e?

MRS. B.

No; nor are they, generally speaking, of great importance. The _prussic
acid_ is, I think, the only one sufficiently interesting to require any
further comment. It can be formed by any artificial process, without the
presence of any animal matter; and it may likewise be obtained from a
variety of vegetables, particularly those of the narcotic kind, such as
poppies, laurel, &c. But it is commonly obtained from blood, by strongly
heating that substance with caustic potash; the alkali attracts the acid
from the blood, and forms with it a _prussiat of potash_. From this
state of combination the prussic acid can be obtained pure by means of
other substances which have the power of separating it from the alkali.

EMILY.

But if this acid does not exist ready formed in blood, how can the
alkali attract it from it?

MRS. B.

It is the triple basis only of this acid that exists in the blood; and
this is developed and brought to the state of acid, during the
combustion. The acid therefore is first formed, and it afterwards
combines with the potash.

EMILY.

Now I comprehend it. But how can the prussic acid be artificially made?

MRS. B.

By passing ammoniacal gas over red-hot charcoal; and hence we learn that
the constituents of this acid are hydrogen, nitrogen, and carbon. The
two first are derived from the volatile alkali, the last from the
combustion of the charcoal.

CAROLINE.

But this does not accord with the system of oxygen being the principle
of acidity.

MRS. B.

The colouring matter of prussian blue is called an acid, because it
unites with alkalies and metals, and not from any other characteristic
properties of acids; perhaps the name is not strictly appropriate. But
this circumstance, together with some others of the same kind, has
induced several chemists to think that oxygen may not be the exclusive
generator of acids. Sir H. Davy, I have already informed you, was led by
his experiments on dry acids to suspect that water might be essential to
acidity. And it is the opinion of some chemists that acidity may
possibly depend rather on the arrangement than on the presence of any
particular principles. But we have not yet done with the prussic acid.
It has a strong affinity for metallic oxyds, and precipitates the
solutions of iron in acids of a blue colour. This is the prussian blue,
or prussiat of iron, so much used in the arts, and with which I think
you must be acquainted.

EMILY.