g been destroyed, or rather decomposed, by the combustion of the
taper.

CAROLINE.

Then why did not the water rise immediately when the oxygen gas was
destroyed?

MRS. B.

Because the heat of the taper, whilst burning, produced a dilatation of
the air in the vessel, which at first counteracted this effect.

Another means of decomposing the atmosphere is the _oxygenation_ of
certain metals. This process is very analogous to combustion; it is,
indeed, only a more general term to express the combination of a body
with oxygen.

CAROLINE.

In what respect, then, does it differ from combustion?

MRS. B.

The combination of oxygen in combustion is always accompanied by a
disengagement of light and heat; whilst this circumstance is not a
necessary consequence of simple oxygenation.

CAROLINE.

But how can a body absorb oxygen without the combination of the two
electricities which produce caloric?

MRS. B.

Oxygen does not always present itself in a gaseous state; it is a
constituent part of a vast number of bodies, both solid and liquid, in
which it exists in a much denser state than in the atmosphere; and from
these bodies it may be obtained without much disengagement of caloric.
It may likewise, in some cases, be absorbed from the atmosphere without
any sensible production of light and heat; for, if the process be slow,
the caloric is disengaged in such small quantities, and so gradually,
that it is not capable of producing either light or heat. In this case
the absorption of oxygen is called _oxygenation_ or _oxydation_, instead
of _combustion_, as the production of sensible light and heat is
essential to the latter.

EMILY.

I wonder that metals can unite with oxygen; for, as they are so dense,
their attraction of aggregation must be very great; and I should have
thought that oxygen could never have penetrated such bodies.

MRS. B.

Their strong attraction for oxygen counterbalances this obstacle. Most
metals, however, require to be made red-hot before they are capable of
attracting oxygen in any considerable quantity. By this combination they
lose most of their metallic properties, and fall into a kind of powder,
formerly called _calx_, but now much more properly termed an _oxyd_;
thus we have _oxyd of lead_, _oxyd of iron_, &c.

EMILY.

And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is
formed by the union of the oxygen with that metal?

MRS. B.

Yes, it is.

CAROLINE