116 BACTERIAL ANALYSIS. 
(a) Transfer 1 c.c. of the water to be examined, 
by means of a sterile graduated pipet, to each of 
three tubes of liquefied gelatin.’ Use a sterile pipet 
for each transfer. 
(4) Shake the tubes, flame the lips, and pour into 
sterile Petri dishes. Place these in the dark at 
20° C. 
(c) Examine the plates from day to day, and 
count the colonies that appear. 
Fic. 50.—Simple microscope for counting colonies, 
It is best to count all the colonies if possible; but when 
they are very numerous, some one of the various methods 
devised for counting colonies must be employed. 
(1) Wolfhiigel’s counting-plate consists of a glass plate 
on which are ruled square centimeters. The Petri dish is 
‘If the water is suspected of containing large numbers 
of bacteria, smaller quantities than 1 c.c. should be added 
to each tube, or, better, dilute the water by the addition of 
a known quantity of sterile water. Many workers prefer 
to mix the water and the gelatin in the Petri dish instead 
of in the tube. 
