Action of Electrolytes on Copper Colloidal Solutions. 583 



containing about 1 gram, and the second with a larger bulb 

 containing about 5 grains of charcoal. The change in the 

 spectrum was only that due to the change in pressure of the 

 gas, as was demonstrated by simply exhausting the apparatus 

 and observing the spectrum. 



So far, then, as this method can decide, the evidence 

 points to the conclusion that hydrogen is really the source of 

 the secondary spectrum. 



I am indebted to Prof. Strutt, of the Royal College of 

 Science, for his various suggestions during this research, 

 and I take this opportunity of acknowledging my thanks to 

 him. 



LIII. The Action of Electrolytes on Copper Colloidal Solutions. 

 ByE. F. Burton, B .A., Demonstrator in Physics, University 

 of Toronto *. 



1. Introduction. 



THE problem of the effect of the presence of very small 

 traces of electrolytes on the stability of colloidal solu- 

 tions is one of the most important unsolved questions relating 

 to colloids. The recent paper by Jacques Duclaux f tends 

 to throw doubt on the theory of the isoelectric point first 

 suggested by Hardy J ; while a later contribution by Lotter- 

 moser § dealing with Duclaux's results shows, at least, that 

 we have yet much to learn regarding the interaction of the 

 colloidal particles and the dissolved salts. 



The experiments detailed in the present communication are 

 a continuation of those performed by the writer || on the 

 action of small traces of electrolytes on silver and gold 

 colloidal solutions prepared by Bredig's method. In this 

 former work it was found that, as minute quantities of 

 aluminium sulphate were added in increasing amounts to 

 either silver or gold solutions, the velocities with which the 

 particles moved in a given electric field were at first reduced 

 to zero and then reversed. As the particles in each case 

 were negatively charged in the pure solution, according to 

 Hardy's theory, the positively charged aluminium ions, and 



* Communicated by Professor J. C. McLennan. 



t Jour, de Ch. phys. vol. v. 1-2-3, p. 29. 



X Proc. Roy. Soc. vol. lxvi. p. 110; Jour, of Physiol, vol. xxix. p. 26. 



§ Zeits. fiir phys. Chem. vol. Ix. 4, p. 451. 



|| Phil. Mag. ser. 6, vol. xii. Nov. 1906. 



