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 growth 

 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 causiag 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 " 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 ot 



