a basis for
continued study, and must be planted and grown upon the different
kinds of media that are obtainable. In this way the slightest
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.

[Illustration: Fig. 5.--Different Kinds of Bacteria Growing in
Gelatin.

A, meager growth, no liquefaction or surface growth; B, profuse
surface growth, radiating filaments from the growth below the
surface; C, a rapid liquefying form; D, a gas producer that grows
equally well in the presence or absence of air; E, form that grows
only in the absence of air, an anaerob.]

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 in a pure state.

=The microscope in bacterial investigations.= 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 high magnifying powers (600 to 800 diameters), combined with
sharp definition.

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 condition, but colored preparations
are made by using solutions of anilin dyes as staining agents. These
are of great service in bringing out almost imperceptible
differences. The art of staining has been carried to the highest
degree of perfection in bacteriology, especially in the detection of
germs that are found in diseased tissues in the animal or human
body.

In studying the peculiarities of any special organism, not only is
it necessary that these cultural and microscopical characters should
be closely observed, but special experiments must be made in
different ways, in order to determine any special properties that
the germ may possess. Thus, the ability of any form to act as a
fermentative organism can be tested by fermentation experiments; the
property of causing disease, studied