12 Agricultural Bacteriology. 



nary house or barn has but little effect. Where the 

 "bacteria are covered by dust or dirt light has no effect. 



Effect of chemicals on bacteria. Many chemical sub- 

 stances are poisonous to plants and animals. When 

 such substances exert a peculiarly marked effect on 

 germ life they are known as disinfectants or germicides. 

 Corrosive sublimate, carbolic acid, and formaldehyde 

 are the best known and strongest disinfectants. When 

 present in very small amounts they do not kill the bac- 

 terial cells but may prevent their growth. Some chem- 

 icals, even in strong solutions, are not poisonous enough 

 to kill the bacteria but may merely check the growth. 

 Such are known as antiseptics or preservatives. All dis- 

 infectants in a dilute form have an antiseptic action, 

 but not all antiseptics, even in concentrated solutions, 

 are disinfectants. For example, lime is an antiseptic, 

 since it dissolves in water to such an extent as to pre- 

 vent all growth of bacteria, but it does not kill many 

 forms of bacteria. Acids, such as vinegar, are anti- 

 septics, and are constantly used in the preservation of 

 human and animal foods. 



Products formed by bacteria. When the bacteria 

 grow in any food-substance, parts of the same are con- 

 mimed. As a result of this growth, the bacteria give 

 off from their bodies various kinds of by-products that 

 are very different from the original food-substance. 

 The changes that take place in any substance in which 

 the bacteria are growing are collectively known as il fer- 

 mentations. " Many kinds of by-products are formed 

 "by the various forms of bacteria, such as the acids pro- 

 duced in the souring of milk, and in the change of cider 

 to vinegar, also in the "working" of canned fruits and 



