144 Proceedings of the Koyal Society of Edinburgh. '[Sess. 
Effect of Silver Nitrate on Bacillus typhosus. 
Concentration of Silver. 
10 Minutes’ 
Exposure. 
15 Minutes’ 
Exposure. 
1 Ag in 3,200,000 . 
0 
0 
1 Ag in 3,400,000 . 
+ 
0 
1 Ag in 4,000,000 . 
+ 
+ 
Compared with this powerful action of free silver ions, the action of 
silver sols is relatively slight. 
Effect of Electrically Prepared Silver Sols on Bacillus typhosus. 
Colloid. 
Concentration 
15 Minutes’ 
30 Minutes’ 
of Silver. 
Exposure. 
Exposure. 
Electrolyte-free (Bredig) 
1 Ag in 25,000 
+ 
0 
Electragol* . 
1 Ag in 32,000 
+ 
0 
The question at issue is how silver in a particulate form can exert 
a bactericidal action. A settlement of the question would enable us to 
understand the mode of action of metal colloids in general ; and it is 
convenient to consider it from the point of view of the general properties 
of colloids. 
Brownian Movement— It is conceivable, although improbable, that the 
bacilli are affected by the impact of the larger submicroscopic particles 
during their incessant movement. If it were permissible to compare 
the effects produced by these minute missiles on bacteria with those which 
would follow similar missiles of the same relative size and moving with 
the same relative velocity on, say, man, we should conclude that the 
bacteria would seriously suffer. No such effect, however, occurs. Bacteria 
may be seen to move actively for long periods in a moderately concentrated 
colloidal metal sol, and they can be seen to be frequently bombarded, 
although it must be confessed that the number of impacts is less than 
would have been expected. In a number of experiments the bacilli and 
the visible submicroscopic particles (above 15 jul/i) were counted, and it 
was found that the bacilli multiplied when the visible particles were in 
the proportion of several thousand to one bacillus. Moreover, if such a 
* Electragol is a commercial electrically prepared silver sol, stabilised by the addition 
of a small quantity of protein, and made isotonic by the addition of sodium chloride. It 
was found to contain more amicrons than the electrolyte-free colloid. 
