48 INTR OD UCTION 



may phagocytosis. (The relation of surface tension to these 

 processes will be considered under the subject of Inflammation.) 



The effect of colloids upon chemical processes 

 going on within their solutions or gels is surprisingly small. 

 Salts in solution in a thick gel of agar or gelatin will diffuse 

 almost as rapidly as in water ; l they will also ionize as rapidly 

 as in watery solutions, and chemical reactions occur with the 

 same speed and completeness as if the colloids were absent. 

 Furthermore it makes little difference whether these processes 

 are measured in a colloid solution that is liquid, or after it has set 

 in the gel form. These facts merely indicate that the colloids 

 do not greatly impede the movements of molecules or ions in 

 solutions. On the other hand, as before mentioned, colloids 

 diffuse little or not at all into each other. Hence, in the cell 

 the colloids are quite fixed in their positions, whereas the crys- 

 talloids may wander about freely, and this arrangement is cer- 

 tainly of great importance in biologic processes. Pauli suggests 

 the probability that the fixation of the colloid causes the cell 

 to have different properties in different parts, and so various 

 reactions may occur independently in different areas of the cyto- 

 plasm. The possibility of the correctness of this view is 

 increased when we consider that the enzymes are colloids, for 

 there is much evidence to show that they are distributed in just 

 such an uneven manner within the cells. 



Although colloids permit the passage of dissolved crystalloids 

 through them, they greatly interfere with the movement of 

 larger particles. This property accounts for the ability of 

 colloids to hold many insoluble substances in such extremely 

 fine suspensions that they seem superficially to be in true solu- 

 tion. If, for example, phosphoric acid is added to a solution of 

 casein in lime-water, the calcium phosphate formed does not 

 precipitate. It is not in solution, however, but rather exists as 

 a suspension of very finely divided particles of the salt which 

 the colloid keeps from aggregating into particles large enough 

 to be visible or to overcome the viscosity of the fluid and sink 

 to the bottom. Probably in this way many substances, includ- 

 ing calcium salts, are carried in the blood, held in permanent 

 suspension by the proteids. Substances thus finely divided 

 will have extremely large surface area for reactions, and, there- 

 fore, will undoubtedly undergo changes with considerable rapid- 

 ity and facility, although not in solution. 



1 The retarding influence of colloids upon diffusion has, however, been gen- 

 erally underestimated, according to the most recent researches. (See Meyer, 

 Hofmeister's Beitr., 1905 (8)," 393; Nell, Ann. d. Phys., 1905 (18), 323; 

 Flexner and Noguchi, Amer. Med., 1906 (1), 154.) 



