c., and by using some substance which acts as a mordant and tends to fix
the stain to the bacteria. Excess of stain is afterwards removed from the
tissues by the use of decolorizing agents, such as acids of varying
strength and concentration, alcohol, &c. Different bacteria behave very
differently to stains; some take them up rapidly, others slowly, some
resist decolorization, others are easily decolorized. In some instances the
stain can be entirely removed from the tissues, leaving the bacteria alone
coloured, and the tissues can then be stained by another colour. This is
the case in the methods for staining the tubercle bacillus and also in
Gram's method, the essential point in which latter is the treatment with a
solution of iodine before decolorizing. In Gram's method, however, only
some bacteria retain the stain, while others lose it. The tissues and
fluids are treated by various histological methods, but, to speak
generally, examination is made either in films smeared on thin
cover-glasses and allowed to dry, or in thin sections cut by the microtome
after suitable fixation and hardening of the tissue. In the case of any
bacterium discovered, observation must be made in a long series of
instances in order to determine its invariable presence.

[Sidenote: Cultivation.]

In cultivating bacteria outside the body various media to serve as food
material must be prepared and sterilized by heat. The general principle in
their preparation is to supply the nutriment for bacterial growth in a form
as nearly similar as possible to that of the natural habitat of the
organisms--in the case of pathogenic bacteria, the natural fluids of the
body. The media are used either in a fluid or solid condition, the latter
being obtained by a process of coagulation, or by the addition of a
gelatinizing agent, and are placed in glass tubes or flasks plugged with
cotton-wool. To mention examples, blood serum solidified at a suitable
temperature is a highly suitable medium, and various media are made with
extract of meat as a basis, with the addition of gelatine or agar as
solidifying agents and of non-coagulable proteids (commercial "peptone") to
make up for proteids lost by coagulation in the preparation. The reaction
of the media must in every case be carefully attended to, a neutral or
slightly alkaline reaction being, as a rule, most suitable; for delicate
work it may be necessary to standardize the reaction by titration methods.
The me