RELATION TO ENVIRONMENT 133 



inert gas, as hydrogen or carbon dioxide. For this purpose 

 special apparatus is necessary, and a great variety of forms 

 of apparatus has been devised. 2. Absorption of oxygen. 

 This can be done by means of certain chemical substances, 

 such as an alkaline solution of pyrogallic acid. Here, too, the 

 forms of apparatus are numerous. A very simple form of 

 apparatus is made by pushing the cotton plug of a test tube 

 culture into the tube and then rilling up with an alkaline solu- 

 tion of pyrogallic acid and closing the test tube with a rubber 

 stopper. 3. Exhaustion of air. This is done by means of an 

 air pump. Any of the above methods are equally good and, 

 in fact, may frequently be combined to advantage. 4. Ex- 

 clusion of oxygen. This is accomplished by pouring on top of 

 the culture either more culture media or sterile oil. 5. Growth 

 in the presence of air. This is done either by growing with 

 some other organism that is aerobic or some reducing sub- 

 stance, such as litmus or formic acid. 



When Pasteur discovered these organisms, he thought 

 that they were the organisms which carried on fermentation, 

 and he defined fermentation as life without air. This on fur- 

 ther study has been found to be without basis of fact, but it is 

 true that a considerable number of fermentations are pro- 

 duced by anaerobic bacteria. Among these may be mentioned 

 the butyric acid fermentation, the fermentation of cellulose, 

 the retting of flax, and putrefaction. Reference has already 

 been made to the fact that these anaerobic bacteria may pro- 

 duce disease. 



