THE COLLOIDAL STATE 



93 



properties of Bence Jones protein, found that in the cold the precipitating effect 

 of certain ions is very marked. 



The mechanism of the action of electrolytes must clearly be related to the 

 neutralisation of the electric charge on the colloidal particles by the opposite 

 charge on the precipitating ion. It was thought at one time that the charged 

 colloidal particles were kept in suspension by the mutual repulsion of their similar 

 charges ; in such a case, their stability should be least at the exact neutralisation 

 point and, when excess of the precipitating electrolyte is added, so as to give the 

 particles a new charge of the opposite sign to their original one, the condition 

 should also be a stable one. Although in many cases this seems to be the case, 

 in others the maximum stability has been found not to be exactly at this point. 

 The presence of the Helmholtz double layer puts theoretical difficulties in the way 

 of accepting the mutual repulsion of particles as being directly responsible for their 

 permanent suspension. However this may be, it is clear that the presence of 



FIG. 42. AGGREGATION BY ELECTROLYTES. Photo-micrographs of blood 

 corpuscles of Scyllium canicula, suspended in half-normal sodium 

 chloride. 



A, effect of addition of 0'0008 molar cerium chloride. 



B, that of 0'08 molar cerium chloride. 



The dilute trivalent ion causes aggregation hy reversing the sign of the charge of a part 

 only of the corpuscles. The concentrated solution causes rapid reversal of the charge 

 to the positive sign on all the corpuscles together. 



(After Mines.) 



electric charges of the same sign is likely to be a hindrance to their mutual 

 coalescence*. 



The mechanism of the precipitation by electrolytes is well illustrated by the 

 following experiment by Mines ( 1 91 2, p. 227). The blood corpuscles of Scyllium are 

 agglutinated (aggregated) by cerium chloride in a concentration of O'OOOS molar, 

 as shown in Fig. 42. In a concentration of 0'08 molar they remain in suspension. 

 Tested by their direction of migration in an electric field, the corpuscles are found 

 to have a negative charge, when in sodium chloride of a strength corresponding to 

 that of the blood plasma of the fish. In the strong cerium solution the charge is 

 completely reversed, and the corpuscles are electro-positive. When the dilute 

 solution is added, certain of them have their charge reversed before others ; these 

 positive ones will unite with negative ones, forming aggregates large enough to fall 

 rapidly under the influence of gravity. 



The fact that excess of electrolyte does actually reverse the sign of the charge on particles 

 can be investigated by an apparatus on the plan of that of Ridsdale Ellis (1912, p. 339) which, 

 by the use of non-polarisable electrodes, avoids the production of gas and other troublesome 

 electrolytic disturbances. 



