﻿478 Action of Electrolytes on Colloidal Solutions. . 



aluminium sulphate are added to the colloidal solution, it 

 appears that all of the aluminium ions go to decrease the 

 charge on the particle, and when aluminium is added in 

 quantities just sufficient to neutralize that charge, coagulation 

 of the particles is most rapid. When, however, the electro- 

 lyte is added at once in excess of this quantity, the particles 

 act as absorbers of the metallic ions, and the charge on the 

 particle is thus changed at once from a large negative one to 

 a large positive ; this positive charge on the particle induces 

 the same surface tension effects as the negative charge, and 

 so maintains the colloidal particles in the state of fine 

 subdivision. 



In this action may lie the explanation of Linder and 

 Picton's results * on the coagulative power of zirconium and 

 platinum mentioned above (p. 473). For the experiments 

 detailed in the present paper the largest quantity of aluminium 



N 

 sulphate added to the colloidal solution was 10 drops 



per 40 c.cs. of colloid. If the tetravalent metals are some 

 30 times as powerful as the trivalent (as the theory of 

 Whetham demands), it is quite probable that Linder and 

 Picton missed the isoelectric point altogether by adding much 

 too large a quantity of the zirconium and platinum ions 

 at first. 



6. Conclusion. 



The following are the points dealt with above : — 



1. The addition of small quantities of aluminium sulphate to 



colloidal solutions of silver and gold has been shown to 

 bring about a decrease in the charge of the particle and 

 even a reversal of the sign of that charge. 



2. The existence of an isoelectric point has been demonstrated, 



and thus the treatment of the case as analogous to Hardy's 

 egg-albumen experiment has been justified. 



3. A calculation of the size of the charge on each particle 



of silver solution has been made. 



In conclusion I wish to thank Prof. J, J. Thomson for his 

 kindly interest and helpful suggestions at all times. 



Cavendish Laboratory, 

 June 1906. 



* Loc. cit. 



