e latter for the isolation of the
micro-organisms of diphtheria and tubercle. The first essential in the
latter process is the concentration of the bacterial contents of a large
volume of the sample into a small compass; but in the case of milk,
thorough centrifugalisation is substituted for filtration.

_Apparatus Required_:

A large centrifugal machine. This machine, to be of real
service in the bacteriological examination of milk, must
conform to the following requirements:

1. The centrifugal machine must be of such size, and should
carry tubes or bottles of such capacity, as to enable from
200 to 500 c.c. of milk to be manipulated at one time.

2. The rate of centrifugalisation should be from 2500 to
3000 revolutions per minute.

3. The portion of the machine destined to carry the tubes
should be a metal disc, of sufficient weight to ensure good
"flank" movement, continuing over a considerable period of
time. In other words, the machine should run down very
gradually and slowly after the motive power is removed, thus
obviating any disturbance of the relative positions of
particulate matter in the solution that is being
centrifugalised.

4. The machine should preferably be driven by electricity,
or by power, but in the case of hand-driven machines--

(a) The gearing should be so arranged that the requisite
speed is obtained by not more than forty or fifty
revolutions of the crank handle per minute, so that it may
be maintained for periods of twenty or thirty minutes
without undue exertion.

(b) The handle employed should be provided with a special
fastening (e. g., a clutch similar to that employed for
the free wheel of a bicycle), or should be readily
detachable so that, on ceasing to turn, the handle should
not, by its weight and air resistance, act as a brake and
stop the machine too suddenly.

One of the few satisfactory machines of this class is shown
in figure 212.

[Illustration: FIG. 212.--Electrically driven centrifugal machine, with
flexible (broken) spindle encircled by the field magnets of the motor.]

Sterile centrifugal tubes, of some 60-70 c.c. capacity,
tapering to a point at the closed end, plugged with
cotton-wool.

Small centrifugal machine to run two tubes of 10 c.c.
capacity at 2500 to 3000 revol