ubate at 37 deg. C.

4. Pour similar plates of wort gelatine and incubate at 20 deg. C.

5. Pick off the individual colonies that appear in the several plates,
subcultivate them on the various media, and identify them.

EXAMINATION OF SOIL.

The bacteriological examination of soil yields information of value to
the sanitarian during the progress of the process of homogenisation of
"made soil" (e. g., a dumping area for the refuse of town) and
determines the period at which such an area may with propriety and
safety be utilised for building purposes; or to the agriculturalist in
informing him of the suitability of any given area for the growth of
crops.

The surface of the ground, exposed as it is to the bactericidal
influence of sunlight and to rapid alternations of heat and cold, rain
and wind, contains but few micro-organisms. Again, owing to the density
of the molecules of deep soil and lack of aeration on the one hand, and
the filtering action of the upper layers of soil and bacterial
antagonism on the other, bacterial life practically ceases at a depth of
about 2 metres. The intermediate stratum of soil, situated from 25 to 50
cm. below the surface, invariably yields the most numerous and the most
varied bacterial flora.

~Collection of Sample.~--A small copper capsule 6 cm. high by 6 cm.
diameter, with "pull-off" cap secured by a bayonet catch, previously
sterilised in the hot-air oven, is the most convenient receptacle for
samples of soil.

[Illustration: FIG. 217.--Soil scoop.]

The instrument used for the actual removal of the soil from its natural
position will vary according to whether we require surface samples or
soil from varying depths.

(a) For ~surface~ samples, use an iron scoop, shaped like a shoe horn,
but provided with a sharp spine (Fig. 217). This is wrapped in asbestos
cloth and sterilised in the hot-air oven. When removed from the oven,
wrap a piece of oiled paper, silk, or gutta-percha tissue over the
asbestos cloth, and secure it with string, as a further protection
against contamination.

On reaching the spot whence the samples are to be taken, the coverings
of the scoop are removed, and the asbestos cloth employed to brush away
loose stones and debris from the selected area. The surface soil is then
broken up with the point of the scoop, scraped up and collected in the
body of the scoop, and transferred to the sterile capsule for
transmission.

[Illustration: FIG. 218.--Frae