MOLECULAR MIXTURES 237 



as a whole and that it is permeable by the molecules of 

 the solvent, but impermeable to those of the solute. 

 The net pressure exerted on the membrane by the dis- 

 solved molecules is of value p\ p 2 , acting from left to 

 right if pi is greater than p z . The membranous piston 

 will then move toward the right. As it does, mole- 

 cules of the solvent pass through it, so that the 

 number on the left increases while that on the right 

 decreases. Since the number of molecules of the solute 

 on each side of the piston remains fixed, the concentra- 

 tion on the left decreases and that on the right in- 

 creases. 1 This will cease when the concentrations 

 have been altered to such an extent that pi p2 = 0, 

 that is, when Ci = c^. 



In the case we have discussed there is no external 

 opposition to the motion of the membrane, which will 

 move into its final position without there being any 

 work done. The case is similar to that of an expand- 

 ing gas. If the two vessels in Joule's experiment 

 (cf. page 219) were separated by a frictionless and 

 weightless piston the piston would have moved with- 

 out any work being done. It is only when a decrease 

 in the concentration of the gas molecules takes place 

 against an external force that work is done. Suppose, 

 however, that we turn the vessel of Fig. 28 on end 

 and oppose the motion of the piston by weights. The 

 piston will move just as before, supposing that the 

 weights are constantly reduced, being always just a 



1 It is customary to speak of this pressure which the molecules 

 of a solute exert as the osmotic pressure of the solution. If the 

 membrane is withdrawn the concentrations would be made alike 

 by diffusion. In other words, osmotic pressure is the agency causing 

 diffusion. 



