170 AGRICULTURAL BACTERIOLOGY 



reduce the bacterial content of surface waters from 95 to 

 99 per cent, and to eliminate practically all danger from 

 typhoid and other water-borne diseases. 



Another method of purifying water supplies consists of 

 producing gelatinous precipitates by the addition of chem- 

 ical agents such as salts of iron or aluminum. When these 

 are added to water containing lime in solution, insoluble 

 compounds of a jellylike nature are formed. All fine par- 

 ticles, including bacteria, are enmeshed in the gelatinous 

 material, and thus can be removed readily by sedimenta- 

 tion or filtration. When water so treated is passed through 

 coarse filters, these gelatinous precipitates are readily re- 

 moved, and the water thereby not only clarified, but its 

 germ content materially reduced. 



The fine turbidity present in wines is removed in a sim- 

 ilar way. When small quantities . of gelatin are added, 

 the tannin naturally present in wines causes to be pro- 

 duced an insoluble gelatinous precipitate that is readily re- 

 moved by filtration. 



The harmful organisms in water may be destroyed by 

 heating it to the boiling-point. They may also be killed by 

 adding to the water a minute quantity of calcium hypo- 

 chlorite or bleaching powder, which has a powerful ger- 

 micidal action in the absence of all but traces of organic 

 matter. The odor and taste of chlorine is at first evident 

 in the treated water. This soon disappears and the hypo- 

 chlorite is changed into harmless substances. One part 

 of the reagent to one million parts of water is often suffi- 

 cient to destroy 99 per cent, of the bacteria therein. The 

 treatment is applicable to any quantity of water. Some 

 large cities chlorinate their entire supply. The water sup- 

 plies of the armies in the recent war were safeguarded by 

 adding hypochlorite to the water-tanks whenever they 

 were filled. 



