170 PRACTICAL BOTANY 



The relation of bacteria to milh and water supply is a sub- 

 ject of great importance. A rather large number of harmless 

 bacteria may often be found in reasonably pure milk and water ; 

 but careless handling of bottles and cans, or the use of tuber- 

 culous cows, may result in widespread disease, and has been 

 known to cause epidemics of tonsilitis, scarlet fever, and ty- 

 phoid fever. If milking is done through absorbent cotton or 

 several layers of cheesecloth, used as a cover for the milk pail, 

 most of the impurities are caught therein. Milk pails and ship- 

 ping cans should always be sterilized before they are used. 

 Milkmen who otherwise were fairly careful in their work have 

 been known to rinse their pails and cans in polluted wells or 

 streams. Bacteria of various diseases have thus been distrib- 

 uted. Either milk or water may be sterilized by boiling, and 

 may be kept so if placed in sterile vessels. Both, however, 

 are better if they can be secured and kept in a pure con- 

 dition without it. An efficient method of preserving milk is 

 by Pasteurization,! in which the vessels containing the milk 

 are placed in water and brought to a temperature of 150° to 

 155° F., and then cooled and kept cool until used. This method 

 kills most of the bacteria in milk and makes less change other- 

 wise than does boiling.^ 



The formation of acetic acid (the acid of vinegar) is due 

 to the growth of several kinds of bacteria. Part or all of the 

 processes of curing tobacco, tanning of leather, preparation 

 of plant fibers as flax and hemp, butter and cheese making, 

 and many other important industries depend upon the growth 

 processes of different kinds of bacteria. 



1 "Directions for the Home Pasteurization of Milk," Circular 152, Bureau 

 of Animal Industry, U. S. Dept. Agr., 1909. 



2 "Care of Milk on the Farm," Farmers' Bulletin 63, U. S. Dept. Agr., 

 1906. 



"Bacteria in Milk," Fai-mers' Bulletin 348, V. S. Dept. Agr., 1900. 



" Sources of Bacteria in Milk," Bulletin 61, .Storrs Agr. Ex. Sta., Storrs, 

 Conn., 1908. 



"Milk and its Products asCarriersof Tuberculosis Infection," iiuHeKni4S, 

 Bureau of Animal Industry, U. S. Dept. Agr., 1909. 



