6 4 



AGRICULTURAL CHEMISTRY 



used for generating steam they produce a large amount of 

 boiler scale. This can be partially prevented by the use 

 of materials containing tannic acid. 



66. Methods of Improving Drinking Waters. (i) 



By boiling, which destroys disease-producing organisms. 



Boiled or sterilized water, however, is not free from the 



poisonous compounds 

 which many organisms 

 produce. In cases of 

 pestilential diseases, 

 water should always be 

 boiled. 



(2) By filtering 

 through charcoal or 

 through disks of un- 

 glazed porcelain-like 

 material, which results 

 in removing a large part 

 of the organic matter. 

 Special care, however, 

 should be taken to keep 

 the filter clean, other- 

 wise it will be a source 

 of contamination. Boil- 

 -. ing the water before fil- 



Fig. 35.-Pasteur water filter. tering also improves its 



sanitary condition. One of the most effectual forms of 

 water filters is the Pasteur filter, in which the water 

 passes through a series of tubes which present a large sur- 

 face area for filtering. 



