three strokes of the
air pump, aspirate a small quantity of gas, so creating a slight vacuum.
Then shut off the stop-cock and disconnect the air pump.

9. Fill the 10 c.c. bulb pipette with water; insert its point into the
rubber tubing on the long tube b as far as the screw clamp. Open the
screw clamp and run in water until stopped by the internal pressure.
Shut off stop-cock. (The water dissolves the soda and pyrogallic acid
converting the latter into alkaline pyro. and so bringing its latent
capacity for oxygen into action).

[Illustration: FIG. 137.--Bulloch's jar.]

10. Reverse the tubes from the tubulures so that they meet, out of
harm's way, over the top of the bell glass; again see that all joints
are tight and transfer the apparatus to the incubator.

This last method is the most satisfactory for anaerobic cultivations, as
by its means complete anaerobiosis can be obtained with the least
expenditure of time and trouble.

FOOTNOTES:

[8] See also method of opening and closing culture tubes, pages 74-76.

[9] If compressed tablets of pyrogallic acid cannot be obtained make up
a stock "acid pyro" solution

Pyrogallic acid, 10 grammes
Hydrochloric acid, 1.5 c.c.
Distilled water, 100 c.c.

and at step 4, run in 10 c.c. of the solution.

XV. METHODS OF ISOLATION.

The work in the preceding sections, arranged to demonstrate the chief
biological characters of bacteria in general, is intended to be carried
out by means of cultivations of various organisms previously isolated
and identified and supplied to the student in a state of purity. A
cultivation which comprises the progeny of a single cell is termed a
"pure culture"; one which contains representatives of two or more
species of bacteria is spoken of as an "impure," or "mixed"
"cultivation," and it now becomes necessary to indicate the chief
methods by which one or more organisms may be isolated in a state of
purity from a mixture; whether that mixture exists as an impure
laboratory cultivation, or is contained in pus and other morbid
exudations, infected tissues, or water or food-stuffs.

[Illustration: FIG. 138.--Haematocytometer cell, showing, a, section
through the centre of the cell, and b, a magnified image of the cell
rulings.]

Before the introduction of solid media the only method of obtaining pure
cultivations was by "dilution"--by no means a reliable method.
"Dilution" consisted in estimati