pour a plate from each tube. Label
each plate with (a) the distinctive number of the sample, (b) the
quantity of water sample it contains, and (c) the date.

12. Pour the contents of a tube of liquefied agar--not inoculated--into
a Petri dish to act as a control to demonstrate the sterility of the
batch of agar employed.

13. Allow the plates to set, and incubate at 37 deg. C.

14. Empty the water chamber of the levelling apparatus and refill it
with ice-water.

15. By means of the sterile 10 c.c. pipette deliver 9.9 c.c. sterile
distilled water into a sterile glass capsule.

16. Add 0.1 c.c. of the water sample to the 9.9 c.c. sterile water in
the capsule. This will give a dilution of 1 in 100.

17. Plant the six tubes of nutrient gelatine in the following manner: To
the first tube add 0.5 c.c. of the water sample direct from the bottle;
to the second, 0.3 c.c.; and to the third, 0.2 c.c.; and pour a plate of
each tube. To the fourth tube add 0.5 c.c. of the diluted water sample
from the capsule; to the fifth, 0.3 c.c.; and to the sixth, 0.2 c.c.;
and pour a plate from each.

18. Label each plate with the quantity of the water sample it
contains--that is, 0.5 c.c., 0.3 c.c., 0.2 c.c., 0.005 c.c., 0.003 c.c.,
and 0.002 c.c.

19. Pour a control (uninoculated) gelatine plate.

20. Allow the plates to set, and incubate at 20 deg. C.

21. To the first tube of liquefied wort gelatine add 0.5 c.c. water
sample; to the second, 0.3 c.c.; and to the third, 0.2 c.c.

22. Label the plates, allow them to set, and incubate at 20 deg. C.

23. Count and record the number of colonies that have developed upon the
agar at 37 deg. C. after forty-eight hours' incubation.

24. Note the number of colonies present on each of the gelatine and wort
gelatine plates after forty-eight hours' incubation.

25. Replace the gelatine and wort plates in the incubator; observe again
at three days, four days, and five days.

26. Calculate and record the number of organisms present per cubic
centimetre of the original water from the average of the six gelatine
plates at the latest date possible up to seven days--the presence of
liquefying bacteria may render the calculation necessary at an earlier
date, hence the importance of daily observations.

_Method of Counting._--The most accurate method of counting the colonies
on each of the plates is by means of either Jeffery's or Pakes' counting
disc. Each of these discs consists of a piece o