ope
of a laboratory handbook, the methods in use for their detection and
separation come into the ordinary routine work and will therefore be
described (_vide_ page 276 _et seq._).

X. NUTRIENT MEDIA.

In order that the life and growth of bacteria may be accurately observed
in the laboratory, it is necessary--

1. To _isolate_ individual members of the different varieties of
micro-organisms.

2. To _cultivate_ organisms, thus isolated, apart from other associated
or contaminating bacteria--i. e., in _pure culture_.

For the successful achievement of these objects it is necessary to
provide nutriment in a form suited to the needs of the particular
bacterium or bacteria under observation, and in a general way it may be
said that the nutrient materials should approximate as closely as
possible, in composition and character, to the natural pabulum of the
organism.

The general requirements of bacteria as to their food-supply have
already been indicated (page 142) and many combinations of proteid and
of carbohydrate have been devised, from time to time, on those lines.
These, together with various vegetable tissues, physiological or
pathological fluid secretions, etc., are collectively spoken of as
_nutrient media_ or _culture media_.

The greater number of these media are primarily _fluid_, but, on account
of the rapidity with which bacterial growth diffuses itself through a
liquid, it is impossible to study therein the characteristics of
individual organisms. Many such media are, therefore, subsequently
rendered solid by the addition of substances like gelatine or agar, in
varying proportions, the proportions of such added material being
generally mentioned when referring to the media; e. g., 10 per cent.
gelatine, 2 per cent. agar. Gelatine is employed for the solidification
of those media it is intended to use in the cultivation of bacteria at
the room temperature or in the "cold" incubator. In the percentages
usually employed, gelatine media become fluid at 25 deg. C.; higher
percentages remain solid at somewhat higher temperatures, but the
difficulty of filtering strong solutions of gelatine militates against
their general use.

Media, on the other hand which have been solidified by the addition of
agar, only become liquid when exposed to 90 deg. C. for about ten
minutes, and again solidify when the temperature falls to 40 deg. C.

When it becomes necessary to render these media fluid, heat is appli