﻿474 Mr. E. F. Burton on the Action of 



colloidal particles are kept in their state o£ fine subdivision in 

 the liquid medium by the surface tension between the particle 

 and liquid, then anything which tends to decrease the 

 potential difference between the particle and the liquid will 

 increase that surface tension. Such lessening of the potential 

 difference is brought about by the absorption by the particles 

 of ions bearing a charge opposite in sign to that on the 

 particle ; now, as the surface tension is increased, the 

 particles tend to decrease the surface exposed to the liquid 

 by uniting with one another and thus bring about 

 coagulation. 



2 . Purpose of Experiments. 



As is quite evident, the whole superstructure of the theory 

 of the coagulation of colloids is built up on the results of 

 Hardy's experiments on egg albumen. According to Noyes*, 

 however, the behaviour of the albumen particles may be due 

 to the fact that albumen is an amphoteric substance, capable of 

 acting as a base towards acids and as an acid towards bases. 

 The important question remains then — what effect have the 

 ions of an added electrolyte on the charge of the colloidal 

 particle in general ? 



In the experiments given below, the velocities of the 

 particles of gold and silver solutions respectively are measured 

 both before and after adding varying quantities of an electro- 

 lyte. Billitzer f, in making similar experiments on colloidal 

 solutions of Pt, Hg, Ag, Au, Pd, to which he added gradually 

 increasing amounts of various electrolytes, found that the 

 velocity of the particle gradually decreased and eventually 

 changed its direction, showing that even the sign of the charge 

 was changed by the addition of the electrolyte. He added 

 gelatine and urea to his solutions in order to prevent 

 coagulation. Whitney and Blake % disagree in toto with the 

 conclusions of Billitzer, and fail to reproduce his results with 

 colloidal solutions of gold and platinum, free from gelatine. 

 They assign Billitzer's change in the direction of migration 

 to the dissolved gelatine. In the following experiments, 

 the addition of traces of aluminium sulphate to colloidal 

 solutions of gold and. silver (without the addition of either 

 gelatine or urea) brought about in each case a change in the 

 direction of migration of the particles in an electric field. 



* Journ. Amer. Chem. Soc. vol. xxvii. No. 2, p. 85. 



t Ann. der Physik. vol. si. p. 903 (1903) . 



J Journ. Amer. Chem. Soc. vol. xxvi, No. 10. p. 1339 (1904). 



