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cess proceeds a very thin, yellowish transparent film will ap-
pear over the surface of the bright blade. This may be care-
fully removed, floated on to a cover glass in a drop of dis-
tilled water, dried and stained for microscopic study. Usually
it will show the streptococcus media abundantly—sometimes
almost pure, but oftener with other micro-organisms abun-
dantly intermingled.
In looking for these plaques, very unclean mouths should
not be chosen, for in these the true microbic plaques of the
caries fungus will be obscured by the abundance of othet
micro-organisms, or may not be present at all. for othei
microbes may also form gelatinous masses. Often we see
very filthy mouths in which no caries whatever occurs, and
in which the caries fungus takes little or no part in the forma-
tion of the gummy masses of micro-organisms. I have not
been able to find these plaques in mouths in which no decay
was progressing, and am of the opinion that the beginnings
of caries upon smooth surfaces of the enamel never occur
without them.
These plaques were first demonstrated 'in ground sec-
tions by Dr. Leon WilHams of London (Cosmos, 1897),
J.
whose skill enabled him to so harden these films in position
upon the teeth that he has succeeded in grinding sections of
carious areas just starting in the enamel, and has demon-
strated the plaques in position and the action of the acid upon
the enamel beneath them. Dr. Williams' work in this direc-
tion has greatly enriched our knowledge of the conditions sur-
rounding the beginning of decay, and of the actual details of
the process as it occurs in the enamel. Although much of
this latter had been fairly made out before. Dr. Williams has
rendered certain many points that had not been fixed by
direct observation, and especially the relation of microbic
gelatinous plaques to the beginnings of decay of the enamel.

Conditions of the Occurrence of Caries.

It requires conditions that are favorable to micro-organ-
isms in order that they may produce caries ; this we want to
keep constantly in mind. The fact that the particular species
of micro-organisms that produces caries may grow in the
mouth plentifully, and yet produce no caries; that we find
them in the mouths of those who are immune to caries, who

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