I will take some lime-water,
and I will blow through; my breath contains carbonic acid, and you will
see the clear liquid become milky owing to separation of insoluble
carbonate of lime, or chalk. I now continue blowing, and at length that
chalk dissolves with the excess of carbonic acid, forming bicarbonate of
lime. This experiment explains how it is that water percolating through
or running over limestone strata dissolves out the insoluble chalk. Such
water, hard from dissolved carbonate of lime, can be softened by merely
boiling the water, for the excess of carbonic acid is then expelled, and
the chalk is precipitated again. This would be too costly for the
softening of large quantities of water, the boiling process consuming
too much coal, and so another process is adopted. Quicklime, or milk of
lime, is added to the water in the proper quantity. This lime unites
with the excess of carbonic acid holding chalk in solution, and forms
with it insoluble chalk, and so all deposits together as chalk. By this
liming process, also, the iron of the water dissolved likewise in
ferruginous streams, etc.
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