as regards solubility or insolubility, of the substances composing the
strata or layers of earth upon which it falls, and through which it
sinks. If it meets with insoluble rocks--for all rocks are not
insoluble--it remains, of course, pure and soft, and in proportion as
the constituents of rock and soil are soluble, in that proportion does
the water become hard. We all know how dangerous acid is in water,
causing that water to act on many substances, the iron of iron vessels,
the lime in soil or rock, etc., bringing iron and lime respectively into
solution. Now the atmosphere contains carbonic acid, and carbonic acid
occurs in the earth, being evolved by decomposing vegetation, etc.
Carbonic acid is also soluble to a certain, though not large extent, in
water. As we shall see, water charged with carbonic acid attacks certain
substances insoluble in pure water, and brings them into solution, and
thus the water soon becomes hard. About the close of the last lecture, I
said that lime is, to a certain extent, soluble in cold water. The
solution is called lime-water; it might be called a solution of caustic
lime. When carbonic acid gas first comes in contact with such a
solution, chalk or carbonate of lime, which is insoluble in water, is
formed, and the lime is thus precipitated as carbonate. Supposing,
however, we continued to pass carbonic acid gas into that water,
rendered milky with chalk powder, very soon the liquid would clear, and
we should get once more a solution of lime, but not caustic lime as it
was at first, simply now a solution of carbonate of lime in carbonic
acid, or a solution of bicarbonate of lime. 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