Ten minutes later connect up the desiccator
to a sulphuric acid wash-bottle interposing an air filter so that only
dry sterile air enters.

[Illustration: FIG. 158.--Petri dish for drying cultivations.]

7. At intervals of five hours open the apparatus, remove one of the
cover-slip films from the Petri dish, and transfer it to the interior of
a culture flask, with every precaution against contamination. Reseal the
desiccator and again exhaust, and subsequently admit dry sterile air as
before.

8. Incubate the culture flask under optimum conditions until the
completion of seven days, if necessary; and determine the time exposure
at which death occurs.

9. Pour plates from those culture flasks which grow, to determine the
absence of contamination.

10. Repeat these observations at hourly intervals for the five hours
preceding and succeeding the death time, as determined in the first set
of experiments.

(B) _Light._--

(a) Diffuse Daylight:

1. Prepare a tube cultivation in nutrient bouillon, and incubate under
optimum conditions, for forty-eight hours.

[Illustration: FIG. 159.--Plate with star for testing effect of light.]

2. Pour twenty plate cultivations, ten of nutrient gelatine and ten of
nutrient agar, each containing 0.1 c.c. of the bouillon culture.

3. Place one agar plate and one gelatine plate into the hot and cold
incubators, respectively, as _controls_.

4. Fasten a piece of black paper, cut the shape of a cross or star, on
the centre of the cover of each of the remaining plates (Fig. 159).

5. Expose these plates to the action of diffuse daylight (not direct
sunlight) in the laboratory for one, two, three, four, five, six, eight,
ten, twelve hours.

6. After exposure to light, incubate under optimum conditions.

7. Examine the plate cultivations after twenty-four and forty-eight
hours' incubation, and compare with the two controls. Record results. If
growth is absent from that portion of the plate unprotected by the black
paper, continue the incubation and daily observation until the end of
seven days.

8. Control the results.

(b) Direct Sunlight:

1. Prepare plate cultivations precisely as in the former experiments and
place the two controls in the incubators.

2. Arrange the remaining plates upon a platform in the direct rays of
the sun.

3. On the top of each plate stand a small glass dish 14 cm. in diameter
and 5 cm. deep.

4. Fill a solution of potash alum (2 per cent. i