160 COMPOUNDS OF THE PROTEINS 



Similar results have been obtained with a variety of other colloids, 

 Electronegative colloids are precipitated, if at all, by cations; electro- 

 positive colloids by anions. 



Whetham explained these phenomena in the following way: He 

 assumes that each colloidal particle carries ah electrical charge, a 

 corresponding and opposite charge being induced upon the surface of 

 the water in immediate contact with the colloid. The effect of this 

 charge is to diminish the Surface-tension at the surface separating 

 the water and the colloid, and therefore, to diminish the tendency of 

 this surface to contract. So long as the colloid is dispersed through 

 the solution in the form of minute suspended or dissolved particles 

 the surface separating the colloid and the water is very large. When 

 the colloid is flocculated and precipitated the surface is, in consequence, 

 contracted. The less the tendency of this surface to contract, 

 Whetham argues, the less will be the tendency for the colloidal particles 

 to adhere to one another and form large flocculi. 



The cations of the added electrolyte, in the case of "electronegative" 

 colloids, or the anions of the added electrolyte, in the case of " electro- 

 positive" colloids, neutralize, according to Whetham, the charges which 

 are carried by the colloidal particles. The electrical double layer at 

 the surface of the colloid and the water thus disappears, and the surface 

 contracts; the finely suspended colloidal particles unite to form large 

 aggregates having a less extended surface and these aggregates finally 

 become so large as to assume the properties of matter in mass, and 

 hence are carried out of solution by the action of gravity. In this 

 way the dependence of the precipitat ing-power of an electrolyte upon 

 its degree of ionization, and also the reversal in the relative precipitat- 

 ing-powers of the ions of the added electrolyte upon reversion of the 

 sign of the electrical charge presumed to be carried by the colloid, 

 found an explanation. In interpreting the Valency Rule discovered 

 by Schultz, Whetham develops his theory as follows : 



" In a solution where ions are moving freely, the probability that an 

 ion is at any instant within reach of a fixed point is, putting certainty 

 equal to unity, approximately represented by a fraction proportional 

 to the ratio between the volume occupied by the spheres of influence 

 of the ions and the whole volume of the solution and may be written 

 as A C, where A is constant and C represents the concentration of the 

 solution. The chance that two such ions should be present together 

 is the product of their separate chances, that is (AC) 2 . Similarly the 

 chance for the conjunction of three ions is (AC) 3 , and for the con- 

 junction of n ions is (AC) n ." 



"In order that three solutions containing trivalent, divalent and 

 univalent ions respectively should have equal coagulative powers, 

 the frequency with which the necessary conjunctions should occur must 

 be the same in each solution. We should then have, the constant being 

 assumed equal in each case: 



. 2n~ 2n 3n~ 3n . 6n~ 6n 



A Ca = A C 2 = A Ci a constant = B 



