84 



HARRY C. JONES 



OSMOTIC PRESSURE AND DIFFUSION 



In concluding let me say a few words with reference to the 

 bearing of osmotic pressure on general chemistry from a somewhat 

 different standpoint. There is no property of solutions with 

 which we are more familiar than with diffusion. Whenever a 

 solution of one concentration is brought in contact with a solution 

 of a different concentration, we have the dissolved substance 

 passing over from the region of greater to the region of lesser 

 concentration, and this continues until homogeneity is established 

 in all parts of the solution, provided the temperature of all parts 

 of the solution is kept the same. 



We are so famiUar with this phenomenon that we are liable not 

 to stop and ask any questions concerning it. Yet a moment's 

 thought will show that we are deahng here with a very remarkable 

 phenomenon. Take a cylinder of water of almost any reasonable 

 height, and place on the bottom of that cylinder some very heavy 

 soluble substance, say lead nitrate, mercuric chloride or platinic 

 chloride. This heavy substance will dissolve and will rise through 

 the comparatively light water up to the top of the cylinder. 



This phenomenon differs in degree, but somewhat resembles 

 the phenomenon that would be presented by our rising directly 

 through the atmospheric air, although we are a great many times 

 heavier than the air. 



We cannot study the phenomenon of diffusion without recog- 

 nizing that we ha\'e some very powerful force at work, driving the 

 heavy dissolved substance upwards against the pull of gravity 

 through the much lighter solvent, and we must ask what is the 

 nature of this force. 



A quantitative study of the phenomena of diffusion shows that 

 they obey the laws of osmotic pressure. Fick's law for diffusion 

 states that the amount of dissolved substance that will pass 

 through a given cross section separating two solutions of different 

 concentration, is proportional to the difference in concentration on 

 the two sides of that section. This will be recognized at once as 

 nothing else than Boyle's law for gases, which states that the 

 pressure exerted between two gases brought in contact, is pro- 

 portional to the difference in concentration of the two gases. 



