Diffusion and Diffusion Tension 13 



posedly constant) increases. Thus, an equilibrium will be 

 established sooner or later, wherein the number of particles 

 escaping from the liquid in unit time will be just equaled by 

 the number re-entering it. That is, evaporation is just 

 equaled by the opposite process, condensation. This is the 

 condition when evaporation apparently ceases. The gas 

 pressure with which the liquid particles escape is termed 

 vapor tension. And when evaporation has apparently ceased, 

 the gas pressure of the vapor in the space above the liquid 

 is equal to the vapor tension which the particles exhibit in 

 leaving the liquid surface. We have thus a means for 

 measuring the vapor tension of any liquid. 



If the temperature rises, the vapor tension rises corre- 

 spondingly, following the principles of gases. If the 

 external pressure upon the supernatant mass of vapor be 

 increased, its gas pressure becomes greater than the vapor 

 tension of the liquid, and condensation surpasses evaporation, 

 thus decreasing the number of vapor particles — and hence 

 the pressure due to them — until equality of tension and 

 pressure is restored. If two such chambers in which the 

 supernatant vapor is at different pressures be connected 

 above the level of the liquid, the substance will distil over 

 and condense in the chamber which has the lower pressure. 

 This will continue until the two pressures have been equal- 

 ized by the resulting change in the relative volumes occupied 

 respectively by the two masses of liquid, and by the diffusion 

 of the vapor particles themselves. 



b) Mixed liquids. — If two different, equally miscible 

 liquids are brought into contact with each other so as to 

 form two horizontal strata, diffusion will take place in both 

 directions, just as in the corresponding case with gases, but 

 much more slowly on account of the friction and interfer- 

 ence of the particles: and there will result a uniform mix- 

 ture in which both kinds of particles are equally distributed 



