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°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° C. 23. Count and record the number of colonies that have developed upon the agar at 37° 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 of paper, upon which is printed a dead black disc, subdivided by concentric circles and radii, printed in white. In Jeffery's counter (Fig. 207), each subdivision has an area of 1 square centimetre; in Pakes' counter (Fig. 208), radii divide the circle into sixteen equal sectors, and counting is facilitated by concentric circles equidistant from the centre. [Illustration: FIG. 207.--Jeffery's disc, reduced.] [Illustration: FIG. 208.--Pakes' disc, reduced.] (a) In the final counting of each plate, place the plate over the counting disc, and centre it, if possible, making its periphery coincide with one or other of the concentric circles. (b) Remove the cover of the plate, and by means of a hand lens count the colonies appearing in each of the sectors in turn. Make a note of the number present in each.