d be
cultivated, and the progeny of this cell increased to such an extent
in a short course of time that the resulting mass of cells would be
visible to the unaided eye. This is done by growing the bacteria on
various kinds of nutrient media that are prepared for the purpose,
but inasmuch as bacteria are so universally distributed, it becomes
an impossibility to cultivate any special form alone, unless the
medium in which they are grown is first freed from all pre-existing
forms of germ life.

=Food materials.= Many kinds of food substances are used for the
cultivation of bacteria in the laboratory. In fact, bacteria will
grow on almost any organic substance, whether it is solid or liquid,
provided the other essential conditions of growth are furnished. The
food substances that are used for culture purposes are divided into
two classes,--solids and liquids.

Solid culture media may be either permanently solid, like potatoes
and coagulated egg, or they may retain their solid properties only
at certain temperatures, like gelatin or agar. The latter two, which
were devised by Robert Koch, are of utmost importance in
bacteriological research, for their use permits the separation of
the different forms of bacteria that may happen to be in any
mixture. Gelatin is advantageously used, because the majority of
bacteria present wider differences, due to growth upon this medium,
than upon any other. It remains solid at ordinary temperatures,
becoming liquid at about 80 deg. F. Agar, a gelatinous product derived
from a Japanese seaweed, has a much higher melting point, and is
used especially with those organisms whose optimum temperature for
growth is above the melting point of gelatin.

Besides these solid culture media, different liquid substances are
extensively used, such as beef broth, milk and infusions of various
vegetable and animal tissues. Skim milk is of especial value in
studying the milk bacteria, and may be used in its natural
condition, or a few drops of litmus solution may be added, in order
to detect any change in its chemical reaction due to the bacteria.

=Sterilization.= The various ingredients that are used in the
preparation of culture media are not free from micro-organisms,
hence the media would soon spoil if they were not destroyed, and the
media subsequently protected from contamination from the air, etc.
The process of rendering the media free from living micro-organisms
is known as _sterilization_.