FERMENTATION. 



217 



the tube, the fluid (it is used only 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 sufficient fluid 

 should be added to bring its level within the bulb just 

 beyond the bend, and the opening of the bulb plugged 



FIG. 37. 



Fermentation-tube. 



with cotton. The tubes thus filled are then to be ster- 

 ilized. During sterilization they are to be maintained 

 in the upright position. Under the influence of heat the 

 tension of the water-vapor in the closed arm forces most 

 of the fluid into the bulb. As the tube cools, 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 



