COLLOIDS 37 



inp: up of a fluid hy a solid body witliout chemical change. Not 

 all colloids possess this i)roperty, but it is shown by most of the 

 organic colloids, particularly the proteins. Fick distinguishes cap- 

 illary, 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 imbibi- 

 tion of water by colloids is more than a simple mechanical process, 

 for it is accompanied by a contraction in the total volume of solid 

 iiiid water, and by the evolution of lieat. The forces developed are 

 far greater than those of osmotic pressure ; e. g., to prevent imbibi- 

 tion of water by starch requires a pressure of over 2500 atmospheres. 

 On the other hand, the ]^hysical properties of an aqueous colloidal so- 

 lution show that the colloid is not chemically combined in the form 

 of a hydrate. To describe this peculiar relation Hofmeister and Os- 

 wald recommend the term "mechanical affinity." Hardy has shown 

 that water held in a gelatin jelly cannot be removed by g-reat pres- 

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

 jelly is so changed by formalin that it is no longer 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 phe- 

 nomenon of colloids is very important in physiological and patho- 

 logical processes. 



Non-diffusibility. — The lack of power to pass through animal and 

 parchment membranes, wiiich was Graham's starting-point in the 

 study of colloids, is also only a relative condition. 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 2.3.3 



Sugar 7.00 



Magnesium sulphate 7.00 



Protein 49.00 



Caramel 98.00 



This difference of time is so great, however, as to pennit of separation 

 of salts from proteins, etc., by dialyzation, a process in constant u.se. 

 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 colloidal 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 readily 

 and permeate diffusion membranes very slowly. For example, if 



