s
flame below the incubator is never extinguished.

The expansion of the metal walls of the capsule, which takes place upon
the boiling of its contents, provides the motive force, transmitted
through the rigid rod to raise the long arm of the lever B D, and as
this expansion only takes place at a predetermined temperature, the
lever will only be acted upon when the critical temperature is reached,
no sensible effect being produced at even 1 deg. C. below that at which
the capsule is destined to act.

W is a weight sliding on the lever rod D; by increasing the distance
between the weight and the fulcrum of the lower increased pressure is
brought to bear upon the walls of the capsule with the result that the
boiling-point of the liquid in the capsule is slightly raised, and a
range of about two degrees can thus be obtained with any particular
capsule.

FOOTNOTES:

[7] Made by the firm of Chas. Hearson & Co., 235 Regent St., London, W.

XIV. METHODS OF CULTIVATION.

Cultivations of micro-organisms are usually prepared in the laboratory
in one of three ways:

~Tube cultures.~
~Plate cultures.~
~Hanging-drop cultures.~

These may be incubated either ~aerobically~ (i. e., in the presence of
oxygen) or ~anaerobically~ (i. e., in the absence of oxygen, or in the
presence of an indifferent gas, such as hydrogen, nitrogen, or carbon
dioxide).

With regard to the temperature at which the cultivations are grown, it
may be stated as a general rule that all media rendered solid by the
addition of gelatine are incubated at 20 deg. C., or at any rate at a
temperature not exceeding 22 deg. C. (that is, in the "cold" incubator);
whilst fluid media and all other solid media are incubated at 37 deg. C.
(that is, in the "hot" incubator). Exceptions to this rule are numerous.
For instance, in studying the growth of the psychrophylic bacteria, the
yeasts and the moulds, the cold incubator is employed for all media.

Tube cultivations are usually packed in the incubator in small tin
cylinders, such as those in which American cigarettes are sold, or in
square tin boxes. Beakers or tumblers may be used for the same purpose,
but being fragile are not so convenient. Metal test-tube racks, long
enough to just fit into the interior of the incubator and each
accommodating two rows of tubes, are also exceedingly useful.

~AEROBIC.~

~The Preparation of Tube Cultivations.~

The preparation of a tube cultivatio