vegetative
forms.)

3. Incubate one tube cultivation of the organism aerobically or
anaerobically, as may be necessary, in each incubator, and examine at
half-hour intervals for from five to eighteen hours.

4. Note that temperature at which growth is first observed
macroscopically (Optimum temperature).

5. Continue the incubation until the completion of seven days. Note the
extremes of temperature at which growth takes place (Range of
temperature).

6. Control these results--if considered necessary arranging the series
of incubators to include each degree centigrade for five degrees beyond
each of the extremes previously noted.

(B) _Optimum._--

1. Prepare a second series of ten tube cultivations under similar
conditions as to reaction of medium.

2. Incubate in a series of incubators in which the temperature is
regulated at intervals of 1 deg. C. for five degrees on either side of
optimum temperature observed in the previous experiment (A, step 4).

3. Observe again at half-hour intervals and note that temperature at
which growth is first visible to the naked eye = Optimum temperature.

(C) _Thermal Death-point (t. d. p.)_--

Moist--Vegetative Forms:

The _t. d. p._ here is that ~temperature~ which with certainty kills a
watery suspension of the organisms in question after an exposure of ~10
minutes~.

[Illustration: FIG. 155.--Hearson's water-bath.]

_Apparatus Required:_

Water-bath. For the purpose of observing the thermal
death-point a special water-bath is necessary. The
temperature of this piece of apparatus is controlled by
means of a capsule regulator that can be adjusted for
intervals of half a degree centigrade through a range of
30 deg., from 50 deg. C. to 80 deg. C. by means of a spring,
actuated by the handle a, which increases the pressure
in the interior of the capsule. A hole is provided for the
reception of the nozzle of a blast pump, so that a current
of air may be blown through the water while the bath is in
use, and thus ensure a uniform temperature of its contents.
Through a second hole is suspended a certified centigrade
thermometer, the bulb of which although completely immersed
in the water is raised at least 2 cm. above the floor of
the bath.

Sterile glass capsules.

Flask containing 250 c.c. sterile normal saline solution.

Case of sterile pipettes, 10 c.c. (in tenths of