teria which
may have escaped the disintegrating action of the beads. Pipette off the
supernatant emulsion for use in the test.

_2. Germicide._--

_a. Disinfectant to be tested._--

The first essential point is to test the unknown disinfectant, which may
be referred to as germicide-x, on the lines set out on page 311 to
determine its inhibition coefficient.

This constant having been fixed, prepare various solutions of
germicide-x with sterilised distilled water by accurate volumetric
methods, commencing with a solution somewhat stronger than that
representing the inhibition coefficient. The solutions must be prepared
in fairly large bulk, not less than 5 c.c. of the disinfectant being
utilised for the preparation of any given percentage solution.

_b. Standard Control._--~Phenol.~

The standard germicide used for comparison should be one which is not
subject to variation in its chemical composition, and the one which has
obtained almost universal use is Phenol.

The following table shows the effect of different percentages of
carbolic acid upon B. coli for varying contact times, compiled from an
experiment conducted under the standard conditions referred to under
Environment. The results closely correspond to those recorded by the
Lancet Commission on Disinfectants, 1909.

---------------------+-----------------------------------
| Contact time in minutes.
Percentage of phenol +------+---+---+---+---+---+---+----
| 2-1/2| 5 |10 | 15| 20| 25| 30| 35
---------------------+------+---+---+---+---+---+---+----
1.20 | - | - | - | - | - | - | - | -
1.10 | - | - | - | - | - | - | - | -
1.0 | + | - | - | - | - | - | - | -
0.9 | + | - | - | - | - | - | - | -
0.85 | + | + | - | - | - | - | - | -
0.80 | + | + | + | - | - | - | - | -
0.75 | + | + | + | + | + | - | - | -
0.7 | + | + | + | + | + | + | - | -
0.65 | + | + | + | + | + | + | + | -
---------------------+------+---+---+---+---+---+---+----

- = No growth, i. e., bacteria killed.
+ = Growth, i. e., bacteria still living.

From this it will be seen that the following percentage solutions will
need to be prepared, namely: 1.1 per cent., 1.0 per cent., 0.9 per
cent., 0.75 per cent., 0.7 per cent., as controls for each experiment.

Prepare