practically insoluble in water under any circumstances. We now arrive at
the important result known to chemists as the precipitation of insoluble
compounds from solutions. In order to define this result, however, we
must, of course, first consider the circumstances of causation of the
result. Let us take a simple case of chemical decomposition resulting in
the deposition or precipitation of a substance from solution in the
insoluble state. We will take a salt you are probably acquainted
with--sulphate of copper, or bluestone, and dissolve it in water, and we
have then the sulphate of copper in solution in water. Now suppose it is
our desire to obtain from that solution all the copper by depositing it
in some insoluble form. We may accomplish this in several different
ways, relying on certain methods of decomposing that sulphate of copper.
One of the simplest and most economical is that adopted in a certain
so-called wet method of extracting copper. It is based on the fact that
metallic iron has a greater tendency to combine in water solutions, with
the acids of copper salts, than the copper has in those salts. We
simply need to place some scraps of iron in the copper sulphate solution
to induce a change which may be represented as follows: Copper sulphate,
consisting of a combination of copper oxide with sulphuric acid, yields
with iron, iron sulphate, a combination of iron oxide with sulphuric
acid, and metallic copper. The metallic copper produced separates in the
form of a red coating on the iron scraps. But we may also, relying on
the fact that oxide of copper is insoluble in water, arrange for the
deposition of 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