Such pure water is called
soft water. But the term is only a comparative one, and water that is
not quite, but nearly pure--pure enough for most practical purposes--is
also called soft water. Now rain is the purest form of natural water,
for it is a kind of distilled water. Water rises in vapour from the
ocean as from a still, and the salt and other dissolved matters remain
behind. Meeting cold currents of air, the vapours condense in rain, and
fall upon the earth. After coming in contact with the earth, the
subsequent condition of that water entirely depends upon the character,
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.
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