nsparency which belongs to their crystalline
state; they are covered with a white powder, occasioned by their having
been exposed to the atmosphere, which has deprived their surface of its
lustre, by absorbing its water of crystallisation. Salts are, in
general, either _efflorescent_ or _deliquescent_: this latter property
is precisely the reverse of the former; that is to say, deliquescent
salts absorb water from the atmosphere, and are moistened and gradually
melted by it. Muriat of lime is an instance of great deliquescence.

EMILY.

But are there no salts that have the same degree of attraction for water
as the atmosphere, and that will consequently not be affected by it?

MRS. B.

Yes; there are many such salts, as, for instance, common salt, sulphat
of magnesia, and a variety of others.

_Sulphat of lime_ is very frequently met with in nature, and constitutes
the well-known substance called _gypsum_, or _plaster of Paris_.

_Sulphat of magnesia_, commonly called _Epsom salt_, is another very
bitter medicine, which is obtained from sea-water and from several
springs, or may be prepared by the direct combination of its
ingredients.

We have formerly mentioned _sulphat of alumine_ as constituting the
common _alum_; it is found in nature chiefly in the neighbourhood of
volcanos, and is particularly useful in the arts, from its strong
astringent qualities. It is chiefly employed by dyers and
calico-printers, to fix colours; and is used also in the manufacture of
some kinds of leather.

Sulphuric acid combines also with the metals.

CAROLINE.

One of these combinations, _sulphat of iron_, we are already well
acquainted with.

MRS. B.

That is the most important metallic salt formed by sulphuric acid, and
the only one that we shall here notice. It is of great use in the arts;
and, in medicine, it affords a very valuable tonic: it is of this salt
that most of those preparations called _steel medicines_ are composed.

CAROLINE.

But does any carbon enter into these compositions to form steel?

MRS. B.

Not an atom: they are, therefore, very improperly called steel: but it
is the vulgar appellation, and medical men themselves often comply with
the general custom.

Sulphat of iron may be prepared, as you have seen, by dissolving iron in
sulphuric acid; but it is generally obtained from the natural production
called _Pyrites_, which being a sulphuret of iron, requires only
exposure to the atmosphere to