CHAPTER V. 

 ABSORPTION. 



Physical Introduction Diffusion. When a solution of a substance 

 is placed in a vessel, and a layer of water carefully run in on the top 

 of it, it is found after a time that the dissolved substance has spread 

 itself through the water, and that the composition of the mixture is 

 uniform throughout. The result is the same when two solutions 

 containing different proportions of the same substance, or containing 

 different substances, are placed in contact. The phenomenon is 

 called diffusion. The time required for complete diffusion is com- 

 paratively short in the case of a substance like hydrochloric acid or 

 sodium chloride, exceedingly long in the case of albumin or gum 

 In both it is more rapid at a high temperature than at a low. 



Osmosis. If the solution be separated from water by a membrane 

 absolutely or relatively impermeable to the dissolved substance, but 

 permeable to water, water passes through the membrane into the 

 solution. This phenomenon is called osmosis. E.g., a membrane of 

 ferrocyanide of copper, nearly impermeable to cane-sugar, can be 

 formed in the pores of an unglazed porcelain pot by allowing potas- 

 sium ferrocyanide and cupric sulphate to come in contact there. If 

 the pot is filled with, say, a i per cent, solution of cane-sugar, closed 

 by a suitable stopper, connected to a manometer, and then placed 

 in a vessel of water, water passes into it till the pressure indicated 

 by the manometer rises to a certain height. With a 2 per cent, 

 solution it reaches twice this height, and in general the osmotic 

 pressure, as it is called, is in any solution proportional to the 

 molecular concentration* of the solution, or, in other words, to the 

 number of molecules of the dissolved substance in a given volume 

 of the solution. If in this sentence we substitute ' gaseous 

 pressure' for 'osmotic pressure,' and 'gas' for 'solution,' we have 

 a statement of Boyle's law, which asserts that the pressure of a 

 gas is proportional to its density. And, indeed, it has been 

 shown that the osmotic pressure of the dissolved substance is the 



* The molecular concentration is strictly defined as the number of 

 grammes of the dissolved substance in a litre of the solution divided by the 

 gramme-molecular weight. The gramme-molecular weight, or gramme- 

 molecule, is the number of grammes corresponding to the molecular 

 weight. Thus, the gramme-molecular weight of sodium chloride (NaCl) is. 

 58-36 grammes, and of cane-sugar (C^H^On), 342 grammes. 



