274 PURIFICATION OF PUBLIC WATER SUPPLIES. 



basin or reservoir, unless its character is such as to de- 

 velop algse growths. Except where the impurities are 

 such as to subside rapidly, as the sand of some v^^estern 

 rivers, the size of reservoirs required renders the method 

 impracticable. It is used to some extent in the west. In 

 1889 a series of experiments were conducted at St. Louis, 

 Mo., to determine the most favorable conditions, from 

 which the conclusion was drawn that filling one basin at 

 a time and allowing it to stand until needed is preferable, 

 from an economic point of view, over continuous flow 

 through a larger basin. 



In 1889 thirteen western towns were using settling 

 basins. 



Precipitation or sedimentation may be greatly facili- 

 tated by the use of some coagulating substance that will 

 group together the finer particles, having nearly the same 

 specific gravity as water, and carry them down. A small 

 quantity of alum, sulphate of alumina, placed in a bottle 

 of water turbid with clay will produce a precipitation 

 in a few hours that without this aid requires many days. 

 It is said to have been in use for centuries in India and 

 China to purify water from suspended matters. Its ef- 

 ficiency is said to be somewhat dependent ujDon the 

 presence of calcium carbonate in the water. By the de- 

 composition of the alum calcium sulphate is formed and 

 a gelatinous aluminum hydrate which, together, sur- 

 round the particles of foreign matter, including bacteria, 

 and carry them down. 



A similar effect may be produced by metallic iron in 

 solution. If in a bottle partly filled with turbid water 

 a quantity of iron borings be placed and the bottle be 

 corked and shaken vigorously for a few minutes and 

 then uncorked and set aside, sedimentation will soon 

 begin and in a comparatively short space of time the 

 water will be clarified. 



The chemical action, it is said by Dr. Leffman, of Phil- 



212 



