CHANGES IN THE REACTION OF MEDIA 217 



This property of fermentation with evolution of gas is of 

 such importance as a differential characteristic that con- 

 siderable attention has been given to it, and those who have 

 been most intimately concerned in the development of our 

 knowledge on the subject do not consider it sufficient to 

 say that the growth of an organism "is accompanied by the 

 production of gas-bubbles/' but that under given condi- 

 tions we should determine not only the amount of gas or 

 gases produced by the organism under consideration, but 

 also their nature. For this purpose, Smith 1 recommends the 

 employment of the fermentation tube. This is a tube bent 

 at an acute angle, closed at one end and enlarged with a bulb 

 at the other. At the bend the tube is constricted. To it 

 a glass foot is attached so that the tube may stand upright. 

 (See Fig. 36.) To fill 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 with cotton. The tubes thus 

 filled are then to be sterilized. 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 and 



1 An excellent and exhaustive contribution to this subject has been 

 made by Theobald Smith in the Wilder Quarter-Century Book, Ithaca, 

 N. Y., 1803. 



