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



Table II. — Gold Solution. 

 Amount of gold per 100 c.cs. = 6*2 mgs. 



No. 



Grrms. of Al. per 

 100 c.cs. 



Spec. Conductivity 

 of Solution at 18°C. 



Velocity at 

 18° C. 



1 





 19xl0~ 6 

 38X10 -6 

 63xl0 -6 



3-6 XlO- 6 



5-2 xlO- 6 



66 XlO" 6 



11-6 XlO -6 



+33 Xl0~ 5 

 +17-1 xlO" 5 

 - 1-7 Xl0~ 5 

 -13-5X10 -5 



2 



3 



4 





With solutions No. 2 and 3, both with silver and gold, 

 the velocities given in the tables are those during the first 

 10 minutes after turning on the current, instead o£ that 

 during the second 20 minutes as in former cases*; with 

 these four solutions the motion was gradually interfered 

 with by coagulation which was hastened by the current. 

 But with Nos. 1 and 4 in both cases the motion was quite 

 steady. 



Samples of each of the solutions to which the electrolyte 

 had been added were enclosed in test-tubes and the rapidity 

 of coagulation observed. With the silver solutions, No. 2 

 coagulated within a few hours, No. 3 had settled slightly 

 after standing all night, while No. 4 took longer to coagulate 

 than No. 3. With the gold, solutions Nos. 2 and 3 both 

 coagulated at the end of a few hours, while No. 4 had not 

 completely coagulated after standing for 4 days. In each case 

 the pure solution was stable for any length of time. 



These results point quite clearly to the existence of an 

 isoelectric point for such solutions, for it is quite apparent 

 that the particle passes through a state of maximum instability 

 at the time when its charge is changing from negative to 

 positive. 



4. Calculation of the Charge on a Particle. 



In the curves given in fig. 1, the velocities of the particles 

 in these solutions are plotted against the amount of aluminium 

 per 100 c.cs. of colloidal solution. From them we may deduce 

 the amount of aluminium which appears to be just necessary 

 to neutralize the charge on the particles in those solutions ; 

 for silver it is about 26 x 10~ 6 grams per 100 c.cs., for gold, 

 37 x 10~ 6 . In such very dilute solutions we may take it that 



* Phil. Mag. [6] vol. xi. April 1906. 



