﻿Electrolytes on Colloidal Solutions. 473 



electric field, and consequently the sign of the charge on the 

 particles, depends on whether the liquid in which it is sus- 

 pended is acidic or alkaline. If acidic, the particles are 

 positively charged ; if alkaline, the particles are negatively 

 charged." At the point (called the isoelectric point). 

 where the sign of the charge changes, coagulation of the 

 albumen took place. From analogy with this phenomenon, 

 the generally accepted theory or the coagulation of all 

 colloidal particles by electrolytes has been proposed : namely, 

 that the particles have their charges neutralized by the 

 absorption of the oppositely charged ions of an electrolytic 

 solution and, at the isoelectric point, where the charge becomes 

 zero, the colloid coagulates. 



Following out this idea, AVhetham * has given an expla- 

 nation of the remarkable valency relations of the coagulative 

 powers of electrolytic solutions. On the supposition " that 

 in order to produce the aggregation of colloidal particles 

 which constitutes coagulation, a certain minimum electro- 

 static charge has to be brought within reach of a colloidal 

 group, and that such conjunctions must occur with a certain 

 minimum frequency throughout the solution,*" he deduces 

 that the coagulative powers of equivalent solutions containing 

 monovalent, divalent, and trivalent ions respectively would 

 be in the ratio of 1 : x : x 2 , where x is an arbitrary constant. 

 If a? is put equal to 32 this ratio reproduces, in a wonderfully 

 exact way, the experimental numbers of Linder and Picton : 

 the results, calculated and observed, are respectively — 

 1 : 32 : 1024 and 1 : 35 : 1023. An apparent contradiction 

 of this view* is found in the recent work o£ Linder and 

 Picton f. who observe that solutions containing tetravalent 

 metallic ions (Pt and Zr) possess very feeble coagulative 

 power towards colloidal arsenious sulphide ; whereas, 

 according to the above theory, solutions of these metals 

 should have a coagulative power 30,000 times that of an 

 equivalent solution of a univalent ion. An explanation of 

 this fact is suggested later in the present paper. 



Correlating Hardy's results with the Lippmann pheno- 

 menon regarding the connexion between surface tension and 

 potential difference, Bredig % suggested a very plausible 

 theory as to the precise causes bringing about coagulation. 

 The surface tension of mercury in contact with an electrolytic 

 solution reaches a maximum when the potential difference 

 between the two phases is zero. If we assume that the 



* Theory of Solutions, p. 396. 



t Journ. of Cheiri. Sec. vol. lxxxvii. p. 1906. 



X Anorganische Fermentc, Leipzig- (1901). 



