496 
Journal of Agricultural Research 
Vol. I, No. 6 
The cultures were incubated for seven days at 30° and titrated cold 
against twentieth-normal sodium hydrate with phenolphthalein as an indi¬ 
cator. From the results so obtained is subtracted the titration of a blank, 
and the result is expressed as the percentage of normal acid. Some objec¬ 
tion may be raised against the use of 30° C. as an incubation temperature 
rather than the more common one of 37 0 . The lower temperature was 
adopted because practically all streptococci will grow at this temperature, 
while a few grow at 37 0 slowly or not at all. 
The fermentation produced by the streptococci is in almost all cases 
so marked that there is very rarely any question about the presence or 
absence of the fermentation. Of all the substances we have used gly¬ 
cerin forms an exception to this rule. The fermentation proceeds 
slowly and in seven days may be slightly above or slightly below 1 per 
cent normal acid, the point selected as marking the line between fer¬ 
mentation and no fermentation. This is illustrated by Table I, which 
shows the progressive rate of fermentation by typical cultures. Three 
cultures fermenting dextrose are included to show the usual course of the 
fermentation in the more easily fermented sugars. Each titration was 
made from a separate tube. A study of this table shows that the 12 
cultures may be divided into three quite distinct types on the basis of 
the rate of fermentation of glycerin. This is shown more clearly in 
figure 3, in which the average titrations for each of the three types are 
plotted. Two of these cultures fermented the glycerin with comparative 
