_Of MARINE ACID AIR._
Being very much struck with the result of an experiment of the Hon. Mr.
Cavendish, related Phil. Trans. Vol. LVI. p. 157, by which, though, he
says, he was not able to get any inflammable air from copper, by means
of spirit of salt, he got a much more remarkable kind of air, viz. one
that lost its elasticity by coming into contact with water, I was
exceedingly desirous of making myself acquainted with it. On this
account, I began with making the experiment in quicksilver, which I
never failed to do in any case in which I suspected that air might
either be absorbed by water, or be in any other manner affected by it;
and by this means I presently got a much more distinct idea of the
nature and effects of this curious solution.
Having put some copper filings into a small phial, with a quantity of
spirit of salt; and making the air (which was generated in great plenty,
on the application of heat) ascend into a tall glass vessel full of
quicksilver, and standing in quicksilver, the whole produce continued a
considerable time without any change of dimensions. I then introduced a
small quantity of water to it; when about three fourths of it (the whole
being about four ounce measures) presently, but gradually, disappeared,
the quicksilver rising in the vessel. I then introduced a considerable
quantity of water; but there was no farther diminution of the air, and
the remainder I found to be inflammable.
Having frequently continued this process a long time after the admission
of the water, I was much amused with observing the large bubbles of the
newly generated air, which came through the quicksilver, the sudden
diminution of them when they came to the water, and the very small
bubbles which went through the water. They made, however, a continual,
though slow, increase of inflammable air.
Fixed air, being admitted to the whole produce of this air from copper,
had no sensible effect upon it. Upon the admission of water, a great
part of the mixture presently disappeared; another part, which I suppose
to have been the fixed air, was absorbed slowly; and in this particular
case the very small permanent residuum did not take fire; but it is
very possible that it might have done so, if the quantity had been
greater.
The solution of _lead_ in the marine acid is attended with the very same
phaenomena as the solution of copper in the same acid; about three
fourths of the generated air disappearing on the admission of water; and
the remainder being inflammable.
The solutions of iron, tin, and zinc, in the marine acid, were all
attended with the same phaenomena as the solutions of copper and lead,
but in a less degree; for in iron one eighth, in tin one sixth, and in
zinc one tenth of the generated air disappeared on the admission of
water. The remainder of the air from iron, in this case, burned with a
green, or very light blue flame.
I had always thought it something extraordinary that a species of air
should _lose its elasticity_ by the mere _contact_ of any thing, and
from the first suspected that it must have been _imbibed_ by the water
that was admitted to it; but so very great a quantity of this air
disappeared upon the admission of a very small quantity of water, that
at first I could not help concluding that appearances favoured the
former hypothesis. I found, however, that when I admitted a much
smaller quantity of water, confined in a narrow glass tube, a part only
of the air disappeared, and that very slowly, and that more of it
vanished upon the admission of more water. This observation put it
beyond a doubt, that this air was properly _imbibed_ by the water,
which, being once fully saturated with it, was not capable of receiving
any more.
The water thus impregnated tasted very acid, even when it was much
diluted with other water, through which the tube containing it was
drawn. It even dissolved iron very fast, and generated inflammable air.
This last observation, together with another which immediately follows,
led me to the discovery of the true nature of this remarkable kind of
air.
Happening, at one time, to use a good deal of copper and a small
quantity of spirit of salt, in the generation of this kind of air, I was
surprized to find that air was produced long after, I could not but
think that the acid must have been saturated with the metal; and I also
found that the proportion of inflammable air to that which was absorbed
by the water continually diminished, till, instead of being one fourth
of the whole, as I had first observed, it was not so much as one
twentieth. Upon this, I concluded that this subtle air did not arise
from the copper, but from the spirit of salt; and presently making the
experiment with the acid only, without any copper, or metal of any kind,
this air was immediately produced in as great plenty as before; so that
this remarkable kind of air is, in fact, nothing more than the vapour,
or fumes of spirit of salt, which appear to be of such a nature, that
they are not liable to be condensed by cold, like the vapour of water,
and other fluids, and therefore may be very properly called an _acid
air_, or more restrictively, the _marine acid air_.
This elastic acid vapour, or acid air, extinguishes flame, and is much
heavier than common air; but how much heavier, will not be easy to
ascertain. A cylindrical glass vessel, about three fourths of an inch in
diameter, and four inches deep, being filled with it, and turned upside
down, a lighted candle may be let down into it more than twenty times
before it will burn at the bottom. It is pleasing to observe the colour
of the flame in this experiment; for both before the candle goes out,
and also when it is first lighted again, it burns with a beautiful
green, or rather light-blue flame, such as is seen when common salt is
thrown into the fire.
When this air is all expelled from any quantity of spirit of salt, which
is easily perceived by the subsequent vapour being condensed by cold,
the remainder is a very weak acid, barely capable of dissolving iron.
Being now in the possession of a new subject of experiments, viz. an
elastic acid vapour, in the form of a permanent air, easily procured,
and effectually confined by glass and quicksilver, with which it did not
seem to have any affinity; I immediately began to introduce a variety of
substances to it; in order to ascertain its peculiar properties and
affinities, and also the properties of those other bodies with respect
to it.
Beginning with _water_, which, from preceding observations, I knew would
imbibe it, and become impregnated with it; I found that 2-1/2 grains of
rain-water absorbed three ounce measures of this air, after which it was
increased one third in its bulk, and weighed twice as much as before; so
that this concentrated vapour seems to be twice as heavy as rain-water:
Water impregnated with it makes the strongest spirit of salt that I have
seen, dissolving iron with the most rapidity. Consequently, two thirds
of the best spirit of salt is nothing more than mere phlegm or water.
