40 Tin: CHEMISTRY Ayo riivfiics of the cell 



logic 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 cytoplasm. The possibility of the correctness of this view is 

 increased when we consider that the enzymes are colloids, for there is 

 inuch evidence to show that they are distributed in just such an un- 

 even 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 solution. If, for example, sodium 

 phosphate is added to a solution of casein in lime-water, tlie calcium 

 phosphate formed does not precipitate. It is not in solution, how- 

 ever, 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, in- 

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

 suspension b3' the proteins. Substances thus finely divided will have 

 extremely large surface area for reactions, and, therefore, will un- 

 doubtedly undergo changes with considerable rapidity and facility, 

 although not in solution. 



Precipitation and Coagulation of Colloids. — Because of the rather 

 slender marghi by which the colloids are separated from the suspen- 

 sions, their persistence in solution is generally in a rather precarious 

 condition. Kelatively slight changes suffice to throw the colloids out 

 of solution, and when once precipitated, they are often incapable of 

 again dissolving in the same solvent. Solutions of albumin may un- 

 dergo spontaneous coagulation on standing for some time, and agita- 

 tion rapidly produces the same effect in many protein solutions. 

 Some inorganic colloids are as readily coagulated as the proteins. 

 A comparatively small rise in temperature, less than to 50° C. with 

 some proteins, renders the protein perfectly insoluble. Further- 

 more, we have coagulation of protein solutions by enzyme action. 

 The inorganic "colloidal suspensions" may be precipitated by the 

 addition of very small quantities of electrolytes. Colloidal solutions 

 of the type of the proteins are not so readily i)recipitated by most 

 electrolytes, but if to the solution large quantities of crystalloids are 

 added, the protein molecules are practically crowded out of solution, 

 as in the "salting-out" process used in separating proteins by am- 

 monium sul])liate and other salts. Tlie effect of heat U])on different 

 folloids is ])ecu]iar, in that some vai'ieties, a.s silicic acid, aluminium 

 liydrate, and many proteins are rendcrcM] so insolnble tliat they can- 

 not again be dissolved in any fluid williont lii-st l)eing modified in 

 some way; whereas colloids of the type of gelatin and agar are made- 



