METHODS OF ELECTRICAL STIMULATION 15 



Vapor pressure. Were it not for the attrac- 

 tive or cohesive force just mentioned, the mole- 

 cules of a liquid would rapidly pass into the 

 space surrounding the liquid and the liquid 

 would soon disappear (evaporation). But the 

 molecules are so close together that their attrac- 

 tion for each other largely prevents escape. 

 Nevertheless, the motion of many of the mole- 

 cules at or near the surface of the liquid is 

 sufficient to break through this force of cohesion. 

 These molecules escape and their places are 

 taken by molecules which may in turn escape. 

 Thus evaporation proceeds. In the vapor above 

 the liquid the escaping molecules collide with 

 other molecules and may be driven by the recoil 

 back into the liquid. If the liquid be in a 

 confined space, the number of vapor molecules 

 recoiling into the liquid will increase, partly 

 because they are brought nearer together and 

 collisions are thus more frequent and partly 

 because the molecules rebound from the con- 

 taining wall. When the number of molecules 

 returning equals the number leaving the liquid, 

 the vapor and the liquid are in equilibrium 

 (saturation). In other words, the vapor tension 

 or pressure or force with which the liquid mole- 

 cules tend to leave the liquid is balanced by the 

 gas pressure or force with which the gas mole- 



