266 CHEMISTRY OF THE PROTEIDS CHAP. 



however, either the kation or the anion is stronger, then the unsatisfied 

 balance of electro-affinity represents available energy which, when 

 brought into contact with colloids, will lead to the precipitation of the 

 latter, provided that the colloid has an electrical sign which is the 

 opposite to that carried by the stronger ion of the * neutral ' salt. 

 This explanation offered by the author has further in its favour the 

 fact that the electro-affinities of colloids are very feeble. 



ON ACCLIMATISATION OF COLLOIDS, AND ON THEIR 'SPONTANEOUS' 



CHANGE 



Before giving further evidence in support of the view that colloids 

 are electrolytes, it is necessary to discuss the so-called 'acclimatisa- 

 tion' of colloids and their 'spontaneous change.' (See p. 323.) 



ACCLIMATISATION. Freundlich l has stated that generally the pre- 

 cipitation of a colloid varies according to the quickness with which 

 a given quantity of salt is added. Suppose it requires the addition of 

 10 ccm. of a given salt solution to produce complete coagulation of 

 a colloid, provided the salt solution be added quickly, then after the 

 gradual addition of 5 ccm. of this salt solution it will take still 1 ccm. 

 to complete the coagulation. Freundlich conies therefore to the con- 

 clusion that in precipitating a colloid it is not a mere question of 

 adding a certain number of ions, and that we are not dealing with 

 chemical equilibria, but that the agency bringing about coagulation 

 is a time-factor, dependent on the rate of diffusion. If all the salt 

 solution is added at once, then a marked diffusion of ions is called forth, 

 owing to great differences in the concentration of the solution, and 

 correspondingly great differences of potential will be established in the 

 biphasic colloid + water system at the limiting surfaces of the two 

 phases, in consequence of which rapid ' coagulation ' will ensue. For 

 further information on the views of Freundlich, see p. 268. 



The author cannot accept this explanation, for the conversion of an 

 electro-positive or an electro-negative colloid into 'an iso-electric, i.e. 

 non-electric, non-colloidal, or insoluble substance, depends directly on 

 the abolishment of hydrolytic dissociation, while the separation of the 

 non-colloidal substance is dependent on entirely different causes. If we 

 imagine a colloidal solution containing a sufficient number of colloidal 

 particles to allow these to be in actual contact with one another when 

 they are distributed throughout the solution, it follows that these 

 particles must aggregate whenever the existing difference of potential 

 between the colloidal particles and the solvent is done away with, 



Herbert Freundlich, Zeit.f. physik. Chem. 44. 129 (1903). 





