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METHODS OF BACTERIOLOGICAL INVESTIGATION. 61

temperature, in consequence of which the bacteria do not develop
so rapidly as they do on the agar-agar plates ; sometimes no
development whatever takes place. The thought naturally pre-
sents itself to every one, that the bacteria growing in the deeper
parts of dentine may not develop under free access of air.
Accordingly, small pieces of decayed dentine should be placed
in tubes of melted gelatine or agar-agar, and allowed to sink
to different distances from the surface, where, on the medium
becoming solid, they obtain but a very limited supply of air.
To obtain a pure culture from a gangrenous pulp, I carefully
cleanse and sterilize the tooth as before, so as to be absolutely
sure that no living germs are present upon the surface; I then
split the tooth by means of a pair of sterilized incising forceps.
Very often the tooth may be split without the forceps coming
into contact with the pulp, in which case the pulp, or portions
of it, may be lifted from its bed by means of a nerve-needle,
and transferred to a drop or two of water or gelatine. It is then
torn or picked to pieces so as to liberate the bacteria, after
which it may be drawn several times across the surface of the
agar-agar, or the inoculations may be made from the water.
For such micro-organisms as grow at low temperatures, gela-
tine forms a much more convenient material than agar-agar;
furthermore, differences of growth are much more readily recog-
nized on gelatine than on agar-agar. Consequently, we should
not rely upon cultures on agar-agar alone, but all of the bacteria
which we succeed in cultivating- on agar-agar we should likewise
attempt to cultivate on gelatine. The majority of them will
grow, though by no means all of them.
The great advantage possessed by the solid culture media
over liquid media, consists in the fact that each germ must re-
main at the point where it is at the time the media becomes
solid. A number of different kinds of micro-organisms, there-
fore, on a plate of gelatine, may develop without mixing together,
each one remaining as a pure culture, whereas in lii^uids they
would naturally mix, thereby causing an impure culture.