_vide_ page 481).

Percentage solutions of pure phenol (_vide_ page 482).

Aqueous emulsion of B. coli (_vide_ page 481).

Tubes of bile salt broth.

~Preliminary Tests.~--

_a. Inhibition Coefficient._--

Determine the lowest percentage of germicide-x which inhibits growth of
B. coli in the bile salt broth, and the highest percentage which fails
to inhibit (page 311). On the result of this experiment determine the
bulk of medium required in the subculture tubes and the percentage
solutions to be employed in the trial trip. Assuming the inhibition
coefficient to be 1:1000, it will be quite safe to employ the ordinary
culture tubes containing 10 c.c. medium in the subsequent experiments.

_b. Trial Trip._--

Determine the lethal effect of a series of five solutions of germicide-x
(say 1:100, 1:250, 1:300, 1:500, 1:600) at contact times of 2-1/2, 5, 25
and 30 minutes in the following manner:

1. Arrange five test-tubes marked A to E in the lower tier of the
test-tube rack.

2. Into tube A pipette 5 c.c. germicide-x 1:100 solution.

Into tube B pipette 5 c.c. germicide-x 1:200 solution.

Into tube C pipette 5 c.c. germicide-x 1:300 solution.

Into tube D pipette 5 c.c. germicide-x 1:500 solution.

Into tube E pipette 5 c.c. germicide-x 1:600 solution.

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

5. Take a sterile 1 c.c. pipette from the case, pick up a sterile rubber
washer with forceps and push the point of the pipette into the central
hole.

6. Put down the forceps on the bench with the sterile points projecting
over the edge. Without taking the tube from the rack remove the
cotton-wool plug from tube A, and lower the pipette, with the rubber
washer affixed, on to the open mouth of the tube; with the help of the
forceps to steady the washer, push the pipette on through the hole until
the point of the pipette has reached to within a few millimetres of the
bottom of the tube (see fig. 219).

7. Adjust in the same way a pipette and a washer in the mouth of each of
the other tubes, B, C, D and E.

8. Set the electric signal clock to ring for the commencement of the
experiment and at subsequent inte