rful pump, such as a foot bicycle pump, fitted with a manometer. The
candle is then immersed in a jar of water and held completely submerged
whilst the internal pressure is gradually raised to two atmospheres by
the action of the pump. Any crack or flaw will at once become obvious by
reason of the stream of air bubbles issuing from it.

The examination for permeability is conducted as follows:

_Apparatus Required_:

Filtering apparatus: The actual filter candle that is used
must be the one it is intended to test and must be
previously carefully sterilised; the arrangement of the
apparatus will naturally vary with each different form of
filter, one or other of those already described (_vide_
pages 42-48).

Plate-levelling stand.

Case of sterile plates.

Case of sterile pipettes, 10 c.c. (in tenths).

Case of sterile pipettes, 1 c.c. (in tenths).

Tubes of nutrient gelatine.

Flask containing sterile normal saline solution.

Sterile measuring flask, 1000 c.c. capacity.

METHOD.--

1. Prepare surface cultivations, on nutrient agar in a culture bottle,
of the Bacillus mycoides, and incubate at 20 deg. C., for forty-eight
hours.

2. Pipette 5 c.c. sterile normal saline into the culture bottle and
emulsify the entire surface growth in it.

3. Pipette the emulsion into the sterile measuring flask and dilute up
to 1000 c.c. by the addition of sterile water.

4. Pour the emulsion into the filter reservoir and start the filtration.

5. When the filtration is completed, pour six agar plates each
containing 1 c.c. of the filtrate.

6. Incubate at 37 deg. C. until, if necessary, the completion of seven
days.

7. If the filtrate is not sterile, subcultivate the organism passed and
determine its identity with the test bacterium before rejecting the
filter--since the filtrate may have been accidentally contaminated.

8. If the filtrate is sterile, resterilise the candle and repeat the
test now substituting a cultivation of B. prodigiosus--a bacillus of
smaller size.

9. If the second test is satisfactory, test the candle against a
cultivation of a very small coccus, e. g., Micrococcus melitensis, in
a similar manner; in this instance continuing the incubation of
cultivations from the filtrate for fourteen days.

TESTING OF DISINFECTANTS.

Methods have already been detailed (page 310) for the purpose of
studying the vital resistance offere