screw clamp is not
touched again during the experiment.

At this rate the aspirator bottle will empty itself in just under three
hours. Shut off the tap and make up the contents of the aspirator bottle
to 10 litres again.

2. Sterilise the fitted rubber cork, with its funnel and tubing, by
boiling in the water steriliser for ten minutes.

3. Remove the cotton-wool plug from the flask, and replace it by the
rubber stopper with its fittings. Make sure that the end of the stem of
the funnel is immersed in the bouillon.

4. Place the flask in a glass or metal vessel and pack it round with
pounded ice. Arrange the flask with its ice casing just above the neck
of the aspirator bottle.

[Illustration: FIG. 216.--Arrangement of apparatus for air analysis.]

5. Connect up the free end of the glass tube from the flask--after
removing the cotton-wool plug--with the air-entry tube in the mouth of
the aspirating bottle (Fig. 216).

6. Open the tap fully, and allow the water to run.

Replenish the ice from time to time if necessary.

(In emptying itself the aspirator bottle will aspirate 10 litres of air
slowly through the water in the Erlenmeyer flask.)

7. When the aspiration is completed, disconnect the flask and remove it
from its ice packing.

8. Liquefy three tubes of nutrient gelatine and add to them 0.5 c.c.,
0.3 c.c., and 0.2 c.c., respectively, of the water from the flask, by
means of a sterile graduated pipette, as in the quantitative examination
of water. Pour plates.

9. Pour a second similar set of gelatine plates.

10. Incubate both sets of plates at 20 deg. C.

11. Enumerate the colonies present in the two sets of gelatine plates
after three, four, or five days and average the results from the numbers
so obtained; estimate the number of micro-organisms present in 1 c.c.,
and then in the 50 c.c. of broth in the flask.

12. The result of air examination is usually expressed as the number of
bacteria present per cubic metre (i. e., kilolitre) of air; and as the
number of organisms present in the 50 c.c. water only represent those
contained in 10 litres of air, the resulting figure must be multiplied
by 100.

Qualitative.--

1. Proceed exactly as in the quantitative examination of air (_vide
supra_), steps 1 to 10.

2. Pour plates of wort agar with similar quantities of the air-infected
water, and incubate at 37 deg. C.

3. Pour plates of nutrient agar with similar quantities of the water and
inc