esent.

~8. Gas Production.~--

(A) _Carbon Dioxide and Hydrogen._--

_Apparatus Required_:

Fermentation tubes (_vide_ page 161) containing sugar
bouillon (glucose, lactose, etc.). The medium should be
prepared from inosite-free bouillon (_vide_ page 183).

_Reagent Required_:

n/2 caustic soda.

METHOD.--

1. Inoculate the surface of the medium in the bulb of a fermentation
tube and incubate.

2. Mark the level of the fluid in the closed branch of the fermentation
tube, at intervals of twenty-four hours, and when the evolution of gas
has ceased, measure the length of the column of gas with the millimetre
scale.

Express this column of gas as a percentage of the entire length of the
closed branch.

3. To analyse the gas and to determine roughly the relative proportions
of CO_{2} and H_{2}, proceed as follows:

Fill the bulb of the fermentation tube with caustic soda solution.

Close the mouth of the bulb with a rubber stopper.

Alternately invert and revert the tube six or eight times, to bring the
soda solution into intimate contact with the gas.

Return the residual gas to the end of the closed branch, and measure.

The loss in volume of gas = carbon dioxide.

The residual gas = hydrogen.

Transfer gas to the bulb of the tube, and explode it by applying a
lighted taper.

(B) _Sulphuretted Hydrogen._--

_Media Required_:

Iron peptone solution (_vide_ page 185).
Lead peptone solution.

1. Inoculate tubes of media, and incubate together with control tubes.

2. Examine from day to day, at intervals of twenty-four hours.

The liberation of the H_{2}S will cause the yellowish-white precipitate
to darken to a brownish-black, or jet black, the depth of the colour
being proportionate to the amount of sulphuretted hydrogen present.

Quantitative: For exact quantitative analyses of the gases produced by
bacteria from certain media of definite composition, the methods devised
by Pakes must be employed, as follows:

[Illustration: FIG. 153.--Gas-collecting apparatus.]

_Apparatus Required_:

Bohemian flask (300 to 1500 c.c. capacity) containing from
100 to 400 c.c. of the medium. The mouth of the flask is
fitted with a perforated rubber stopper, carrying an
L-shaped piece of glass tubing (the short arm passing just
through the stopper). To the long arm of the tube is
attached a piece of pressure tubing some 8 cm.