in length,
plugged at its free end with a piece of cotton-wool. Measure
accurately the total capacity of the flask and exit tube,
also the amount of medium contained. Note the difference.

Gas receiver. This is a bell jar of stout glass, 14 cm. high
and 9 cm. in diameter. At its apex a glass tube is fused in.
This rises vertically 5 cm., and is then bent at right
angles, the horizontal arm being 10 cm. in length. A
three-way tap is let horizontally into the vertical tube
just above its junction with the bell jar.

An iron cylinder just large enough to contain the bell jar.

About 15 kilos of metallic mercury.

Melted paraffin.

An Orsat-Lunge working with mercury instead of water, provided with two
gas tubes of extra length (capacity 120 and 60 c.c. respectively and
graduated throughout, both being water-jacketed) or other gas analysis
apparatus, capable of dealing with CO_{2}, O_{2}, H_{2}, and N_{2}.

METHOD.--

1. Inoculate the medium in the flask in the usual manner, by means of a
platinum needle, taking care that the neck of the flask and the rubber
stopper are thoroughly flamed before and after the operation.

[Illustration: FIG. 154.--Orsat-Lunge gas analysis apparatus.]

2. Fill the iron cylinder with mercury.

3. Place the bell jar mouth downward in the mercury--first seeing that
there is free communication between the interior of the jar and the
external air--and suck up the mercury into the tap; then shut off the
tap.

4. Plug the open end of the three-way tap with melted wax.

5. Connect up the horizontal arm of the culture flask with that of the
gas receiver by means of the pressure tubing (after removing the
cotton-wool plug from the rubber tube), as shown in Fig. 153.

6. Give the three-way tap half turn to open communication between flask
and receiver, and seal _all_ joints by coating with a film of melted
wax. When the tap is turned, the mercury in the receiver will naturally
fall.

7. Place the entire apparatus in the incubator. (Two hours later, by
which time the temperature of the apparatus is that of the incubator,
mark the height of the mercury on the receiver.)

8. Examine the apparatus from day to day and mark the level of the
mercury in the receiver at intervals of twenty-four hours.

9. When the evolution of gas has ceased, remove the apparatus from the
incubator; clear out the wax from the nozzle of the three-way