36 THE CHEMISTRY AND PHYSICS OF THE CELL 



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

 very slowly into each other. Hence, in the cell the colloids are quite 

 fixed in their positions, whereas the crystalloids may wander about 

 freely, and tliis arrangement is certainly of great importance in bio- 

 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 

 much evidence to show that thej^ 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 solution. If, for example, sodium 

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

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

 ever, but rather exists as a suspension of ver}^ 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 by the proteins. Substances thus finely chvided w^ill have 

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

 edly undergo changes with considerable rapidity and facility, although 

 not in solution. 



Precipitation and Coagulation of Colloids. — Because of the 

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

 sions, their persistence in solution is generally in a precarious con- 

 dition. Relatively 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 under- 

 go spontaneous coagulation on standing for some time, and agitation 

 rapidly produces the same effect in many protein solutions. Some 

 inorganic colloids are as readily coagulated as the proteins. A com- 

 paratively small rise in temperature, less than to 50° C. with some 

 proteins, renders the protein perfectly insoluble. Furthermore, we 

 have coagulation of protein solutions by enzyme action. The inor- 

 ganic "colloidal suspensions" may be precipitated by the addition of 

 very small quantities of electrolytes. Colloidal solutions of the tj'pe 

 of the proteins are not so readily jireciintaled by most clectrolj'tes, 

 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 ammonium sul- 

 phate and other salts. The effect of heat upon different colloids is 



