hich it will be convenient to consider first, amounts to
about 110 c.c. It is collected in the manner previously described
(_vide_ page 416); similar bottles are used, and if four are filled the
combined contents, amounting to about 240 c.c., will provide ample
material for both the qualitative and quantitative examinations. Unless
the examination is to be commenced at once, the ice-box must be
employed, otherwise water bacteria and other saprophytes will probably
multiply at the expense of the microbes indicative of pollution, and so
increase the difficulties of the investigation.

In the routine examination of water supplies it is customary to limit
the qualitative examination to a search for

A. B. coli and its near allies.

B. Streptococci,

organisms which are frequently spoken of as microbes of indication, as
their presence is held to be evidence of pollution of the water by
material derived from the mammalian alimentary canal, and so to
constitute a danger signal.

C. Some observers still attach importance to the presence of B.
enteritidis sporogenes, but as the search for this bacterium,
(relatively scarce in water) necessitates the collection of a fairly
large quantity of water it is not usually included in the routine
examination.

In the case of water samples examined during the progress of an
epidemic, of new supplies and of unknown waters the search is extended
to embrace other members of the coli-typhoid group; and on occasion the
question of the presence or absence of Vibrio cholerae or (more rarely)
such bacteria as B. anthracis or B. tetani, may need investigation.

When pathogenic or excremental bacteria are present in water, their
numbers are relatively few, owing to the dilution they have undergone,
and it is usual in commencing the examination, to adopt one or other of
the following methods:

A. _Enrichment_, in which the harmless non-pathogenic bacteria may be
destroyed or their growth inhibited, whilst the growth of the parasitic
bacteria is encouraged.

This is attained by so arranging the environment, (i. e., Media,
incubation temperature, and atmosphere) as to favor the growth of the
pathogenic organisms at the expense of the harmless saprophytes.

B. _Concentration_, whereby all the bacteria present in the sample of
water, pathogenic or otherwise, are concentrated in a small bulk of
fluid.

This is usually effected by filtration of the water sample through a
porcelain filter can