184 



BACTERIOLOGY. 



FIG. 34. 



upright. (See Fig. 34.) To fill the tube the fluid 

 (they are only used with fluid media) is poured into the 

 bulb until this is about half full. The tube is then 

 tilted until the closed arm is nearly horizontal, so that 

 the air may flow out into the bulb and the fluid flow 

 into the closed arm to take its place. When this has 

 been completely filled enough fluid should be added to 

 cover the lowest expanding portion of the bulb, and the 

 opening of the bulb plugged with cotton. They are 

 then to be sterilized. During sterilization they are to 

 be maintained in the upright position. Under the in- 

 fluence of heat the tension of water- 

 vapor in the closed arm forces most 

 of the fluid into the bulb. As the 

 tubes cool the fluid returns to its 

 place in the closed arm and fills it 

 again with the exception of a small 

 space at the top which is occupied by 

 the air originally dissolved in the 

 liquid and which has been driven 

 out by the heat. The air-bubble 

 should be tilted out after each ster- 

 ilization, and finally, after the third 

 exposure to steam this arm of the 

 tube will be free from air. 



The medium employed is bouillon 

 containing some fermentable carbo- 

 's fermentation- hydrate, as glucose, lactose, or sac- 

 charose. After inoculation the flasks 

 are placed in the incubator and the amount of gas that 

 collects in the closed arm is, from day to day, noted. 



From studies that have been made this gas is found to 

 consist usually of about one part by volume of carbonic 



