.

The word oxyd, then, simply means a metal combined with oxygen?

MRS. B.

Yes; but the term is not confined to metals, though chiefly applied to
them. Any body whatever, that has combined with a certain quantity of
oxygen, either by means of oxydation or combustion, is called an _oxyd_,
and is said to be _oxydated_ or _oxygenated_.

EMILY.

Metals, when converted into oxyds, become, I suppose, negative?

MRS. B.

Not in general; because in most oxyds the positive energy of the metal
more than counterbalances the native energy of the oxygen with which it
combines.

This black powder is an oxyd of manganese, a metal which has so strong
an affinity for oxygen, that it attracts that substance from the
atmosphere at any known temperature: it is therefore never found in its
metallic form, but always in that of an oxyd, in which state, you see,
it has very little of the appearance of a metal. It is now heavier than
it was before oxydation, in consequence of the additional weight of the
oxygen with which it has combined.

CAROLINE.

I am very glad to hear that; for I confess I could not help having some
doubts whether oxygen was really a substance, as it is not to be
obtained in a simple and palpable state; but its weight is, I think,
a decisive proof of its being a real body.

MRS. B.

It is easy to estimate its weight, by separating it from the manganese,
and finding how much the latter has lost.

EMILY.

But if you can take the oxygen from the metal, shall we not then have it
in its palpable simple state?

MRS. B.

No; for I can only separate the oxygen from the manganese, by presenting
to it some other body, for which it has a greater affinity than for the
manganese. Caloric affording the two electricities is decomposed, and
one of them uniting with the oxygen, restores it to the aeriform state.

EMILY.

But you said just now, that manganese would attract oxygen from the
atmosphere in which it is combined with the negative electricity; how,
therefore, can the oxygen have a superior affinity for that electricity,
since it abandons it to combine with the manganese?

MRS. B.

I give you credit for this objection, Emily; and the only answer I can
make to it is, that the mutual affinities of metals for oxygen, and of
oxygen for electricity, vary at different temperatures; a certain degree
of heat will, therefore, dispose a metal to combine with oxygen, whilst,
on the contrary, the former will be c