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,

having an alkaline carbonate in solution. Professor Clark of Aberdeen,* (Scotland) has recently patented a plan for the purification of water, by the addition of

lime. The lime unites with the excess of carbonic acid in the water, and forms carbonate of lime (chalk) which precipitates, along with the carbonate of lime held

* Repository of Patent Inventions, for October, 1841.

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previously in solution in the water. The effect of this process is similar to that of ebullition, -- as the hardness of water is, however, owing to the sulphate and not the

carbonate of lime,* this plan can have little or no influence in rendering hard water soft. Alkaline carbonates soften water, decompose all the earthy salts (calcareous and magnesian carbonates, sulphates, and chlorides) and precipitate the earthy matters. They leave, however, in solution, an alkaline salt, but which does not communicate to water the property of hardness. Sea-water includes the waters of the ocean and of those lakes, called island seas, which possess a similar composition. The Dead Sea, however, varies so much from ordinary sea-water, as to rank amongst mineral waters. The quantity of solid matter varies considerably in the waters of different seas, as the following statement proves