g the mixture from the
bath.

5. Make cultures upon suitable media; incubate.

[Illustration: FIG. 140.--Benzole bath.]

7. Differential Atmosphere Cultivation.--

(a) By adapting the atmospheric conditions to the particular organism
it is desired to isolate, it is comparatively easy to separate a strict
aerobe from a strict anaerobe, and _vice versa_. In the first case,
however, it is important that the cultivations should be made upon
solid media, for if carried out in fluid media the aerobes multiplying
in the upper layers of fluid render the depths completely anaerobic, and
under these conditions the growth of the anaerobes will continue
unchecked.

(b) When it is desired to separate a facultative anaerobe from a
strict anaerobe, it is generally sufficient to plant the mixture upon
the sloped surface agar, incubate aerobically at 37 deg. C., and examine
carefully at frequent intervals. At the first sign of growth,
subcultivations must be prepared and treated in a similar manner. As a
result of these rapid subcultures, the facultative anaerobe will be
secured in pure culture at about the third or fourth generation.

(c) If, on the other hand, the strict anaerobe is the organism
required from a mixture of facultative and strict anaerobes, pour plates
of glucose formate agar (or gelatine) in the usual manner, place them in
a Bulloch's or Novy's jar, and incubate at a suitable temperature. Pick
off the colonies of the required organism when the growth appears, and
transfer to tubes of the various media.

Incubate under suitable conditions as to temperature and atmosphere.

~8. Animal Inoculation.~--

Finally, when dealing with pathogenic organisms, it is often advisable
to inoculate some of the impure culture (or even some of the original
_materies morbi_) into an animal specially chosen on account of its
susceptibility to the particular pathogenic organism it is desired to
inoculate. Indeed, with some of the more sensitive and strictly
parasitic bacteria this method of animal inoculation is practically the
only method that will yield a satisfactory result.

XVI. METHODS OF IDENTIFICATION AND STUDY.

In order to identify an organism after isolation, tube, plate, and other
cultivations must be prepared, incubated under suitable conditions as to
temperature and environment, and examined from time to time (a)
~macroscopically~, (b) by ~microscopical methods~, (c) by ~chemical
methods~, (d) by ~phy