utions per minute preferably
driven by electricity, of the type figured on page 327 (Fig.
162).

Sterile centrifugal tubes of 10 c.c. capacity with the
distal extremity contracted to a narrow tube and graduated
in hundredths of a cubic centimetre (Fig. 213).

Sterilised cork borer.

Case of sterile pipettes, 10 c.c. (in tenths of a cubic
centimetre).

Case of sterile pipettes, 1 c.c. (in tenths of a cubic
centimetre).

Sterile teat pipettes.

Flask of sterile normal saline solution.

METHOD.--

1. Fill 50 c.c. of the milk sample into each of four tubes, and replace
the cotton-wool plugs by solid rubber stoppers (sterilised by boiling),
and fit the tubes in the centrifugal machine.

NOTE.--One or two cubic centimetres of paraffinum liquidum
introduced into the buckets of the centrifuge before the
glass tubes are inserted will obviate any risk of breakage
to the latter.

[Illustration: FIG. 213.--Milk sedimenting tubes.]

[Illustration: FIG. 214.--Milk in centrifuge tube.]

2. Centrifugalise the milk sample for thirty minutes at a speed of 2500
revolutions per minute.

3. Remove the motive power and allow the machine to slow down gradually.

4. Remove the tubes of milk from the centrifuge. Each tube will now show
(Fig. 214):

(a) A superficial layer of cream (varying in thickness with different
samples) condensed into a semi-solid mass, which can be shown to
contain some organisms and a few leucocytes.

(b) A central layer of separated milk, thin, watery, and opalescent, and
containing extremely few bacteria.

(c) A sediment or deposit consisting of the great majority of the
contained bacteria and leucocytes, together with adventitious matter,
such as dirt, hair, epithelial cells, faecal debris, etc.

5. Withdraw the rubber stopper and remove a central plug of cream from
each tube by means of a sterile cork borer; place these masses of cream
in two sterile capsules. Label C^{1} and C^{2}.

6. Remove all but the last one or two c.c. of separated milk from each
tube, by means of sterile pipettes.

7. Mix the deposits thoroughly with the residual milk, pipette the
mixture from each pair of tubes into one sterile 10 c.c. tube
(graduated) by means of sterile teat pipettes, then fill to the 10 c.c.
mark with sterile normal saline solution and mix together. Label D^{1}
and D^{2}.

8. Place the two tubes of mixed deposit