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20 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

of ruminants ; also two kinds of cellulose fermentation in artificial
mixtures. The former took place in neutral 1 per cent, beef extract
solutions, in which purified cotton or paper pulp was suspended,
carbonic acid, marsh gas, and small quantities of sulphuretted hj--
drogen, aldehyde, isobutyric acid, and acetic acid being formed
;
the latter in alkaline beef-extract solutions, carbonic acid and
hydrogen being formed, with the same by-products as before.
ISTothing at all is known concerning the occurrence of such fer-
mentations in the human mouth. They might possibly come
into consideration in the decomposition of cotton used in the
treatment of teeth.
a. Lactic Acid Fermentation.
Lactic acid fermentation of carbohydrates takes place sponta-
neously in milk, in the juice of the sugar-beet, in the accumu-
lations in the oral cavity, etc., and may be artificially induced by
a large number of diiferent bacteria in saccharine solutions. It
proceeds most rapidly at a temperature of from 30°-38° C. The
course of the fermentation varies greatly, according to the bac-
terium by which it is produced. Sometimes there seems to be a
perfectly even decomposition of the molecules of sugar into two
molecules of lactic acid, according to the equation CgHj^Oy (grape
sugar)=2C3Hg03 (lactic acid), no appreciable quantity of car-
bonic acid or hydrogen being formed. At other times the fer-
mentation is of a more violent nature, large quantities of carbonic
acid and hydrogen being formed, with other by-products, as
acetic, formic, succinic, and possibly butyric acid.
It is usually assumed that lactic acid fermentation must always
be accompanied by a development of carbonic acid. I was, how-
ever, struck by the fact that in certain cultures an evolution of
gas always failed to take place. In order to arrive at a positive
conclusion in regard to this point, I undertook an experiment
in a large glass vessel containing a liter of beef-extract-sugar-
solution, which I inoculated with a small piece of carious den-
tine. Then a rubber cork, carrying a bent glass rod, was coated
with sealing-wax and pressed into the heated opening of the
glass vessel, thus hermetically sealing it. The vessel, being
placed in the incubator, exhibited after a few hours a rapid de-