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MOUTH-BACTERIA AS EXCITERS OF FERMENTATION. 107

By this simple reaction the detection of lactic acid, which is
otherwise often extremely difficult, is yery much facilitated. Un-
fortunately, howeyer, it cannot he applied under all conditions,
since not only lactic acid and its salts, hut also tartaric acid,
malic acid, acetic acid, oxalic acid and its salts, as far as I haye
examined them, and possibly some other substances not yet ex-
amined, give the same reaction. In order to produce the yellow
color, it is, in fact, only necessary to add to the \'iolet solution a
small piece of fruit (apple, grape, plum, tomato), a few drops of
white wine (Mosel, etc.). All these acids must, of course, be
first excluded before the presence of lactic acid can be assumed
with absolute certainty.
In order to determine more accurately the acid formed in a
mixed fermentation of carbohydrates, as it occurs in the mouth,
a chemical analysis was made.
The method of carrying out such an analysis will now l)e giyen :
Two hundred cc. of fi'esh saliya are mixed with 2.0 starch and
allowed to stand forty-eight liours at blood temperature; the
mixture is then filtered, and heated to 100° C, to stop the
fermentation. This process is repeated until about a liter of the
solution has accumulated. It is then placed in a retort and
reduced to a yolume of about 75 cc. It will be yery strongly
acid. A few drops of this liquid added to a yery thin solution
of methyl-yiolet, leaye it unchanged; from this we conclude
that we have to deal with an organic acid, as an inorganic acid
would turn it first blue and then green. Since the acid did not
distill during the prolonged boiling, we may set it down as non-
yolatile, hence a non-yolatile organic acid. The distillate was
very slightly acid; we will call it distillate Xo. 1, as we wish to
refer to it again.
The solution was further reduced in volume to about 40 cc.
over the water-bath, and then transferred to a large glass vessel,
briskly shaken with one and one-half to two liters of sulphuric
ether, and allowed to stand until the ether became perfectly
transparent. This was tlien filtered into a large retort and dis-
tilled, proper precautions being observed to prevent accidents.
When the volume had been reduced to al)Out 50 cc. the solution
was filtered into a porcelain vessel, and still further reduced oyer