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240 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

of observing the action of copper amalgam fillings, that they do
possess a preserving action upon tooth-substance. I, along with
most others, formerly accounted for this action upon the sup-
position that copper amalgam does not shrink while setting. I
meet almost daily wnth amalgam fillings, not containing copper,
which admit of the point of an excavator being inserted between
the filling and the margin of the cavity, whereas copper amal-
gam fillings appear to hug the walls of the cavity perfectly.

Fig. 107.

An IXOCULATED GELATINE PLATE containing pieces of freshly-mixed cement, a, o.xychlo-
ride; h, oxyphosphate. A very marked hindrance in the development of the bacteria is noted
around the pieces of oxychloride : around the pieces of oxyphosphate it is scarcely percep-
tible. Plate twenty-four hours old.
Elliott,* however, found by a very extended series of experiments
that copper amalgams do contract, and some of them to a sur-
prising degree. Elliott's results are corroborated by the evidence
of J. Boyd 'W^allis,t who claims that the slight contraction is a
distinct advantage in the case of soft and sensitive teeth, because
of the more speedy formation of the oxide or sulphide, wdiich,
being absorbed by the surrounding dentine, protects it from
further progress of decay. " Pulps d}ing under copper amalgam

* Transactions of the Odontological Society of Great Britain, December, 1888.
^Dental Record, February, 1889.