212 BACTERIOLOGY. 



media to which 1 to 2 per cent, of grape-sugar (glucose) 

 has been added. A convenient method of demonstrat- 

 ing this projjerty is to employ a tube about half full of 

 agar-agar containing the necessary amount of grape- 

 sugar. The medium is to be liquefied on a water-bath, 

 and then cooled to about 42° C, when a small quantity 

 of a pure culture of the organism under consideration 

 should carefully be distributed through it. The tube is 

 then placed in ice-water and rapidly solidified in the 

 vertical position. When solid it is placed in the incu- 

 bator. After twenty-four to thirty-six hours, if the 

 organism possesses the property of causing fermentation 

 of glucose, the medium will be dotted everywhere with 

 very small cavities containing the gas that lias resulted. 

 This property of fermentation with evolution of gas 

 is of such importance as a differential characteristic 

 that considerable 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 organ- 

 ism "is accompanied by the production of gas-bub- 

 bles," but that under given conditions we should deter- 

 mine not only the amount of gas or gases produced 

 by the organism under consideration, but also their 

 nature. For this purpose, Smith^ 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. 39.) To fill 



' An excellent and exhaustive contribntion to this subject has been 

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

 N. Y„ 1893. 



