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172 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. :

in a large quantity of chloroform; then they came for an equal
length of time in a diluted solution of Canada balsam in chloro-
form, to which, after about twenty-four hours, was added as
much hardened Canada balsam as would dissolve in it.
AVeil now placed the teeth in a porcelain vessel, added enough
of the solution to cover them, and then kept the solution over a
water-bath at 60°-70° C.,—later at 80°-90° C. until the mass
when cooled became as hard as glass; the teeth were then care-
fully taken out, the superfluous balsam removed, and the speci-
mens were ready for grinding.
Sections of dentine are also frequently prepared as follows
Eather thick ground sections are made comprising both the nor-
mal and softened tissue; these are decalcified in diluted acid
(chromic acid, etc.), and cut into microscopically thin sections by
the microtome. Such preparations have the great disadvantage
that we are not able to determine with certainty just where the
decayed tissue stops and the artificiall}- decalcified begins; nor
can we tell exactly what changes of the dentine may have been
caused by the artificial decalcification.
Having made and examined several thousand mieroseoi^ic prep-
arations of decayed dentine, I have found the following method
to be the most recommendable, both on account of its extreme
simplicity as on account of the excellence of the preparations
which it yields and the few reagents to which it is necessary to
subject the tissue.
Selecting a freshly-extracted decayed tooth, we wash out the
cavity only sufliciently to remove the particles of food, and
break away the margins of enamel so as to expose the softened
dentine as much as possible. Then with a sharp instrument we
dissect, so to speak, the decayed from the sound dentine, keep-
ing hard upon the latter; by this means we may easily shell out
nearly the whole of the softened dentine in one piece. AYhere
the decay has approached near to the pulp, it is very easy to ex-
tend the cut quite to the pulp-chamber, by which means a thin
layer of hard dentine will be contained in the mass removed.
The material thus gained is immediately cut on the freezing
microtome. It is well to freeze the tissue in an aqueous solu-
tion of gum-arabic instead of in water. Such a solution has a