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FILLING TEETH 145

becoming wet, either during or immediately after
insertion, the rubber dam should be used. An
oxyphosphate filling will become perfectly hard in
water, but it is generally considered advantageous to
varnish the surface. The majority of these cements
are " hydraulic " ; this is to say, that as soon as
they have become reasonably hard, the absorption
of a little moisture tends to improve them. This
is probabl}^ why those operators who smooth their
cement fillings with an instrument dipped in vaseline
or oil, and well rub the lubricant over the surface
of the filling, find they obtain as good results as if
they used varnish. There is no doubt that the
majority of oxyphosphate fillings become harder in
time : a filling, the surface of which is tested twenty-
four hours after insertion, will be found not quite so
hard as it would be in a week, and probably not so
hard in a week as it would be in a month, and this
increase of hardness is taking place in the presence
of moisture. There is one practical point that must
not be overlooked, namely, that an oxyphosphate
filling must usually be cut out and replaced within
two years. Some cements are much harder to cut
out than others, and the durability of a cement
filling by no means depends on its hardness.
There are many good cements that may be
comparatively easily and rapidly removed, and are