perature of
seventy-two degrees colder than that at which water freezes.

MRS. B.

Exactly so.

CAROLINE.

But is the temperature of the atmosphere ever so low as that?

MRS. B.

Yes, often in Siberia; but happily never in this part of the globe.
Here, however, mercury may be congealed by artificial cold; I mean such
intense cold as can be produced by some chemical mixtures, or by the
rapid evaporation of ether under the air-pump.*

[Footnote *: By a process analogous to that described, page 155.
of this volume.]

CAROLINE.

And can mercury be made to boil and evaporate?

MRS. B.

Yes, like any other liquid; only it requires a much greater degree of
heat. At the temperature of six hundred degrees, it begins to boil and
evaporate like water.

Mercury combines with gold, silver, tin, and with several other metals;
and, if mixed with any of them in a sufficient proportion, it penetrates
the solid metal, softens it, loses its own fluidity, and forms an
_amalgam_, which is the name given to the combination of any metal with
mercury, forming a substance more or less solid, according as the
mercury or the other metal predominates.

EMILY.

In the list of metals there are some whose names I have never before
heard mentioned.

MRS. B.

Besides those which Sir H. Davy has obtained, there are several that
have been recently discovered, whose properties are yet but little
known, as for instance, titanium, which was discovered by the Rev. Mr.
Gregor, in the tin-mines of Cornwall; columbium or tantalium, which has
lately been discovered by Mr. Hatchett; and osmium, iridium, palladium,
and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed in
minute quantities with crude platina, and the distinct existence of
which they proved by curious and delicate experiments.

CAROLINE.

Arsenic has been mentioned amongst the metals. I had no notion that it
belonged to that class of bodies, for I had never seen it but as a
powder, and never thought of it but as a most deadly poison.

MRS. B.

In its pure metallic state, I believe, it is not so poisonous; but it
has such a great affinity for oxygen, that it absorbs it from the
atmosphere at its natural temperature: you have seen it, therefore, only
in its state of oxyd, when, from its combination with oxygen, it has
acquired its very poisonous properties.

CAROLINE.

Is it possible that oxygen can impart poisonous qualities? That valuable
subst