e from the atmosphere without an elevation of
temperature, some light and heat, I suppose, must be disengaged, though
not in sufficient quantities to be sensible.

MRS. B.

Undoubtedly; and, indeed, it is not surprising that in this case the
light and heat should not be sensible, when you consider how extremely
slow, and, indeed, how imperfectly, most metals oxydate by mere exposure
to the atmosphere. For the quantity of oxygen with which metals are
capable of combining, generally depends upon their temperature; and the
absorption stops at various points of oxydation, according to the degree
to which their temperature is raised.

EMILY.

That seems very natural; for the greater the quantity of caloric
introduced into a metal, the more will its positive electricity be
exalted, and consequently the stronger will be its affinity for oxygen.

MRS. B.

Certainly. When the metal oxygenates with sufficient rapidity for light
and heat to become sensible, combustion actually takes place. But this
happens only at very high temperatures, and the product is nevertheless
an oxyd; for though, as I have just said, metals will combine with
different proportions of oxygen, yet with the exception of only five of
them, they are not susceptible of acidification.

Metals change colour during the different degrees of oxydation which
they undergo. Lead, when heated in contact with the atmosphere, first
becomes grey; if its temperature be then raised, it turns yellow, and a
still stronger heat changes it to red. Iron becomes successively a
green, brown, and white oxyd. Copper changes from brown to blue, and
lastly green.

EMILY.

Pray, is the white lead with which houses are painted prepared by
oxydating lead?

MRS. B.

Not merely by oxydating, but by being also united with carbonic acid. It
is a carbonat of lead. The mere oxyd of lead is called red lead.
Litharge is another oxyd of lead, containing less oxygen. Almost all the
metallic oxyds are used as paints. The various sorts of ochres consist
chiefly of iron more or less oxydated. And it is a remarkable
circumstance, that if you burn metals rapidly, the light or flame they
emit during combustion partakes of the colours which the oxyd
successively assumes.

CAROLINE.

How is that accounted for, Mrs. B.? For light, you know, does not
proceed from the burning body, but from the decomposition of the oxygen
gas?

MRS. B.

The correspondence of the colour of the light with