CHAPTER XIV. 

 DISINFECTION AND STERILIZATION (Continued). 



CHEMICAL AGENTS. 



A VERY large number of chemical substances might be 

 used for destroying bacteria or preventing their growth 

 either through direct injurious action or by the effect of 

 concentration. Those which are practically useful are rela- 

 tively few, though this is one of the commonest methods of 

 disinfecting and the word "disinfectant" is frequently 

 wrongly restricted to chemical agents. 



Chemical agents act on bacteria in a variety of ways. 

 Most commonly there is direct union of the chemical with 

 the protoplasm of the cell and consequent injury. Some- 

 times the chemical is first precipitated on the surface of the 

 cell without penetrating at once. If removed soon enough, 

 the organism is not destroyed. This is true of bichloride of 

 mercury and formaldehyde. If bacteria treated with these 

 agents in injurious strength be washed with ammonia or 

 ammonium sulphate, even after a time which would other- 

 wise result in their failure to grow, they will develop. Some 

 chemicals change the reaction of the material in a direction 

 unfavorable to growth, and if the change is enough, may 

 even kill the bacteria. Some agents remove a chemical 

 substance necessary to the growth of the organism and 

 hence inhibit it. Such actions are mainly preventive (anti- 

 septic) and become disinfectant only after a long time. 



ELEMENTS. 



Oxygen. — Oxygen as it occurs in the air is probably not 

 injurious to living bacteria but aids them with the excep- 



