INTESTINAL GROUP 81 



(ij per cent.) dextrose, lactose, and saccharose, respec- 

 tively, in each part, and filter again, if necessary. 



6. Fill fermentation-tubes, taking care to label each 

 one properly, and sterilize in Arnold on 3 consecutive 

 days for 20 minutes. 



All gas must be carefully tilted out of the closed arm 

 of the tube while the fluid is warm. When sterilization 

 is completed, inoculate one set of the fermentation-tubes 

 with B. coli A, another set with B. coli B, and a third set 

 with B. lactis aero genes. Inoculate with the straight or 

 looped needle. 



After 24 hours, note the percentage of gas formed. 

 This is done by means of Frost's fermentation-chart, a 

 model of which is represented by Fig. 33. This chart is 

 placed between the open and closed arms of the tube, 

 resting on the neck and moved along until the extreme 

 upper end of the closed arm is level with the top of the 

 chart and parallel with the vertical lines. The percent- 

 age may then be read off by the figures marked at each 

 end of the chart. 



After taking note of the percentages, replace the fer- 

 mentation-tubes in the thermostat, and repeat the meas- 

 urement after another 24 hours' incubation. Many 

 organisms are able to obtain the oxygen necessary for 

 life by attacking compounds as, for instance, carbo- 

 hydrates and decomposing them. In such cases a tur- 

 bidity is developed in the medium. The closed arm offers 

 strict anaerobic conditions, all the atmospheric oxygen 

 having been driven out by the heat of sterilization. The 

 bulb allows the organism to come in contact with atmos- 

 pheric oxygen, and therefore offers aerobic conditions. 



Gas-production is by no means a constant phenom- 



