METHODS OF PURIFICATION 77 



thus accumulated the waste of pasture and stable and, it may 

 be, of dumping ground. 



It is not necessary that water should be absolutely free 

 from all foreign substances in order to be safe for daily use 

 in drinking; a limited amount of mineral matter is not 

 injurious and may sometimes be really beneficial. It is the 

 presence of animal 'and vegetable matter that causes real 

 danger, and it is known that typhoid fever is due largely to 

 such impurities present in the drinking water. 



70. Methods of Purification. Water is improved by any 

 of the following methods: 



(a) Boiling. The heat of boiling destroys animal and 

 vegetable germs. Hence water that has been boiled a few 

 minutes is safe to use. This is the most practical method of 

 purification in the home, and is very efficient. The boiled 

 water should be kept in clean, corked bottles ; otherwise for- 

 eign substances from the atmosphere reenter the water, and 

 the advantage gained from boiling is lost. 



(b} Distillation. By this method pure water is obtained, 

 but this method of purification cannot be used conveniently in 

 the home (Section 25). 



(c) Filtration. In filtration, the water is forced through 

 porcelain or other porous substances which allow the passage 

 of water, but which hold ba^ck the minute foreign particles 

 suspended in the water. (See Laboratory Manual.) The fil- 

 ters used in ordinary dwellings are of stone, asbestos, or char- 

 coal. They are often valueless, because they soon become 

 choked and cannot be properly cleaned. 



The filtration plants owned and operated by large cities 

 are usually safe; there is careful supervision of the filters, 

 and frequent and effective cleanings are made. In many 

 cities the filtration system is so good that private care of the 

 water supply is unnecessary. 



