166 PHYSIOLOGY 



their particles are of varying size according to the conditions in which 

 they exist, and carry varying charges of electricity, whereas an ion 

 such as Na or 01 has a mass which is constant for the ion in question, 

 and always carries the same electric charge. The charged particles of an 

 acid- or alkali- globulin may be distinguished therefore as pseudo-ions. 

 In these adsorption combinations, although the chemical nature of 

 the colloidal molecules is concerned, there is an absence of definite 

 equilibrium points, such as we are accustomed to in most chemical 

 reactions. The inertia of the system and the large size of the molecules 

 determine the occurrence of false equilibria and of delayed reaction, 

 so that the condition and behaviour of a colloidal system at any 

 moment are determined, not entirely by the quantitative relations 

 of its components, but also by the past history of the system. 



COMBINATIONS BETWEEN COLLOIDS 



Besides the compounds between colloids and electrolytes, com- 

 bination, or at least interaction, takes place between different col- 

 loids. 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 re- 

 actions between toxin and antitoxin, between ferment and substrate, 

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

 pounds 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 par- 

 ticles aggregating to form larger complexes. These aggregations may 

 settle to the bottom of the fluid as a precipitate, or may form a species 



