CULTIVATION OF BACTERIA. 127

For the preparation of bouillon, as well as gelatin,
agar-agar, and glycerin agar still to be described, beef-
extract (Liebig's) may be employed, but for the most
delicate work this is rather undesirable, because of its
unstable composition and because of the precipitation of
meat-salts, which can scarcely be filtered out of the agar-
agar, owing to the fact that they only crystallize when
the solution cools. When it is desirable to prepare the
bouillon from beef-extract, the method is very simple.
To 1000 c.cm. of clean water 10 grams of Whitte's dried
beef-peptone, 5 grams of sodium chlorid, and about 2
grams of beef-extract are added. The solution is boiled
until the constituents are dissolved, neutralized, if neces-
sary, and filtered when cold. If it is filtered while hot,
there is always a subsequent precipitation of meat-salts,
which clouds it.

Bouillon and other liquid culture-media are best dis-
pensed and kept in small receptacles—test-tubes or flasks
—in order that a single contaminating organism, should
it enter, may not spoil the entire bulk. A very con-
venient simple apparatus used by bacteriologists for fill-
ing tubes with liquid media is shown in Figure 21. It
need not be sterilized before using, as the culture-medium
will be sterilized by the intermittent method after the
tubes are filled. The test-tubes and flasks into which
the culture-medium is filled must, however, be previously
sterilized by dry heat. The dry-heat sterilization is done,
of course, after the cotton plugs are in place.

Bouillon is the basis of most of the culture-media.
The addition of 10 per cent of gelatin makes it " gela-
tin ;n that of i per cent, of agar-agar makes it ''agar-
agar.n The preparation of these media, however, varies
somewhat from that of the plain bouillon.

Gelatin.—The culture-medium known as gelatin has de-
cided advantages over the bouillon, not only because it is
an excellent food for bacteria, and, like the bouillon, trans-
parent, but because it is also solid. Nor is this all : it is
a transparent solid which can be made liquid or solid at