172 BACTERIOLOGK. 



When solid it is placed in the incubator. After twenty- 

 four to thirty-six hours, if the organism possesses the 

 property of causing fermentation of sugar, the medium 

 will be dotted everywhere with very small cavities con- 

 taining the gas that has resulted. 



This property of fermentation with production of gas 

 is of such importance as a differential means that latterly 

 considerable attention has been given to it, and those 

 who have been most intimately concerned in the devel- 

 opment of our knowledge on the subject do not consider 

 it enough to say that the growth of ■ an organism " is 

 accompanied by the production of gas-bubbles," but 

 that under given conditions we should determine not 

 only the amount of gas produced by the organism under 

 consideration but also its quality. For this purpose 

 Smith* recommends the employment of the fermenta- 

 tion-tube used by Einhorn in the quantitative fermenta- 

 tion test for sugar in the urine. It 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. 33.) To iill 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. 



1 An excellent and exhaustive contribution to this subject has been made 

 by Theobald Smith in "The Wilder Quarter-Century Book," Ithaca, N. Y., 



