CHAPTER VII 

 ABSORPTION 



SECTION I. PRELIMINARY PHYSICO-CHEMICAL DATA. 



Imbibition, or molecular imbibition, is the term applied to the en- 

 trance 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 epidermic 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 comparatively 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 solu- 

 tion. This phenomenon is called osmosis. E.g., a membrane of ferro- 

 cyanide of copper, nearly impermeable to cane-sugar, can be formed 

 in the pores of an unglazed porcelain pot by allowing potassium ferro- 

 cyanide 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, 



* 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 molecular weight. 

 Thus, the gramme-molecular weight of sodium chloride (NaCl) is 58-36 

 grammes, and of cane-sugar (Cj^H^Oj!), 342 grammes. 



420 



