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238 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

cement being, as far as I know, the only material which was
introduced with this object in view. That it does not accom-
plish its object will, I think, be apparent from the experiments
recorded below.
Methods.—I.
Various methods may be employed for determining the anti-
septic action of filling-materials. The two which I have made
use of are exceedingly simple, and at the same time very instruc-
tive. In applying the first of these methods we proceed as fol-
lows A tube of ordinary nutritive gelatine is infected with a bac-
:
terium from the oral cavity, which grows rapidly at room tem-
perature without liquefying the gelatine. The gelatine is then
melted, slightly shaken, so as to distribute the fungi equally
throughout the solution, and poured upon a horizontal sterilized
glass plate, upon which we drop pieces of the filling-material or
other substances whose antiseptic action we wish to determine.
As soon as the gelatine becomes stifi' we place the plate in a
damp chamber. A plate prepared in this way, without the addi-
tion of any material having an antiseptic action, will become
cloudy and opaque in the course of twenty-four to forty-eight
hours, through the development of innumerable colonies of bac-
teria. If, however, the pieces of filling-materials which we have
dropped upon the plate possess an antiseptic action, the devel-
opment of the fungi in their neighborhood will be retarded or
altogether prevented, and each piece will appear surrounded by
an area of transparent gelatine whose size will depend upon the
activity of the antiseptic employed. Most of the filling-materials
in use were tested by this method in respect to the antiseptic
action, with the result that the only one which possesses such
action and retains it for an indefinite time after it has been in-
serted is copper amalgam.* Not only freshly-mixed fillings, but
pieces of old, half-worn-out fillings, taken from teeth extracted
in the polyclinic of the Dental Institute, and even pieces of den-
tine from teeth which had been filled with copper amalgam, in-
* Regarding an unexpected antiseptic action of certain preparations of gold
which might appear to furnish an exception to this rule, see the experiments
described below.