Nitrate of silver is the best test for the presence of chloride of soda or

common salt. By adding a small quantity of this to the common well water of New-York, a copious, white, flocculent precipitate is immediately formed, which is the chloride of soda. The same test, however, applied to the Croton water, produces no discoloration whatever. Purification of Common water. By filtration, water may be deprived of living beings and of all suspended impurities ; but substances held in solution, cannot

thus be separated. Ebullition destroys the vitality of both animals and vegetables ;

expels air, or carbonic acid, and causes the precipitation of carbonate of lime, but

the water should be afterwards subjected to the process of filtration. Distillation,

when properly conducted is the most effectual method of purifying water. But distilled water is in general contaminated by traces of organic matter. The addition of chemical agents is another mode which has been proposed and practised, for freeing water from some of its impurities. Alum is often used by the common people to cleanse muddy water, and ashes and pearl-ash to destroy its hardness. When alum is used, two or three grains are sufficient for a quart of water. The alum decomposes the carbonate of lime ; sulphate of lime is formed in solution, and the alumina precipitates in flocks, carrying with it mechanical impurities. This agent, however, adds nothing to the chemical purity of the water, but by converting the carbonate into sulphate of lime augments its hardness, Caustic alkalies added to lime saturate the excess of carbonic acid, and throw down the carbonate of lime,