ed again so that these
culture plates are made with different amounts of seed with the
expectation that in at least one plate the seeding will not be so thick
as to prevent further study. For transferring the culture a loop made of
platinum wire is used. By passing this through a gas flame, it can be
sufficiently sterilized.

[Illustration: FIG. 3. Profile view of gelatin plate culture; _b_, a
liquefying form that dissolves the gelatin; _c_ and _d_, surface
colonies that do not liquefy the gelatin.]

To further study the peculiarities of different germs, the separate
colonies are transferred to other sterile tubes of culture material and
thus _pure cultures_ of the various germs are secured. These cultures
then serve as a basis for continued study and must be planted and grown
upon all the different kinds of media that are obtainable. In this way
the slight variations in the growth of different forms are detected and
the peculiar characteristics are determined, so that the student is able
to recognize this form when he meets it again.

These culture methods are of essential importance in bacteriology, as it
is the only way in which it is possible to secure a quantity of germs of
the same kind.

~The microscope in bacterial investigation.~ In order to verify the purity
of the cultures, the microscope is in constant demand throughout all the
different stages of the isolating process. For this purpose, it is
essential that the instrument used shall be one of strong magnifying
powers (600-800 diameters), combined with sharp definition.

[Illustration: FIG. 4. Pure cultures of different kinds of bacteria in
gelatin tubes. _a_, growth slight in this medium; _b_, growth copious at
and near surface. Fine parallel filaments growing out into medium
liquefying at surface; _c_, a rapid liquefying form; _d_, a
gas-producing form that grows equally well in lower part of tube as at
surface (facultative anaerobe); _e_, an obligate anaerobe, that develops
only in absence of air.]

The microscopical examination of any germ is quite as essential as the
determination of culture characteristics; in fact, the two must go hand
in hand. The examination reveals not only the form and size of the
individual germs, but the manner in which they are united with each
other, as well as any peculiarities of movement that they may possess.

In carrying out the microscopical part of the work, not only is the
organism examined in a living conditio