CHANGES IN THE REACTION OF MEDIA 199 



chapter on Media, and the changes in their colors studied 

 with different bacteria. Milk and litmus tincture or peptone 

 solution to which rosohc acid has been added are excellent 

 media for this experiment. 



Fermentation. — The production of gas as an indication of 

 fermentation is an accompaniment of the growth of certain 

 bacteria. This is best studied in media to which 1 to 2 per 

 cent, of grape-sugar (glucose) has been added. A convenient 

 method of demonstrating this property 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 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 containing the 

 gas that has resulted. 



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 sufiicient 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^ recommends the 



' An excellent and exhaustive contribution to this subject has been 

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

 N. Y., 1803. 



