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THE ANTISEPTIC ACTION OF FILLING-MATERIALS. 243

1-1000, then in a larger quantity of sterilized water to remove
the sublimate, after which they are dried with sterilized bibulous
paper. AVe then take the teeth by the root or roots, rest the
side of the crown upon a small anWl, and strike a sharp blow
upon it with a hammer. The filling flies out, exposing the un-
touched surface of carious dentine. ~\Ve now with a sterilized
spoon-shaped excavator remove a small piece of the carious den-
tine and place it upon a previously prepared plate of sterile
nutritive agar-agar. The plate is then put away in a moist

Fig. 109.

A STERILE AGAR-AG.\R PLATE, containing in the left half pieces of dentine from a cavity
which had been filled with copper amalgam, in the right half pieces from a cavity which had
been filled with gold amalgam. The former have remained sterile, whereas an extensive
growth of bacteria has taken place around the latter. Plate throe days old.
chamber at or near the temperature of the human body. If now
the bacteria in the carious dentine have been killed by the action
of the filling-material, or if the dentine has been so acted upon
by the material as itself to become antiseptic, no growth will
develop around it ; otherwise we will find in the course of forty-
eight to sixty hours that the piece of dentine becomes surrounded
by a growth of varying extent.
In examining the plates, a low power of the microscope
should be used in cases where a growth is not visible to the
naked eye. Furthermore, a slight cloudiness or precipitate
which sometimes forms around pieces impregnated with copper
salts must not be mistaken for micro-organisms ; and lastly, a
development of bud-fungi (yeast-fungi, Saccliaromycetes), or
mould-fungi (Hyphomycetes), which is very frequently observed,
must not be mistaken for bacteria (Schizomycetes).