gnated with fixed air readily dissolves iron, as Mr.
Lane has discovered; so that if a quantity of iron filings be put to it,
it presently becomes a strong chalybeate, and of the mildest and most
agreeable kind.

I have recommended the use of _chalk_ and oil of vitriol as the
cheapest, and, upon the whole, the best materials for this purpose. But
some persons prefer _pearl ashes_, _pounded marble_, or other calcareous
or _alkaline substances_; and perhaps with reason. My own experience has
not been sufficient to enable me to decide in this case.

Whereas some persons had suspected that a quantity of the oil of vitriol
was rendered volatile by this process, I examined it, by all the
chemical methods that are in use; but could not find that water thus
impregnated contained the least perceivable quantity of that acid.

Mr. Hey, indeed, who assisted me in this examination, found that
distilled water, impregnated with fixed air, did not mix so readily with
soap as the distilled water itself; but this was also the case when the
fixed air had passed through a long glass tube filled with alkaline
salts, which, it may be supposed, would have imbibed any of the oil of
vitriol that might have been contained in that air[2].

Fixed air itself may be said to be of the nature of an acid, though of a
weak and peculiar sort.----Mr. Bergman of Upsal, who honoured me with a
letter upon the subject, calls it the _aerial acid_, and, among other
experiments to prove it to be an acid, he says that it changes the blue
juice of tournesole into red. This Mr. Hey found to be true, and he
moreover discovered that when water tinged blue with the juice of
tournesole, and then red with fixed air, has been exposed to the open
air, it recovers its blue colour again.

The heat of boiling water will expel all the fixed air, if a phial
containing the impregnated water be held in it; but it will often
require above half an hour to do it completely.

Dr. Percival, who is particularly attentive to every improvement in the
medical art, and who has thought so well of this impregnation as to
prescribe it in several cases, informs me that it seems to be much
stronger, and sparkles more, like the true Pyrmont water, after it has
been kept some time. This circumstance, however, shews that, in time,
the fixed air is more easily disengaged from the water; and though, in
this state, it may affect the taste more sensibly, it cannot be of so
much use in the stoma