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ACCOMPANYING PHENOMENA OF DENTAL DECAY. 159

dead deutine. Through the kindness of the late Dr. Franz, I
obtained no fewer than three hundred human teeth which had
been worn in the mouth on plates. Nearly all of them showed
different degrees of decay. I split about sixty which seemed
specially suited for the purpose. One case only revealed a phe-
nomenon resembling transparency, but even in this one case it
was not possible to say that the change was not brought about
while the tooth was still living. Again, the appearance in ques-
tion is by no means peculiar to decay. We have seen that it
does not accompany decay of dead teeth, whereas it is very com-
mon in sound teeth which have been worn off, also in teeth
whose approximal surfaces have been slightly worn away by
friction. It is found particularly in senile teeth, whose entire
roots frequently become transparent, likewise in roots in the pro-
cess of resorption, etc. ; everywhere, in short, where the living
dentinal fibers are slightly irritated. I have noticed a high degree
of transparency in the teeth of old dogs which have been ex-
tensively worn away. It does not show the remotest similarity
to decay, and, in fact, bears no direct relation to it, inasmuch as
it is absent in dead teeth, and very frequently occurs in a marked
degree and in typical forms in places where there is not a trace of
decay, and where decay, on the whole, very seldom occurs. The
same agents which produce decay may also occasion transparency
of dentine, which explains the frequent simultaneous appearance
of both phenomena.
The opacity of normal dentine is produced by the different
re coefficients of refraction of its component parts. If two sub-
stances, whether transparent or not, of different refractive power
be mixed, an opaque substance will result, unless the one is
soluble in the other. Thus water and oil are both transparent,
but when mixed they are opaque. In a like manner the
opaque foam is composed of two transparent substances, air and
water. Furthermore, both glass and air are transparent, but
when glass is pulverized, that is to say mixed with air, an opaque
substance will result. The powdered glass can be made trans-
parent again, when another substance having the same coefficient
of refraction as glass is substituted for the air. Oil of cedar is
such a body (approximately). This poured upon powdered glass