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THE CEMENTS. 133

these have not been in sufficient number, nor have they been
sufficiently varied as to conditions to give certain working data.
These cements are not impervious to moisture. Some
experiments have recently been made as to the qualities of
the cements with regard to exclusion of moisture, and it has
been found that none of them are actually impermeable, while
many of those on the market admit moisture readily. As yet,
we do not know with any degree of certainty upon what these
differences depend. They are, however, very important. Other
things being equal, the cement that is nearest moisture-tight is
the best for use. Any one may try cements by making experi-
mental fillings and placing them for a day, or two or three, in a
solution of aniline dye, and then breaking them open and
observing the penetration of the color into the mass of the
material. Some of the cements will be penetrated through and
through in a very short time, while others will resist for a
longer time.
It appears from experiments already made that those cements
which are not permeable by moisture shrink badly while setting.
This shrinkage renders them a poorer material for use than some
of those that are in a degree penetrable by moisture.
When made into fillings in teeth the cements do not
resist perfectly the solvent action of the oral secretions.
Thisy are slowly dissolved, or waste away. They are, therefore,
not very permanent fillings. In this regard great differences are
observed. In a few cases cement fillings have done excellent
service for many years, seemingly resisting the action of the oral
secretions perfectly. In the majority of cases, however, they
dissolve away within two or three years, and in many instances
within a few months. We have not yet such a knowledge of the
conditions of these variations as will enable us to control them,
and must therefore regard cement fillings as temporary in their
nature and qualities, expecting to renew them at frequent inter-
vals.
Use of the Cements.

The cements are used for setting crowns and bridges, for
temporary fillings, and for preserving for a time teeth that are
very badly broken down, or in other conditicHis which seem to
render the use of the metallic fillings undesirable at the time.