THE BACTERIA OF SOIL, AIR, WATER, ETC. 1 41 



Storage basins in connection with sand filtration, to permit of 

 settling of part of the solid matter before filtration. 



The results obtained by filtration depend greatly upon the 

 intelligence displayed in operation. 



Mechanical Filtration. —This method of filtration is also 

 called the American system. It is more rapid than the pre- 

 ceding method and does not require a large area for filter beds. 

 Although sand is required also, filtration is accomplished by 

 a jelly-like layer of aluminum hydroxide. This product is 

 formed by adding to the water a small quantity of aluminum 

 sulphate or of alum. The carbonates in the water decompose 

 the aluminum salt and produce aluminum hydroxide. It 

 precipitates as a white, flocculent deposit, entangling solid par- 

 ticles, including bacteria, as coffee is cleared with white of egg. 

 Only a trace of aluminum should appear in the water. This 

 method of filtration has not been tested so extensively as slow 

 sand filtration, but seems likely to prove efficient. With water, 

 poor in carbonates, these may have to be added.* 



Whipple and Longleyt found that the efficacy of mechani- 

 cal filters with the addition of alum depends somewhat upon 

 the character of the alum. They find that the alum shall be 

 shown by analysis to contain 17 per cent, of alumina (A12 

 03) soluble in water, and of this amount at least 5 per cent, 

 shrill be in excess of the amount necessary theoretically to 

 combine with the sulphuric acid present. It shall not contain 

 more than i per cent of insoluble substances, and shall be 

 free from extraneous debris of all kinds. It must not contain 

 more than 0.5 per cent, of iron (Fe^Oj) and the iron shall be 

 preferably in the ferrous state. 



Various methods for the purification of water by means of 

 chemicals have been proposed. The use of copper sulphate 

 to disinfect drinking water was recommended by Moore 



*See Fuller. Jourtml American Medical Association. Oct. 31, 1903 

 f/oMrre. Inject. Diseases. Supplement No. x, Feb., igo6, pp. 166-171 



