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12 - THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

even absolutel}' demand the exclusion of air for tlieir develop-
ment; they are called anaerohie. The third group of bacteria
flourish either with or without oxvgen, at least for a certain
length of time ; Hueppe characterizes them as facultatively an-
aerobic. Most bacteria appear to belong to the first group, but
comparatively few purely anaerobic bacteria being at present
known.
The capability of certain bacteria to j^roliferate and to mani-
fest their specific action without access of air, may explain the
progress of tooth caries under air-tight fillings, in cases where
the softened dentine was not thoroughly removed before insert-
ing the filling.
c. Action of Acids and Alkalies.
Acids and alkalies, especially the former, even in very dilute
solutions, retard the development of bacteria. Some species,
however, show important deviations from this rule; thus, for
example, the acetic acid bacterium grows best in an excess of one
to two per cent, of acetic acid, while Micrococcus ure?e will
bear a high degree of alkalinity. A bacterium which I have
examined produces in solutions of carbohydrates 0.7.3 per
cent, of lactic acid, but perishes in this solution in a few days,
through the action of its own product. With comparatively
few exceptions, a neutral medium is best adapted to the develop-
ment of bacteria.

(/. Action of Light, Elcctricitij, and. Pressure.

Light, electricity, and pressure have very little or no influence
upon the development of bacteria, nor upon their specific vital
manifestations. (See Cohn and Mendelsohn.^) The assertion
that combined fillings of tin and gold, or of amalgam and gold,
etc., prevent the development of bacteria by means of the elec-
tricity which they produce, is utterly without scientific founda-
tion.
That even weak electric currents sometimes appear to have a
slight retarding action upon the development of bacteria, may
be easily demonstrated by the following simple experiment: