STERILIZATION 



183 



into main rivers. Care and certain precautions have, there- 

 fore, to be adopted. The presence of excess of chlorine, or its 

 oxides, is tested for by adding a solution of iodide of potassium 

 and starch, which is turned blue. 



The soluble hypochlorites are alkaline ; when acidified they 

 give off chlorine or hypochlorous acid in vapour, so that the 

 walls of sewers and culverts can be thoroughly disinfected in 

 special cases, as in the drains from hospitals. On the other 

 hand, the action of unacidified hypochlorites is very slow, 

 ^especially on organic colouring matters as derived from trade 

 [effluents. In the Manchester report of i8g8 a portion of the 

 Swinton sewage is thus described : 



" Raw Sewage. — Pink colour ; slight purple suspended matter; 

 ^smells of bleach liquor; neutral to litmus." 



" Tank Effluent. — Slight pink colour ; brown precipitate of 

 ^ferric hydroxide "; lime and copperas had been added (see 

 |p. 159); " smells of bleach ; neutral." 



There was an excess of chlorine compounds, since " on the 

 addition of acid, chlorine was liberated equivalent to o*o8 grain 

 per gallon of oxygen" (0*114 part per 100,000). The incu- 

 bator tests given show a slight, but distinct, effect of the chlorine 

 in the bleach, indicated by the " three minutes oxygen absorp- 

 tion ": 



Sterilization of septic tank effluents has been successfully 

 [effected by chlorinated lime in some experiments carried out by 

 the Indian Government on the Hooghly River. This investi- 

 gation showed that 5 grains per gallon of bleaching powder, 

 » equal to less than 2 grains of available chlorine, rendered the 

 [fluid virtually sterile, and that a tank effluent from 2,000 people 

 :ould be rendered inodorous and sterile at from Rs. 10 to 14 per 

 Imonth. The time of contact to complete the action was found 

 to be about one hour, as little or no improvement was noticed 

 Iwhen plates were taken after a longer period. At the Tittagurh 

 installation, near Calcutta, this treatment was applied to latrines. 



