IMPORTANCE OF WATER 



105 



Fig. 41. — A dialyzer. 



soids and suspensoids. A large number of organic colloids belong 

 to the first group; the majority of inorganic colloids belong to the 

 second. Water solutions of emulsoids are very stable. When 

 dried, they absorb water with great force, and thus may be readily 

 dissolved again. They are called, therefore, hydrophilous or lyo- 

 philic colloids. Once having been dried, solutions of suspensoids, 

 on the contrary, cannot be restored to the original state. 



Not being molecular but micellar dispersed systems, solutions 

 of colloidal substances differ considerably from true solutions. 

 They have been called, therefore, pseudo-solutions, or sols. A 

 most conspicuous difference is their relation to highly porous plant 

 or animal membranes, such as bladder or parchment paper. 



Colloidal sols are not able to pass 

 through membranes, as their large 

 micellae are retained in the pores, 

 while true solutions pass readily. Upon 

 this difference depends the well-known 

 method of dialysis, by which colloids 

 may be separated from crystalloids. A 

 vessel covered with a bladder mem- 

 brane or with parchment paper is filled with the liquid to be dia- 

 lyzed and is then immersed in another vessel in which the water is 

 continually renewed. The crystalloids diffuse through the bladder 

 and are carried away by the water current. The colloidal 

 solution remains in the dialyzer (Fig. 41)., 



Another difference between crystalloids and colloids lies in 

 their property of lowering the freezing point of the solvent. In 

 true solutions this lowering is very marked. Thus with water 

 solutions of non-electrolytes it may be as high as 1.85° C. for every 

 gram-molecule per liter of solvent. In colloidal solutions, where 

 the particles of the dispersion phase are large and their number 

 relatively small, the lowering of the freezing point is usually so 

 insignificant that it cannot be conveniently measured. 



Separated from the solvent, the molecules of the dissolved sub- 

 stance usually form structures of regular crystalline nature, where- 

 fore these substances have been called crystalloids. Not so with 

 colloids. Separated from water, their micellae turn into amorphous 

 bodies termed "gels." They always retain more or less water or 

 any other substance that served as the dispersion medium for the 

 colloid. The structure of gels is to a certain degree the inverse 



