es in the incubator at 37 deg. C.

19. Examine the tubes at intervals of 24 hours, and record the results
in tabular form as shown in Table page 491 (the figures in the squares
indicate the number of hours at which the changes in the medium due to
the growth of B. coli first appeared).

20. If a consideration of the tabulated results indicates strengths of
Germicide-x lethal at 2-1/2 and 30 minutes the final test can be
arranged, but if this result has not been attained, sufficient evidence
will probably be available to enable a second trial test to be planned
which will give the required information.

~Final Test.~--

c. _Determination of Phenol Coefficient._--

_X-Disinfectant._--This comprises two distinct tests, one of the
Germicide-x, the other of the standard phenol.

1. Arrange five test-tubes clearly marked in the lower tier of the rack.

2. Pipette into each 5 c.c. respectively of the five percentage
solutions of x-disinfectant which the trial run has already shown will
include those affording lethal values at 2-1/2 and 30 minutes.

3. Arrange 20 tubes of bile salt broth in the upper tier of the
test-tube rack in two rows, those in the front row numbered
consecutively from left to right 1-10, those in the back row 11-20.

4. Arrange further 20 tubes of bile salt broth numbered 21-40 in two
rows in a second smaller rack which can be stood on the upper tier of
the rack as soon as the first 20 tubes have been inoculated.

5. Place a square wire basket of about 50 tube capacity close to the
left of the test-tube rack, for the reception of the inoculated tubes.

6. Adjust a sterile 1 c.c. pipette in the mouth of each of the tubes, A,
B, C, D and E, by means of a washer, as previously described.

7. Set the electric signal clock to ring for the commencement of the
experiment and subsequently at 2-1/2, 5, 10, 15, 20, 25, 30 and 35
minutes.

8. Complete precisely as indicated in Trial Runs, steps 9-19.

_Control Phenol._--

Immediately the subculture tube from the 30-minute contact period have
been inoculated, carry out a precisely similar experiment, in which
five percentage strengths of Phenol, (e. g., 1.1, 1.0, 0.9, 0.75, 0.7)
are arranged in the lower tier of the test-tube rack in place of the
five strengths of Germicide-x.

Calculate the phenol coefficient by the following method:

(a) Divide the figure representing the percentage strength of the
weakest lethal dilution of the carbolic acid c