LEWIS. 



THERMAL PRESSURE. 



147 



In general, therefore, an increase in the total pressure upon a liquid 

 will cause an increase in the vapor pressure, and the ratio of the two 

 changes will be the ratio of the specific gravities of lic^uid and vapor. 

 This relation, which has been stated before in several different forms, has 

 an importance which has been hitherto overlooked, probably because, 

 under ordinary circumstances, the calculated effect upon the vapor pres- 

 sure has been too small to be measurable, and also because the result 

 has usually been obtaiued by assuming the applicability of the gas law to 

 the vapor, thus making the result seem only an approximation. 



Although ordinarily the magnitude of the effect is extremely small, in 

 some cases it must be of considerable significance. Equation (1) shows 

 that the influence of external pressure upon the vapor pressure depends 

 upon the relative densities of vapor and liquid. Therefore, for liquids of 

 high molecular weight, as a rule, the effect will be considerable, and also 

 in the case of a liquid whose vapor is under high pressure. But espe- 

 cially must the effect be considered in the study of critical phenomena 

 and the influence of foreign substances in the determination of the critical 

 constants, for in the region about the critical point the densities of liquid 

 and vapor approach identity. The proof of equation (1) given above 

 shows that its validity rests upon no assumption as to the specific nature 

 of the two phases considered. The probability is immediately suggested 

 that equation (1) is simply a special statement of a general law applying 

 to all heterogeneous equilibrium. That this is true may be shown in the 

 following way. 



In figure 2, let A B C and A' B' C represent 

 two similar enclosures. In the first, Xi and 

 Xj are two different phases of a simple sub- 

 stance X ; Pi and P2 are the pressures exerted 

 by an inert gas on the two sides and are such 

 that equilibrium exists ; B is .1 membrane per- 

 meable only to the vapor of X. The specific 

 volumes in phases Xi and X2 are o-j and o-o 

 respectively. The enclosure A' B' C is the 

 same as A B C except that here there is another 

 state of equilibrium in which the pressures 

 upon the phases Xi and X2 are P^ -\- d P^ and 

 P2 + dPo, and the specific volumes are ai — dcri 

 and (To — da-o respectively. 



Now let the following process occur reversi- 

 bly and isothermally : (1) One gram of Xi is 



FiGURK 2. 



