CHAPTER V 

 ABSORPTION 



Physical Introduction. Imbibition, or molecular imbibition, is the 

 term applied to the entrance of liquid into a colloid, without the loss 

 of its properties as a solid, when no preformed capillary spaces are 

 present. The entrance of water into a piece of gelatin, or an epi- 

 dermic scale, is an example of molecular imbibition. Most animal 

 and vegetable tissues possess this property, which is believed to be 

 of importance in such physiological processes as absorption, secretion, 

 and the excretion of water from the lungs and skin. The process by 

 which liquid passes into a solid with preformed capillary spaces- - 

 e.g., a sponge is sometimes spoken of as capillary imbibition. 



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 compara- 

 tively 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 mole- 

 cular 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 ' os- 



* The molecular concentration is strictly denned 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 mole- 

 cular weight. Thus, the gramme-molecular weight of sodium chloride 

 (NaCl) is 58-36 grammes, and of cane-sugar (C 12 H^O n ), 342 grammes. 



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