Hard water, so called on account of its soap-destroying property, owes its peculiarities to the presence of compounds of lime and magnesia, chiefly the carbonates and sulphates, but occasionally also the chlorides and nitrates. Carbonate qf Lime, or calcium carbonate, which is the principal constituent of chalk water, owes its presence almost entirely to dissolved carbonic acid. Distilled water, from which every trace of carbonic acid has been expelled by boiling, can hold in solution only about 1. 3 grain of carbonate of lime per gallon; but if the water be saturated with carbonic acid i t can dissolve as much as 60 to 70 grains, the whole of which, except the above 1. 3 grain, is precipitated when the water is boiled and the carbonic acid expelled. All natural water contains carbonic acid, obtained partly from the air and partly from the soil ; and therefore all such water which has been in contact with chalk contains carbonate of lime in solution, and possesses the property of destroying soap and of forming an incrustation in vessels in which it is boiled. The use of slaked lime, or hydrate of lime, for softening chalk water was first proposed by Thomas Henry, F.R.S., about a century ago. Its action depends upon the fact that the lime removes from the water the dissolved carbonic acid, combining with it to form carbonate of lime, which precipitates, and the carbonate of lime previously held in solution by the carbonic acid precipitates also, leaving thc clear water soft, except for the very small quantity of carbonate of lime which remains permanently dissolved, and which in natural watcr usually amounts to aboult lfr to 2$ grains per galloil.
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