FEEMEN1ATION. 183 



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

 has been added. 



In this experiment the test-tube should be filled to 

 about one-half its volume with agar-agar. The medium 

 is then liquefied, and when at the proper temperature, 

 a small quantity of a pure culture of the organism under 

 consideration should be carefully 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 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 containing 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 or gases produced by the organism under 

 consideration but also their nature and quality. For this 

 purpose Smith 1 recommends the employment of the fer- 

 mentation-tube used by Einhorn in the quantitative fer- 

 mentation 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 



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

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



