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.