934 The American Naturalist. [October, 
growth or a narrow clear zone followed by one in which the colonies 
were not as thick as on the rest of the plate. Gold.—Purified gold, 
especially if recently glowed, had no inhibitory effect. In those cases 
where inhibition was noticed (some plates of anthrax) the gold had 
not been glowed for several weeks. Miller showed that velvet gold 
has no antiseptic properties but that certain gold preparations used by 
dentists, e. g., Pack’s pellets, Quarter Century gold foil, and Abbey’s 
non-cohesive foil, inhibited the growth for about 5 mm. all around. 
Magnesium.—Tests were made only on Staphylococcus pyogenes aureus 
and the cholera bacillus. With both these organisms there was a clear 
inhibitory zone, followed by a zone of increased growth, sharply 
marked off from the clear zone and gradually fading out on the out- 
side. Zine—Many experiments were made with ordinary scrap zinc, 
cast into a sheet, but no note was kept of these. There was a clear 
zone, however, in every case, and there was probably not much differ- 
ence between the action of this and of pure zinc. With the latter, all 
the organisms tested gave a broader or narrower clear zone, surrounded 
by an intensified zone. With Staphylococcus p. a. the clear zone aver- 
aged 7 mm. With the cholera bacillus there is a wide clear zone 
about 1.5 centimeters, and the effect of the zinc is seen as far as 3 em. 
away from the metal. With other organisms the clear zone is usually 
5 mm., or more, broad, followed by a broad intensified zone that is not 
sharply marked. Cadmium.—With this metal the reactions obtained 
differ quite strikingly, as a rule. The most peculiar zone observed in 
the whole set of experiments is that obtained with the micro-organism 
of anthrax and the pure metal cadmium. In this case there is a per- 
fectly clear zone 5 mm. wide, then an intensified zone of 2 mm. breadth, 
and a second inhibitory zone 1 mm. wide. In some cases this second in- 
hibitory zone is not entirely free from colonies, but it can always be made 
out very distinctly. Mereury.—There is considerable difference in the 
behavior of different micro-organisms towards mercury. With Staphy- 
lococeus p. a. there is a clear zone, about 7 mm. around the metal, fol- 
lowed by a slightly intensified zone which in different cases varies in 
width from 1 to 3 mm. With Bacillus pyocyaneus there is a clear zone 4 
mm. broad around the metal and outside an intensified zone, sharply 
marked toward the clear zone and falling off gradually on the 
outside. With the cholera bacillus there is a clear zone, 2 mm. 
around the metal, then a very narrow intensified zone that is well 
marked. With the bacillus of anthrax there is a broad clear zone, 
9 mm. around the metal, surrounded by a very slightly intensified 
zone that is not sharply marked. With the colon bacillus there 
