f
the salt rapidly entering into a closer combination with the tin.

There is also a beautiful green salt too curious to be omitted; it is
produced by the combination of cobalt with muriatic acid, which has the
singular property of forming what is called _sympathetic ink_.
Characters written with this solution are invisible when cold, but when
a gentle heat is applied, they assume a fine bluish green colour.

CAROLINE.

I think one might draw very curious landscapes with the assistance of
this ink; I would first make a water-colour drawing of a winter-scene,
in which the trees should be leafless, and the grass scarcely green:
I would then trace all the verdure with the invisible ink, and whenever
I chose to create spring, I should hold it before the fire, and its
warmth would cover the landscape with a rich verdure.

MRS. B.

That will be a very amusing experiment, and I advise you by all means to
try it.

[Transcriber's Note:
Several cobalt compounds, including the cobalt chloride described
here, are still in use as invisible ("sympathetic") inks. They are
safe if used appropriately.]

Before we part, I must introduce to your acquaintance the curious metals
which Sir H. Davy has recently discovered. The history of these
extraordinary bodies is yet so much in its infancy, that I shall confine
myself to a very short account of them; it is more important to point
out to you the vast, and apparently inexhaustible, field of research
which has been thrown open to our view by Sir H. Davy's memorable
discoveries, than to enter into a minute account of particular bodies or
experiments.

CAROLINE.

But I have heard that these discoveries, however splendid and
extraordinary, are not very likely to prove of any great benefit to the
world, as they are rather objects of curiosity than of use.

MRS. B.

Such may be the illiberal conclusions of the ignorant and narrow-minded;
but those who can duly estimate the advantages of enlarging the sphere
of science, must be convinced that the acquisition of every new fact,
however unconnected it may at first appear with practical utility, must
ultimately prove beneficial to mankind. But these remarks are scarcely
applicable to the present subject; for some of the new metals have
already proved eminently useful as chemical agents, and are likely soon
to be employed in the arts. For the enumeration of these metals, I must
refer you to our list of simple bodies; they are d