from iron,
copper, brass, tin, silver, quicksilver, bismuth, and nickel, by the
nitrous acid only, and from gold and the regulus of antimony by _aqua
regia_. The circumstances attending the solution of each of these metals
are various, but hardly worth mentioning, in treating of the properties
of the _air_ which they yield; which, from what metal soever it is
extracted, has, as far as I have been able to observe, the very same
properties.

One of the most conspicuous properties of this kind of air is the great
diminution of any quantity of common air with which it is mixed,
attended with a turbid red, or deep orange colour, and a considerable
heat. The _smell_ of it, also, is very strong, and remarkable, but very
much resembling that of smoking spirit of nitre.

The diminution of a mixture of this and common air is not an equal
diminution of both the kinds, which is all that Dr. Hales could observe,
but of about one fifth of the common air, and as much of the nitrous air
as is necessary to produce that effect; which, as I have found by many
trials, is about one half as much as the original quantity of common
air. For if one measure of nitrous air be put to two measures of common
air, in a few minutes (by which time the effervescence will be over, and
the mixture will have recovered its transparency) there will want about
one ninth of the original two measures; and if both the kinds of air be
very pure, the diminution will still go on slowly, till in a day or two,
the whole will be reduced to one fifth less than the original quantity
of common air. This farther diminution, by long standing, I had not
observed at the time of the first publication of these papers.

I hardly know any experiment that is more adapted to amaze and surprize
than this is, which exhibits a quantity of air, which, as it were,
devours a quantity of another kind of air half as large as itself, and
yet is so far from gaining any addition to its bulk, that it is
considerably diminished by it. If, after this full saturation of common
air with nitrous air, more nitrous air be put to it, it makes an
addition equal to its own bulk, without producing the least redness, or
any other visible effect.

If the smallest quantity of common air be put to any larger quantity of
nitrous air, though the two together will not occupy so much space as
they did separately, yet the quantity will still be larger than that of
the nitrous air only. One ounce measure of