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materials for filling. 91

ture, when cooled, into dust, combining the latter
with mercury in sufficient proportion to give the
requisite plasticity, and then thoroughly washing the
whole in alcohol or boiling water, to remove the
oxyds formed during the combination of the metals.
If there is a redundance of mercury, it may be re-
moved by pressing the paste in a piece of chamois skin.
This preparation may in some cases be used for
filling with considerable success ; but in no case can

it be relied upon as a durable material, its destructi-
bility being no less than that of tin or silver in any
circumstances, and being greater where all the mer-
cury is not removed from the surface of the filling,
and the surface not burnished down solid and
smooth. Mercury oxydates with considerable rapid-
ity when exposed to air and moisture, and with

increased energy, under the influence of heat, especially
when some acid is present. This facility of oxyda-
tion is still increased when other metals are com-
bined with mercury. Oxydation of such fillings will
in some cases be confined to the surface, wherever
there is contact of moisture ; in others, it will per-
vade the whole mass, rendering it black and spongy
throughout.
Amalgam fillings, in a short time after their inser-
tion, undergo a hardening process, occasioned by crys-

tallization of the mass, as well as by evaporation of the