IN PROTEIN SOLUTIONS 15 



the disperse phase. This conception is quantitatively worked 

 out by H. Freundlich in his theory of adsorption of ions. 



When a complex salt is dealt with (as, for example, most 

 metallic hydroxide sols), two kinds of electric neutralisation, 

 by electrolytes take place : (i) suppression of ionisation and 

 reduction in solubility due to a considerable quantity of added 

 ions, and (2) low solubility product of one ion, which precipi- 

 tates in very low concentrations. The action of chlorides, 

 nitrates, etc., on ferric hydroxide sol or eerie hydroxide sol is 

 an example of the first, while that of oxalates, tartrates, sul- 

 phates, etc., is an example of the second method of neutralisa- 

 tion.* Analogous behaviour is found with proteins. 



Bredig's theory of the process of precipitation is most widely 

 known. According to this view, neutralisation removes the 

 electrostatic repulsion between the particles, and owing to the 

 tendency of surface energy to sink to a minimum, a reduction 

 in surface occurs by the particles joining together. It cannot 

 be doubted that this is an important factor in the process of 

 precipitation. In particular, the fact that there is a power of 

 association between the neutral particles in a molecular solution 

 must not be overlooked. It comes into play in the formation 

 of precipitates and in crystallisation, and hence can scarcely be 

 neglected in the case of colloids. But our knowledge in this 

 direction is very slightly developed, and any precise connec- 

 tion between surface tension and molecular association is 

 lacking, though both depend on chemical constitution. Develop- 

 ments in this border-line field are likely to come in large part 

 from the study of colloid chemistry. 



Zsigmondy, again, considers the question of whether a dried 

 colloid is again dispersed by mere addition of the dispersion 

 medium as a basis of classification. Colloids which redisperse 

 he calls resoluble ; those that do not, irresoluble. This criterion 

 is also useful up to a point in facilitating the general compre- 

 hension of colloidal behaviour. The extreme tyophobe colloids 

 are irresoluble, while typical hydro-colloids are resoluble. The 

 redispersion of a dried colloid is conditioned by the ionising 

 power of the continuous medium, and the tendency of the 



* See Wo. Pauli and J. Matula, loc. cit. 



