142 
IOWA ACADEMY OF SCIENCES. 
such estimates are only valuable when carried out in large 
numbers, and that a misleading conclusion may be easily 
reached from a few isolated experiments. The data pre- 
sented is conclusive enough that a few general deductions 
may safely be made. 
In separating the colonies on a particular Petri dish into 
the general classes given, which is based on their relation 
to milk, a portion of the dish was divided off which con- 
tained about the number of colonies desired, usually from 
40 to 50. Then every colony which could be found by 
using a hand lense was taken with a platinum needle and 
put into a tube of sterile milk. After about three days in 
the incubator at 35 r C the milk cultures were examined. 
Those which showed a solid acid coagulation, with or 
without gas, with no dissolving of the curd, were put into 
the acid class. Those which did not coagulate the milk 
within that time were classified as producing no effect. 
It is probable a few of these would show coagulation later, 
but it would not be of the acid class, and probably all 
cause more or less complicated chemical changes in the 
milk without changing the appearance. 
Those which coagulated milk without producing acid, or 
caused the curd to show signs of dissolving after coagula- 
tion were classed as enzyme producing. A consideration 
of the data as bearing upon the points under investigation 
as stated, shows that regarding the first point, the evidence 
is very conclusive. In no case does the number develop- 
ing upon the peptone agar approach the number appear- 
ing upon the lactose agar. The greatest difference being 
found in the buttermilk where the lactose agar shows over 
twenty times as many as the peptone agar. 
The comparison between the peptone and lactose gela- 
tin, although less extreme, is sufficient to show conclu- 
sively that the former does not show near as high a devel- 
opment as the latter. As between peptone agar and pep- 
tone gelatin the results indicate that the latter will show 
a considerably greater number of colonies than the former. 
