1918-19.] On the Mode of Action of Metal Sols. 
143 
XII. — On the Mode of Action of Metal Sols. By Professor 
C. R. Marshall. 
(Read May 5, 1919. MS. received May 10, 1919.) 
During the last twenty years colloidal solutions of various metals have 
been largely used in therapeutics. Few investigations, however, have 
been made on their mode of action, and most of these have been made 
on mammals. As a contribution to the subject some experiments made 
under simpler conditions seem worthy of record. The reaction em- 
ployed was the effect of an electrolyte-free silver sol on bacteria. Some- 
what similar experiments have been made by Henri and coadjutors. 
These, which are referred to later, were unknown to me at the time of 
the investigation. 
The sol was prepared by Bredig’s electrical method, using conductivity 
water made in Bourdillon’s apparatus, pure silver wire T25 mm. thick, 
and a current of seven amperes. It was greenish brown in colour, and 
contained 0*004 per cent. Ag. The conductivity was the same as that of 
the conductivity water exposed under the same conditions, namely I x 10 & 
(telephone method). 
Preliminary tests on the antiseptic action of this and other silver 
sols showed that Bacillus typhosus was the organism most susceptible, 
and this was therefore chiefly used. The order of susceptibility of 
the strains of bacilli investigated was — B. typhosus, B. pestis, B. 
paratyphosus A, B. paratyphosus B, B. enteritidis (Gaertner), B. coli 
communis. 
The bacillary emulsion was made by adding 1 to 3 c.c. of sterile 
conductivity water to a twenty-four hours’ growth of B. typhosus on 
agar medium, agitating gently and pouring off. The number of bacilli 
in a known volume was then counted. As nearly as possible a definite 
number of bacilli was added to a known concentration of the sol, and at 
definite intervals a platinum loopful of the mixture was added to 5 c.c. 
of a slightly modified Douglas broth and incubated. 
The minimum lethal concentration of silver in solution was determined 
with silver nitrate. For a ten-minutes action it was found to be thirty- 
thousandth normal. 
