S. B.

Very well. I shall now show you how this takes place in chemistry.

Let us suppose that we wish to decompose the compound we have just
formed by the combination of the two ingredients, copper and nitric
acid; we may do this by presenting to it a piece of iron, for which the
acid has a stronger attraction than for copper; the acid will,
consequently, quit the copper to combine with the iron, and the copper
will be what the chemists call _precipitated_, that is to say, it will
be thrown down in its separate state, and reappear in its simple form.

In order to produce this effect, I shall dip the blade of this knife
into the fluid, and, when I take it out, you will observe, that, instead
of being wetted with a bluish liquid, like that contained in the glass,
it will be covered with a thin coat of copper.

CAROLINE.

So it is really! but then is it not the copper, instead of the acid,
that has combined with the iron blade?

MRS. B.

No; you are deceived by appearances: it is the acid which combines with
the iron, and, in so doing, deposits or precipitates the copper on the
surface of the blade.

EMILY.

But, cannot three or more substances combine together, without any of
them being precipitated?

MRS. B.

That is sometimes the case; but, in general, the stronger affinity
destroys the weaker; and it seldom happens that the attraction of
several substances for each other is so equally balanced as to produce
such complicated compounds.

CAROLINE.

But, pray, Mrs. B., what is the cause of the chemical attraction of
bodies for each other? It appears to me more extraordinary or unnatural,
if I may use the expression, than the attraction of cohesion, which
unites particles of a similar nature.

MRS. B.

Chemical attraction may, like that of cohesion or gravitation, be one of
the powers inherent in matter which, in our present state of knowledge,
admits of no other satisfactory explanation than an immediate reference
to a divine cause. Sir H. Davy, however, whose important discoveries
have opened such improved views in chemistry, has suggested an
hypothesis which may throw great light upon that science. He supposes
that there are two kinds of electricity, with one or other of which all
bodies are united. These we distinguish by the names of _positive_ and
_negative_ electricity; those bodies are disposed to combine, which
possess opposite electricities, as they are brought together by the
attraction wh