THE CHEMISTRY AND PHYSICS OF THE CELL 45 



the taking up of a fluid by a solid body without chemical 

 change. Not all colloids possess this property, but it is shown 

 by most of the organic colloids, particularly the proteids. Fick 

 distinguishes capillary, osmotic, and molecular imbibition, the 

 latter of which is the form exhibited by colloids, and it occurs 

 independent of the existence of pores or other preformed spaces 

 in the imbibing body. The imbibition of water by colloids is 

 more than a simple mechanical process, for it is accompanied 

 by a contraction in the total volume of solid and water, and by 

 the evolution of heat. On the other hand, the physical proper- 

 ties of an aqueous colloidal solution show that the colloid is 

 not chemically combined in the form of a hydrate. To describe 

 this peculiar relation Hofmeister and Oswald recommend the 

 term " mechanical affinity. " Hardy has shown that water 

 held in a gelatin jelly cannot be removed by great pressures 

 (400 pounds to the square inch), but after the nature of the 

 jelly is so changed by formalin that it is no more liquefiable by 

 heat, the water can be easily expressed from the loose meshwork 

 that is formed. It would seem from this that the imbibition 

 and retention of water by colloids may be closely related to 

 surface phenomena. Hofmeister has shown that organized 

 animal tissues obey the same laws of imbibition as do simple 

 gelatin plates, and probably this phenomenon of colloids is 

 very important in physiological processes. 



Non=diffusibility. The lack of power to pass through 

 animal and parchment membranes, which was Graham's start- 

 ing-point in the study of colloids, is also only a relative condi- 

 tion. This is shown by the following figures giving the relative 

 time required by the same amount of different substances to 

 pass through a certain diffusion membrane : 



Sodium chloride 2.33 



Sugar (< 7.00 



Magnesium sulphate 7.00 



Proteid 49.00 



Caramel , 98.00 



This difference of time is so great, however, as to permit of 

 separation of salts from proteids, etc., by dialyzation, a process 

 in constant use. Primarily the ability to diffuse through a 

 given membrane requires that the diffusing substance be soluble 

 in the membrane. Diffusion membranes are always composed 

 of colloids, e. g., animal bladders, or parchment, which is a col- 

 loidal cellulose. Crystalloids are generally soluble in colloids, 

 while colloids are little or not at all soluble in other colloids, 

 and hence do not diffuse through one another and therefore they 