Iron filings, being admitted to this air, were dissolved by it pretty
fast, half of the air disappearing, and the other half becoming
inflammable air, not absorbed by water. Putting chalk to it, fixed air
was produced.
I had not introduced many substances to this air, before I discovered
that it had an affinity with _phlogiston_, so that it would deprive
other substances of it, and form with it such an union as constitutes
inflammable air; which seems to shew, that inflammable air universally
consists of the union of some acid vapour with phlogiston.
Inflammable air was produced, when to this acid air I put spirit of
wine, oil of olives, oil of turpentine, charcoal, phosphorus, bees-wax,
and even sulphur. This last observation, I own, surprized me; for, the
marine acid being reckoned the weakest of the three mineral acids, I did
not think that it had been capable of dislodging the oil of vitriol from
this substance; but I found that it had the very same effect both upon
alum and nitre; the vitriolic acid in the former case, and the nitrous
in the latter, giving place to the stronger vapour of spirit of salt.
The rust of iron, and the precipitate of nitrous air made from copper,
also imbibed this air very fast, and the little that remained of it was
inflammable air; which proves, that these calces contain phlogiston. It
seems also to be pretty evident, from this experiment, that the
precipitate above mentioned is a real calx of the metal, by the solution
of which the nitrous air is generated.
As some remarkable circumstances attend the absorption of this acid air,
by the substances above-mentioned, I shall briefly mention them.
Spirit of wine absorbs this air as readily as water itself, and is
increased in bulk by that means. Also, when it is saturated, it
dissolves iron with as much rapidity, and still continues inflammable.
Oil of olives absorbs this air very slowly, and at the same time, it
turns almost black, and becomes glutinous. It is also less miscible with
water, and acquires a very disagreeable smell. By continuing upon the
surface of the water, it became white, and its offensive smell went off
in a few days.
Oil of turpentine absorbed this air very fast, turning brown, and almost
black. No inflammable air was formed, till I raised more of the acid
air than the oil was able to absorb, and let it stand a considerable
time; and still the air was but weakly inflammable. The same was the
case with the oil of olives, in the last mentioned experiment; and it
seems to be probable, that, the longer this acid air had continued in
contact with the oil, the more phlogiston it would have extracted from
it. It is not wholly improbable, but that, in the intermediate state,
before it becomes inflammable air, it may be nearly of the nature of
common air.
Bees-wax absorbed this air very slowly. About the bigness of a hazel-nut
of the wax being put to three ounce measures of the acid air, the air
was diminished one half in two days, and, upon the admission of water,
half of the remainder also disappeared. This air was strongly
inflammable.
Charcoal absorbed this air very fast. About one fourth of it was
rendered immiscible in water, and was but weakly inflammable.
A small bit of _phosphorus_, perhaps about half a grain, smoked, and
gave light in the acid air, just as it would have done in common air
confined. It was not sensibly wasted after continuing about twelve
hours in that state, and the bulk of the air was very little diminished.
Water being admitted to it absorbed it as before, except about one fifth
of the whole. It was but weakly inflammable.
Putting several pieces of _sulphur_ to this air, it was absorbed but
slowly. In about twenty-four hours about one fifth of the quantity had
disappeared; and water being admitted to the remainder, very little more
was absorbed. The remainder was inflammable, and burned with a blue
flame.
Notwithstanding the affinity which this acid air appears to have with
phlogiston, it is not capable of depriving all bodies of it. I found
that dry wood, crusts of bread, and raw flesh, very readily imbibed this
air, but did not part with any of their phlogiston to it. All these
substances turned very brown, after they had been some time exposed to
this air, and tasted very strongly of the acid when they were taken out;
but the flesh, when washed in water, became very white, and the fibres
easily separated from one another, even more than they would have done
if it had been boiled or roasted[9].
When I put a piece of _saltpetre_ to this air it was presently
surrounded with a white fume, which soon filled the whole vessel,
exactly like the fume which bursts from the bubbles of nitrous air, when
it is generated by a vigorous fermentation, and such as is seen when
nitrous air is mixed with this acid air. In about a minute, the whole
quantity of air was absorbed, except a very little, which might be the
common air that had lodged upon the surface of the spirit of salt within
the phial.
A piece of _alum_ exposed to this air turned yellow, absorbed it as fast
as the saltpetre had done, and was reduced by it to the form of a
powder. Common salt, as might be expected, had no effect whatever on
this marine acid air.
I had also imagined, that if air diminished by the processes
above-mentioned was affected in this manner, in consequence of its being
saturated with phlogiston, a mixture of this acid air might imbibe that
phlogiston, and render it wholesome again; but I put about one fourth of
this air to a quantity of air in which metals had been calcined, without
making any sensible alteration in it. I do not, however, infer from
this, that air is not diminished by means of phlogiston, since the
common air, like some other substances, may hold the phlogiston too
fast, to be deprived of it by this acid air.
I shall conclude my account of these experiments with observing, that
the electric spark is visible in acid air, exactly as it is in common
air; and though I kept making this spark a considerable time in a
quantity of it, I did not perceive that any sensible alteration was made
in it. A little inflammable air was produced, but not more than might
have come from the two iron nails which I made use of in taking the
sparks.
FOOTNOTES:
[9] It will be seen, in the second part of this work, that, in some of
these processes, I had afterwards more success.