BACTERIA AKD THEIR RELATION TO DAIRYING. 11 



action of the nutritive substratum is especially favorable to 

 the development of the moulds. 



Finally, two factors must here be noted which have the 

 greatest influence on bacterial life, viz., air and light. 



Bacteria and the Air. A large number of bacteria are 

 entirely dependent on the oxygen of the air, and perish if 

 they do not have access to the same. These are called 

 aerobic bacteria. On the other hand, oxygen acts as a 

 poison, or at least as a narcotic, to other forms (ana- 

 erobic bacteria). Some bacteria can further exist both 

 in an atmosphere of oxygen and in its absence. Many of 

 the latter organisms differ in behavior according to 

 whether they have access to air or not. 



Bacteria and Light. The large majority of bacteria 

 like total darkness, and only a few thrive in direct light. 

 Direct sunshine as a rule acts as a poison on all organisms 

 included under the term bacteria; and if ordinary day- 

 light is allowed to act on them for a sufficiently long time 

 and with proper intensity, not only most of the vegetative 

 forms but even their spores will be destroyed. Sunlight 

 is therefore the most common, the cheapest, and the most 

 effective means at our disposal of fighting the bacteria. 

 Light exercises its power of destruction of bacteria on a 

 large scale. In nature the destruction is most intense 

 during the summer months. A putrefactive liquid, the 

 condition of which, as is well known, is due to a fermen- 

 tation brought about by bacteria, can even become sterile 

 through the action of sunlight alone. The more generous 

 the supply of air while the bacteria are exposed to the 

 sunlight, the more rapidly they and their spores are 

 destroyed. 



Bacteria and Disinfectants. It must furthermore be 



