132 BACTERIOLOGY. 



spraying of water. Bacteria are very variously 

 distributed in the air according to the degree 

 of atmospheric purity ; their number decreases 

 at high altitudes, and B. Fischer found that 

 the air over the sea at some distance from land 

 was germ-free. 



Practical Applications. 



Many bacterial discoveries are of direct prac- 

 tical importance. As the fermentation indus- 

 tries have been built up through the investiga- 

 tions of Pasteur and Hansen upon yeasts, so 

 the fundamental work of Duclaux and Hueppe 

 upon the fermentations of milk and cheese has 

 paved the way for a complete revolution in the 

 dairy industry. 1 



A well-known practical result of bacterio- 

 logical research is the sterilization of milk. 

 Before the discovery was made that it is possi- 

 ble to sterilize milk by temperatures below the 

 boiling point and it was known under what 

 conditions this could be done, this method had 

 been declared by Pasteur to be inapplicable to 

 milk, and its impracticability, as was thought, 

 firmly established. Tyndall is largely re- 

 sponsible for the working out of this method of 



1 To forestall the criticism that this conception is onesided, it is 

 necessary merely to mention the fact that other important improve- 

 ments have been wrought on the dairy farm by the introduction of 

 the separator, and by better methods of housing and feeding stock. 



