n distilled water) into
each dish to the depth of 2 cm. to absorb the heat of the sun's rays and
so eliminate possible effects of temperature on the cultivations.

5. After exposures for periods similar to those employed in the
preceding experiment, incubate and complete the observation as above.

(c) Primary Colours: Each colour--violet, blue, green and red--must be
tested separately.

1. Prepare plate cultivations, as in the previous "light" experiments,
and incubate controls.

2. Fasten a strip of black paper, 3 cm. wide, across one diameter of the
cover of each plate.

3. Coat the remainder of the surface of the cover with a film of pure
photographic collodion which contains 2 per cent. of either of the
following aniline dyes, as may be necessary:

Chrysoidin (for red).
Malachite green (for green).
Eosin, bluish (for blue).
Methyl violet (for violet).

4. Expose the plates, thus prepared, to bright daylight (but not direct
sunlight) for varying periods, and complete the observations as in the
preceding experiments. The bactericidal action of light appears to
depend upon the more refrangible rays of the violet end of the spectrum
and is noted whether the red yellow rays are transmitted or not.

5. Control the results.

NOTE.--The ultra-violet rays obtained from a quartz mercury
vapour lamp destroy bacterial life with great rapidity under
laboratory conditions.

(C) _Heat._--(_Vide_ Thermal Death-point, page 298.)

(D) _Antiseptics and Disinfectants._--The resistance exhibited by any
given bacterium toward any specified disinfectant or germicide should be
investigated with reference to the following points:

(A) ~Inhibition coefficient~--i. e., that _percentage of the
disinfectant_ present in the nutrient medium which is sufficient to
prevent the growth and multiplication of the bacterium.

(B) ~Inferior lethal coefficient~--i. e., the _time exposure_ necessary
to kill _vegetative forms_ of the bacterium suspended in water at
20 deg. to 25 deg. C, in which the disinfectant is present in _medium_
concentration (concentration insufficient to cause plasmolysis). And if
the bacterium is one which forms spores,

(C) ~Superior lethal coefficient~--i. e., the _time exposure_ necessary
to kill the _spores_ of the bacterium under conditions similar to those
obtaining in B.

The example here detailed only specifically refers to certain of the
disinfectants:

viz:--Bichlo