212 ELEMENTARY AGKiCULTURAL CHEMISTRY 



of correlating chemical and physical properties with germicidal 

 action is known. The following classification is perhaps one 

 of the best suggested : 



1. Free acids or salts of acid reaction retard the growth of 

 micro-organisms. 



2. Soluble salts of many heavy metals — e.g., mercury and 

 copper — precipitate albuminoids. Such compounds probably 

 act upon the protoplasm in the organisms 



3. Such salts or other substances — e.g., charcoal — by ren- 

 dering albuminoids insoluble, may deprive the bacteria of 

 food, and thus kill them by starvation. 



4. Reducing agents — e.g., sulphites, ferrous salts — remove 

 oxygen from the medium, and thus destroy aerobic organisms 

 — i.e., those which require oxygen. 



5. Oxidising agents — e.g., chlorine, ozone, hydrogen- per- 

 oxide, permanganates — destroy by oxidation both the 

 bacteria and their food, and thus form the most perfect 

 disinfectants. 



6. Some metallic salts are assimilated by bacteria, and the 

 metal is deposited within their tissues. In this way gold and 

 silver salts act as disinfectants, provided they are present in 

 sufficient quantity. 



7. Some substances act as germicides for no apparent 

 chemical reason. To this class belong boric acid, the borates, 

 and some of the aromatic compounds. 



By a disinfectant we usually mean a substance used for 

 killing noxious micro-organisms in substances which are not 

 used for food, while an antiseptic is used to arrest putrefactive 

 changes without rendering the substance to which it is applied 

 injurious to animals. 



A large number of substances possessing disinfecting pro- 

 perties exists, and new ones are continually being introduced. 

 On the farm disinfectants are chiefly used for destroying the 

 risk of infection after outbreaks of contagious diseases. The 

 following are among the most important substances which can 

 be used for this purpose. 



