354 AGRICULTURAL BACTERIOLOGY 



missible disease of human beings, this process is of the great- 

 est importance. The use of dry heat is limited, except when 

 material is to be destroyed by burning, a method that should 

 be widely used in the disposal of carcasses of animals and 

 litter. 



Chemical disinfectants. A large number of chemicals 

 have an injurious action on microorganisms. It is cus- 

 tomary to divide them into two classes, which differ in the 

 intensity of action. Those that have a relatively weak 

 action and tend to prevent the growth of organisms rather 

 than to destroy them are termed antiseptics, or preserva- 

 tives when used in foods. Those that have a more pro- 

 nounced action and destroy the organisms are called dis- 

 infectants. It will be apparent that when a disinfectant is 

 present in small amounts, it will have an antiseptic action, 

 since cessation of growth will always precede death of the 

 organism ; but certain of the antiseptics can not exert a dis- 

 infecting action, even if used in concentrated form. For 

 example, boric acid is but slightly soluble in water, and in a 

 saturated solution exerts an antiseptic action; but, owing 

 to its limited solubility, it can never be classed as a disin- 

 fectant. Some of the disinfectants have the power of over- 

 coming offensive odors by combining with specific sub- 

 stances, and are often called deodorants. Some deodorants 

 have little or no disinfecting action, and of course are of 

 little or no value, since they do not act on the source of 

 the trouble. Again, some chemicals used as disinfectants 

 have an injurious action on insects, such as lice on animals. 

 In the use of these agents the farmer should first have in 

 mind what he desires to accomplish, and then choose the 

 agent that is most likely to be effective as a disinfectant, a 

 deodorant, or an insecticide. He must also have informa- 

 tion as to the resisting power of the organisms he is attempt- 

 ing to kill, and as to the effectiveness of the disinfecting 



