.

7. Pour a set of gelatine plates from the contents of each capsule,
three plates in a set, and containing respectively 0.2, 0.3, and 0.5
c.c. of the dilution. Label carefully; incubate at 20 deg. C. for three,
four, or five days.

8. Enumerate the organisms present in those sets of plates which have
not liquefied, probably those from dilution III or IV, and calculate
therefrom the number present per cubic centimetre of the original sample
of sewage.

Qualitative.--The qualitative examination of sewage is concerned with
the identification and enumeration of the same bacteria dealt with under
the corresponding section of water examination; it is consequently
conducted on precisely similar lines to those already indicated (_vide_
pages 426 to 441).

EXAMINATION OF AIR.

Quantitative.--

_Apparatus Required_:

Aspirator bottle, 10 litres capacity, fitted with a delivery
tube, and having its mouth closed by a perforated rubber
stopper, through which passes a short length of glass
tubing.

Erlenmeyer flask, 250 c.c. capacity (having a wide mouth
properly plugged with wool), containing 50 c.c. sterile
water.

Rubber stopper to fit the mouth of the flask, perforated
with two holes, and fitted as follows:

Take a 9 cm. length of glass tubing and bend up 3 cm. at one
end at right angles to the main length of tubing. Pass the
long arm of the angle through one of the perforations in the
stopper; plug the open end of the short arm with
cotton-wool.

Take a glass funnel 5 or 6 cm. in diameter with a stem 12
cm. in length and bend the stem close up to the apex of the
funnel, in a gentle curve through a quarter of a circle;
pass the long stem through the other perforation in the
rubber stopper.

A battery jar or a small water-bath to hold the Erlenmeyer
flask when packed round with ice.

Supply of broken ice.

Rubber tubing.

Screw clamps and spring clips, for tubing.

Water steriliser.

Retort stand and clamps.

Apparatus for plating (as for enumeration of water
organisms, _vide_ page 420).

METHOD.--

1. Fill 10 litres of water into the aspirating bottle and attach a piece
of rubber tubing with a screw clamp to the delivery tube. Open the taps
fully and regulate the screw clamp, by actual experiment, so that the
tube delivers 1 c.c. of water every second. The