MOLECULAR MIXTURES 235 



those of the vapor and the gas. As the piston is moved 

 into the cylinder the gas pressure rises. (The vapor 

 pressure does not change at constant temperature.) 

 Some of the gas molecules are thus forced into the 

 liquid. At any instant and for any temperature there 

 will be a definite number of them in each c. c., and this 

 amount of dissolved gas is proportional to the " partial 

 pressure" of the gas. 



The surface of the liquid, which looks so smooth 

 and continuous, is formed by moving molecules. There 

 are always spaces through which a gas molecule may 

 penetrate. The volume thus formed by the inter- 

 spaces of the liquid molecules communicates at the 

 surface with the volume occupied by the "undis- 

 solved" molecules of the gas. On the average the 

 effect is that of two communicating vessels for the 

 gas, one free from obstructions and the other much 

 reduced in actual volume because of obstructions. 

 That the latter are moving makes no difference on the 

 average. The pressures in the two vessels are equal, 

 for otherwise more molecules would pass from one 

 vessel than enter it from the other. Since the pres- 

 sures are the same, the number of molecules per c. c. of 

 free space will be the same and will increase directly 

 with an increase in pressure. 



According to this picture the molecules of the dis- 

 solved gas behave exactly like the gas molecules which 

 we discussed in arriving at the equation PV = RT. 

 In applying this it is usual to measure the volume in 

 liters (1000 c. c.) instead of c. c. and hence the value of 

 R must be 1/1000 of its former value. We are of 

 course dealing with 1 mole of the gas, and hence if the 



