Caroline--

MRS. B.

There is no such danger from the sulphureous; but the experiment must be
made with great caution, for, during the formation of sulphureous acid
by combustion, there is always some sulphuric produced.

CAROLINE.

But where is your sulphureous acid?

MRS. B.

We may easily prepare some ourselves, simply by burning a match; we must
first wet the stain with water, and now hold it in this way, at a little
distance, over the lighted match: the vapour that arises from it is
sulphureous acid, and the stain, you see, gradually disappears.

EMILY.

I have frequently taken out stains by this means, without understanding
the nature of the process. But why is it necessary to wet the stain
before it is exposed to the acid fumes?

MRS. B.

The moisture attracts and absorbs the sulphureous acid; and it serves
likewise to dilute any particles of sulphuric acid which might injure
the linen.

Sulphur is susceptible of a third combination with oxygen, in which the
proportion of the latter is too small to render the sulphur acid. It
acquires this slight oxygenation by mere exposure to the atmosphere,
without any elevation of temperature: in this case, the sulphur does not
change its natural form, but is only discoloured, being changed to red
or brown; and in this state it is an oxyd of sulphur.

Before we take leave of the sulphuric acid, we shall say a few words of
its principal combinations. It unites with all the alkalies, alkaline
earths and metals, to form compound salts.

CAROLINE.

Pray, give me leave to interrupt you for a moment: you have never
mentioned any other salts than the compound or neutral salts; is there
no other kind?

MRS. B.

The term _salt_ has been used, from time immemorial, as a kind of
general name for any substance that has savour, odour, is soluble in
water, and crystallisable, whether it be of an acid, an alkaline, or
compound nature; but the compound salts alone retain that appellation in
modern chemistry.

The most important of the salts, formed by the combinations of the
sulphuric acid, are, first, _sulphat of potash_, formerly called _sal
polychrest_: this is a very bitter salt, much used in medicine; it is
found in the ashes of most vegetables, but it may be prepared
artificially by the immediate combination of sulphuric acid and potash.
This salt is easily soluble in boiling water. Solubility is, indeed,
a property common to all salts; and they always produce