14G 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



In order to introduce a general discussion of this problem, which leads to 

 very notable and instructive results, let us consider first a simple, special 

 case, namely, the question of the effect upon the vapor pressure of a liquid 

 caused by a change in the total pressure on the surface of the liquid. 



Figure 1 represents a ring-shaped enclos- 

 ure containing a liquid X in the part BCD, 

 and the vapor of X throughout the remain- 

 ing space. The space D E contains also an 

 infinitesimal layer of some inert and insoluble 

 gas, which is prevented from diffusing into 

 the space B A E by a membrane at E, 

 which is permeable only to the vapor of X. 

 The foreign gas thus enclosed exerts a 

 pressure upon the liquid at D and maintains 

 a difference of level, d 11. between B and 

 D*. This pressure, moreover, must have 

 an effect upon the vapor pressure of the liquid, for, on account of its 

 weight, the pressure of the vapor is greater at I) than at B, but the 

 liquid is in equilibrium with the vapor at both points, therefore the 

 vapor pressure of the liquid is greater at D than at B. If d P x represent 

 the difference in vapor pressure between B and D, and s { the specific 

 gravity of the vapor, then we may write, 



dP 1 = s i dII. 



If d P, represent the difference in the total pressure upon the liquid at 

 B and I), and s., the specific gravity of the liquid, then 



,1 P., = s ..d If. 

 From these two equations, 



dj\ 



.1 /'. 



«i 



or if -r, and <r., represent the specific volumes of vapor and liquid respec 



tively, then 



dp 



,//■ 



,T., 



<u 



0) 



■ ijection . raisi <\ to proofa of tlii- kiml ; thus, in this case, it would 



be argued thai in reality the liquid will distil from I> to the Bpace above E. Thai 

 this may happen in no iray invalidates the proof, for it ia nol necessary thai il 

 should happen. By keeping E A free from drops "f the liquid the system ia in 

 perfect equilibrium, the i quilibrium of auperaaturation. That there is another 

 more stable equilibrium possible ia ol no concern. 



