* 

 16 CONDITIONS OF STABILITY 



colloid to form ions, as well as by the solvation and aggregating 

 power of the disperse particles due to their varied molecular 

 association and surface tension.* 



Having cleared the way by this discussion, we will now briefly 

 collect the facts which bear on the stability of proteins. 



Some proteins are hydrophobe, some have hydrated particles ; 

 and the conditions under which they are stable are different 

 in the two cases. Proteins of the first group exhibit Hardy's 

 law of instability at the neutral point when treated with 

 electrolytes. To this group belong heat-coagulated albumin, 

 casein, salt-free globulin, acid-albumin, etc. On the other hand, 

 glutin, natural albumin, and the other proteins of the second 

 group are stable in solution, even when the particles are not 

 electrically charged. The electric charge is, however, quite 

 an important factor in the stability of the lyophile or hydrated 

 proteins, for it is an accepted fact that only neutral particles 

 are precipitated from solution by electrolytes. Accordingly, 

 all means of displacing albumins from solution will show an 

 optimum at the point of electric neutrality, a condition which 

 will be dealt with in detail later on. Generally, the difference 

 between lyophobe and lyophile albumins lies in the fact that 

 in the first case mere discharge of the particles (e.g., by an electric 

 field) leads to precipitation, while in the second case discharge 

 must be combined with effects which lead to an alteration in 

 the relation of the particles to the solvent, so that their lyophile 

 character is reduced or lost altogether. Saturation of the 

 solution with certain alkali salts, the addition of alcohol or 

 phenols, etc., brings about the latter conditions. 



Such variations in stability as are indicated finally by a 

 visible coagulation form yet another class of changes to which 

 colloids are prone. There is no fixed nomenclature for these 

 changes, which lie on the border line of the physical and the 

 chemical, and they are termed in general the alteration of state 

 of colloids. 



Such changes are classed as reversible or irreversible coagula- 



* Some irresoluble colloids, after precipitation by excessive dialysis, 

 can be redispersed by addition of some electrolyte. This effect, often 

 called peptisation, really depends on the formation of complex salts, as 

 Zsigmondy has shown in the case of stannic acid gel. 



