118 BACTERIAL ANALYSIS. 
(2) Colonies in circular dishes may be counted by means 
of Jeffer’s plate. In this each circle is marked with its 
area in square centimeters, and each division equals 1 
square centimeter. 
(3) The colonies appearing under the microscope in the 
field of a low-power objective may be counted, in several 
parts of the dish, the average taken, and the number in the 
whole dish estimated by the equation : 
Number of colonies in the field _ (4 diam. of field)? 
Whole number of colonies (4 diam. of dish)? 
* (ad) If a qualitative analysis is required, isolate 
and cultivate the different kinds of colonies.’ 
4. Test for the Presence of Bacillus coli.— 
(a) Prepare 10 fermentation-tubes of sterile bouil- 
lon containing I per cent. glucose. 
(6) ‘To each tube add 1 cc. of the water to be 
tested. 
(c) Place the tubes in the incubator at 37.5° C. 
for three days. 
(2d) Note the amount of gas which forms on each 
of the three days. 
If gas-forming bacteria are present, gas will col- 
lect in the closed tube. The number of tubes 
showing the presence of gas gives a rough idea of 
the number of gas-producing bacilli present. 
Bacillus coli, if present, will fill the closed tube 
by the second day. ‘Too little or too much gas 
does not point to the presence of Bacillus coli. 
Bacillus coli forms most of its gas during the first 
twenty-four hours. The liquid in the bulb must 
1See p. 112 for Fuller and Johnson’s groups of water 
bacteria, 
