BACTERIA. 163 



the free, open air. A moderate degree of heat 

 seems to be all that is needed. 



From this fact has come the method of cooling 

 milk to keep it sweet. Vessels containing fresh 

 milk, if placed in the cold water of wells or springs, 

 will attain a temperature low enough to keep it 

 sweet for a day or two, according to the other 

 influencing conditions. Ice, too, is used for cool- 

 ing milk, and will keep it fresh for a longer time 

 than ordinary cold water. Milk kept on ice, or in 

 iced water, under favorable conditions, will stay 

 sweet for several days. 



The cooling process does not destroy the germs, 

 but simply retards their action and holds it in 

 check. The germs are still there, and will multiply 

 and grow, causing the milk to sour, as soon as a 

 sufficiently high temperature is restored. 



When it is desired to destroy the life of bacteria 

 in milk, boiling from a few minutes to an hour or 

 more will accomplish the result. This is often 

 done when harmful species, such as disease-pro- 

 ducing germs, are present. If the milk is to be 

 used immediately, no further care is necessary; 

 but if it is to be kept for some time, it should be 

 quickly cooled, and kept cool until needed. By 

 sealing in air-tight cans while hot, milk will keep 

 sweet for a very long time; in fact, almost indefi- 

 nitely, providing proper care is taken in sealing. 

 The only objection to this method of killing the 

 bacteria in milk is that the boiling produces a 



