' 

 i8 CONDITIONS OF STABILITY 



mostly in an alteration from a state of high dispersion to one of 

 lesser subdivision and of slight stability which in the end brings 

 the colloid or crystalloid impurities in the solution into activity, 

 so that part of the disperse phase is precipitated. This effect is 

 known as the ageing of the colloid ; and the fact that the 

 response of the colloid to any action often continues to appear 

 for some time after the action has ceased, the hysteresis of the 

 colloid. This effect occurs equally with a stabilising or 

 destabilising influence. 



Sols of metallic hydroxides, such as ferric hydroxide sol or 

 eerie hydroxide sol, show on ageing a continuous decrease in 

 viscosity and an increase in electrical conductivity.* As no 

 change in the concentration of electrolytes can be demonstrated, 

 a gradual decrease in the hydration of the particles must be 

 assumed to occur with lapse of time. This effect is also noticed 

 in another research on hydroxide gels.f 



Ageing effects are very important when dealing with proteins. 

 Serum, which has been freed from insoluble globulins by pro- 

 longed dialysis and subsequent nitration, must be allowed to 

 stand for months until the further precipitation of globulin, 

 which invariably occurs, is complete, and the serum is quite 

 clear. The effect of storage and the changes in activity which 

 depend on it are particularly important in the colloidal toxins 

 and antitoxins. 



Graham used the term sol for a colloid system in the liquid 

 state, while the name gel is applied when a partial or complete 

 separation of the disperse phase has occurred, or when the 

 colloid has set as a whole to a jelly. The term gel is, however, 

 very difficult to define accurately, and is used in many different 

 senses in the literature. A closer consideration of the gels, which 

 we would differentiate as systems with a structure from the 

 jellies, in which a demonstrable structure is not assumed and 

 which correspond to the sols^Lottermoser), will follow later. 



In the following chapters we shall again make use of the 

 views on the classification and alteration of state of colloids 

 which we have explained above. 



* A. Fernau and Wo. Pauli, Kolloid Zeitsch., 1917, 20, 20. 

 f Wo. Pauli and J. Matula, Kolloid Zeitsch., 1917, 21, 49. 



