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Chalk or carbonate of lime, when dissolved in water by the 
action of carbonic acid, becomes what chemists term bicarbonate 
of lime—one of the chief causes of the hardness of water. 
Lime stone, or chalk when burnt in a kiln, becomes quick lime 
or caustic lime, having parted with its carbonic acid ; a solution of 
this lime is called lime water and is of frequent use under this 
name medicinally. 
Any lime water may be mixed with another, and any solution 
of bicarbonate of lime mixed with another, without any change 
being produced. If, however, lime water be mixed with a 
solution of bicarbonate of lime, the mixture becomes cloudy and 
white, and in time the white matter subsides in the form of chalk, 
leaving the water perfectly clear. The application of lime water 
to water containing bicarbonates forms the principle of Clark’s 
process for softening water. 
The process more in detail is this :— 
One pound of chalk, when burnt, loses seven ounces in weight of 
carbonic acid driven off in the form of gas. This caustic lime 
when mixed with the hard water, recovers this seven ounces of 
carbonic acid from the water, and is deposited in the form of 
chalk, but the water in parting with this seven ounces of carbonic 
acid also leaves a deposit of one pound of chalk, thus two pounds 
of chalk will be found at the bottom of the cistern. 
Water of the average hardness of that of Bath, contains nearly 
half a pound of chalk to 200 gallons of water ; this amount may 
be assumed as the daily consumption of an ordinary household, 
thus in addition to the water supplied by the Corporation of this 
City we are presented besides with half a pound of chalk, which, 
for my part, I would gladly dispense with. 
The amount of lime in the water supplied daily in this City is 
probably not less than from one to one-and-a-half tons. 
It is perhaps not so generally known that the mere boiling of 
hard water very much reduces its hardness. This is fortunate in 
some respects, otherwise, in many places, the water would be 
quite unfit for culinary and household purposes. 
