f the copper in that form. This we can do by adding caustic
soda to a hot solution of copper sulphate, when we get the following
change: Copper sulphate, consisting of a combination of copper oxide
with sulphuric acid, yields with caustic soda, sulphate of soda, a
combination of soda with sulphuric acid and oxide of copper. Oxide of
copper is black, and so in this decomposition what is called a "black
precipitate" of that oxide is produced on adding the caustic soda. But
it might not suit us thus to deposit the copper from our solution; we
might desire to remove the sulphuric acid from the copper sulphate, and
leave the copper dissolved, say in the form of a chloride. We select,
then, a compound which is a chloride, and a chloride of a metal which
forms an insoluble combination with sulphuric acid--chloride of barium,
say. On adding this chloride of barium to sulphate of copper solution,
we get then a change which we might represent thus: Copper sulphate,
consisting of a combination of copper oxide with sulphuric acid, yields
with barium chloride, which is a combination of barium and chlorine,
insoluble barium sulphate, a combination of barium oxide with sulphuric
acid, and soluble copper chloride, a combination of copper and chlorine.
This is called a double interchange. Now these are a few illustrations
to show you what is meant by chemical decompositions. One practical
lesson, of course, we may draw is this: We must have a care in
dissolving bluestone or copper sulphate, not to attempt it in iron pans,
and not to store or put verdigris into iron vessels, or the iron will
be acted upon, and to some extent the copper salt will become
contaminated with iron. It will now be clear to you that, as a solvent
for bodies usually soluble in water, water that is perfectly pure will
be most suitable and not likely to cause any deposition or precipitation
through chemical decompositions, for there are no salts or other
compounds in pure water to cause such changes. 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