tence of adequate controls in all
experimental work cannot be too urgently insisted upon. Every batch of
plates that is poured should include at least one of the presumably
"sterile" medium; plate or tube cultures should be made from the various
diluting fluids; every tube of carbohydrate medium that is inoculated
should go into the incubator in company with a similar but uninoculated
tube, and so on.

BACTERIOLOGICAL EXAMINATION OF WATER.

The bacteria present in the water may comprise not only varieties which
have their normal habitat in the water and will consequently develop at
20 deg. C., but also if the water has been contaminated with excremental
matter, varieties which have been derived from, or are pathogenic for,
the animal body, and which will only develop well at a temperature of
37 deg. C. In order to demonstrate the presence of each of these classes
it will be necessary to incubate the various cultivations at each of these
temperatures.

Further, the sample of water may contain moulds, yeasts, or torulae, and
the development of these will be best secured by plating in wort
gelatine and incubating at 20 deg. C.

~1. Quantitative.~--

_Collection of the Sample._--The most suitable vessels for the reception
of the water sample are small glass bottles, 60 c.c. capacity, with
narrow necks and overhanging glass stoppers (to prevent contamination of
the bottle necks by falling dust). These must be carefully sterilised in
the hot-air steriliser (_vide_ page 31).

(a) If the sample is obtained from a ~tap~ or ~pipe~, turn on the water
and allow it to run for a few minutes. Remove the stopper from the
bottle and retain it in the hand whilst the water is allowed to run into
the bottle and three parts fill it. Replace the stopper and tie it down,
but _do not seal it_.

(b) If the sample is obtained from a ~stream~, ~tank~, or ~reservoir~,
fasten a piece of stout wire around the neck of the bottle, remove the
stopper, and retain it in the hand. Then, using the wire as a handle,
plunge the bottle into the water, mouth downward, until it is well
beneath the surface; then reverse it, allow it to fill, and withdraw it
from the water. Pour out a few cubic centimetres of water from the
bottle, replace the stopper, and tie it down.

[Illustration: FIG. 203.--Esmarch's collecting bottle for water
samples.]

(c) If the sample is obtained from a ~lake~, ~river~ or the ~sea~; or when
it is desired to compare samples