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 otherwise 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 (antiseptic) 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 exception of the anaërobes. In the nascent state especially as liberated from ozone (O₃) hydrogen peroxide (H₂O₂) and hypochlorites (Ca(ClO)₂) it is strongly bactericidal. =Chlorine.=--Chlorine is actively disinfectant and is coming into use for sterilizing water on a large scale in municipal plants (Fig. 108). [Illustration: FIG. 108.--Apparatus for sterilizing water with liquid chlorine.] _Iodine_ finds extended use in aseptic surgical operations and antiseptic dressings. Bromine, mercury, silver, gold, nickel, zinc and copper are markedly germicidal in the elemental state but are not practical. COMPOUNDS. =Calcium Oxide.=--Calcium oxide (CaO), _quick lime_, is an excellent disinfectant for stables, yards, outhouses, etc., where it is used in the freshly slaked condition as “white wash;” also to disinfect carcasses to be buried. It is very efficient against the typhoid bacillus in water, where it is much used to assist in the softening. =Chloride of Lime.=--Chloride of lime, _bleaching powder_, which consists of calcium hypochlorite, the active agent, and chloride and some unchanged quicklime is one of the most useful disinfectants. It is employed to sterilize water for drinking purposes on a large scale and to disinfect sewage plant effluents. A 5 per cent. solution is the proper strength for ordinary disinfection. Only a supply which is fresh or has been kept in air-tight containers should be used, as it rapidly loses strength on exposure to the air. The active agent is nascent oxygen liberated from the decomposition of the hypochlorite. =Sodium Hypochlorite.=--Sodium hypochlorite prepared by the electrolysis of common salt has been used to some extent.