elasticity in a direction towards the liquid. 

 Hence, although at perceptible distances from the surface of the 



