52 MODERN DAIRY PRACTICE. 



by scientific investigations that bacteria do not multiply in 

 the air; they lack there the moisture so essential for their 

 development. The air can therefore be filled with bac- 

 teria only by the drying and subsequent reduction to a 

 powder-like dust of fluids and other media where these 

 organisms are found, the dust being later on set in motion 

 by currents of the air. The bacteria of the air are thus 

 closely connected with the dust, and appear most numerous 

 in air where a good deal of dust is set in motion. When 

 the air is not stirred, both bacteria and the dust will of 

 course sink to the ground or the floor. It has been found 

 that the air in a closed living-house, when left undisturbed 

 will become free from bacteria, in one to two hours. 



During the hot season the outside air contains most 

 bacteria, while during snowy winters it contains very few 

 such in our climate. In the atmosphere of cities con- 

 siderably more bacteria are usually found than in the air 

 in the country. On high mountains and out on the sea 

 far from land, or deep down into the earth, the air seems 

 to be sterile. 



Importance of Pure Air. It is evident that the air in a 

 cow-stable must be highly infected with bacteria a large 

 portion of the day, it being usually in strong motion and 

 an enormous number of them being scattered at feeding- 

 time. As the bacteria obey the law of gravitation they are 

 of course, as we have seen, most numerous in the lower 

 layers of the air, i.e., just where the milking takes place. 

 The bacterial content of the atmosphere in a stable varies, 

 however, greatly at different times of the day, as I have 

 often had an opportunity of proving in my investigations. 

 While a sterilized gelatine plate placed in the stable im- 

 mediately after the feeding within two minutes became 



