FERMENTATION. 191 



It is essential tliat these peculiarities should be care- 

 fully noted in studying an organism. 



Fermentation. — The production of gas as an in- 

 dication of fermentation is an accompaniment of the 

 growth of some bacteria. This is best studied in 

 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 reduced to the proper tem- 

 perature a small quantity of a pure culture of the organ- 

 ism 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 glucose, the medium will 

 be dotted everywhere with very small cavities contain- 

 ing 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 

 development of our knowledge on the subject do not 

 consider it enough 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 

 and quality. For this purpose Smith'^ recommends the 



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

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

 1893. 



