148 PHYSIOLOGY 



quantitative relations of its components, but also by the past history of the 

 system. 



COMBINATIONS BETWEEN COLLOIDS 



Besides the compounds between colloids and electrolytes, combination, 

 or at least interaction, takes place between different colloids. Many colloids 

 are precipitated by other colloidal solutions. This effect is always found to 

 occur when the colloidal solutions carry different charges. Thus ferric 

 hydrate in colloidal solution is precipitated by colloidal silica or colloidal 

 gold, both colloids being thrown out of solution. On the other hand, certain 

 colloids may exercise a protective influence on other colloidal solutions. 

 Thus, as Faraday first showed, colloidal gold is much more stable in the 

 presence of a little gelatin. The colloids of serum can dissolve a considerable 

 amount of purified globulin. Although the latter in solution shows a drift 

 in the electric field, the resulting solution is quite unaffected by the passage 

 of a current through it. In these cases the protective colloids carry no 

 charge, but the same protective effect may be observed if a large excess 

 of, e.g. an electro-positive colloid be added to an electro-negative colloid. 

 This interaction between different colloids probably plays an important 

 part in many physiological phenomena. We have reason to believe that 

 the reactions between toxin and antitoxin, and between ferment and sub- 

 strate, which we shall study later, are of this character, and that the 

 compounds formed belong to the class of adsorption combinations. 



THE COAGULATION OF COLLOIDS 



Most colloidal solutions are unstable, and the relations between the 

 suspended particle or molecule and the surrounding fluid may be upset by 

 slight changes of reaction or the presence of minute traces of salts. As a 

 result the hydrosol is destroyed, the suspended particles aggregating to 

 form larger complexes. These aggregations may settle to the bottom of 

 the fluid as a precipitate, or may form a species of network, the result 

 varying according to the nature of the colloid and its concentration. Thus 

 gelatin changes from the condition of hydrosol to hydrogel with fall of 

 temperature. The same is true of agar. On the other hand, by adding 

 calcium chloride to an alkaline solution of casein, we obtain a mixture which 

 sets to a jelly on warming, but becomes fluid again on cooling. Other agen- 

 cies may lead to the production of changes which are irreversible. Thus 

 a strong solution of colloidal silica sets to a solid jelly on the addition of 

 a trace of neutral salt, and it is not possible to reform the hydrosol, however 

 long the jelly is submitted to dialysis. 



Two methods of bringing about coagulation of hydrosols deserve special 

 mention. The first of these is heat-coagulation. If a solution of egg- 

 albumin or globulin be heated in neutral or slightly acid medium and in the 

 presence of neutral salt, the whole of it is thrown down in an insoluble form. 

 This coagulated protein is insoluble in dilute acids or alkalies. The same 

 coagulative effect of heating is observed in the metallic sols. With con- 



