116 BACTERIOLOGY. 



Many of them can only be treated with water, or but 

 for a few seconds with alcohol, without losing their 

 color. 



It is essential that these peculiarities should be care- 

 fully noted in studying an organism. 



FERMENTATION. The production of gas as an indica- 

 tion of fermentation is an accompaniment of the growtli 

 of some organisms. This is best studied in media to 

 which 1 to 2 per cent, of grape sugar has been added. 



In this experiment the test-tube should be filled to 

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

 dium is then liquefied, and when at the proper tem- 

 perature, a small quantity of a pure culture of the 

 organism under consideration should be carefully dis- 

 tributed through it. The tube is then placed into 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 con- 

 taining the gas that has resulted. 



Where it is important that the nature of the gas thus 

 produced should be studied, it must be collected, and a 

 special form of apparatus is employed. The cultivation 

 is now to be conducted in a fluid medium. The fermen- 

 tation flasks of somewhat the pattern of that used by 

 Einhorn in the fermentation test for sugar in the urine 

 serve very well for this purpose. 



CULTIVATION WITHOUT OXYGEN. As we have 

 already learned, there is a group of organisms to which 

 the name u anaerobic organisms" has been given, which 

 are characterized by their inability to grow in the pres- 

 ence of oxygen. For the cultivation of the members oi 



