_Of AIR in which a CANDLE, or BRIMSTONE, has burned out._
It is well known that flame cannot subsist long without change of air,
so that the common air is necessary to it, except in the case of
substances, into the composition of which nitre enters, for these will
burn _in vacuo_, in fixed air, and even under water, as is evident in
some rockets, which are made for this purpose. The quantity of air which
even a small flame requires to keep it burning is prodigious. It is
generally said, that an ordinary candle _consumes_, as it is called,
about a gallon in a minute. Considering this amazing consumption of air,
by fires of all kinds, volcanos, &c. it becomes a great object of
philosophical inquiry, to ascertain what change is made in the
constitution of the air by flame, and to discover what provision there
is in nature for remedying the injury which the atmosphere receives by
this means. Some of the following experiments will, perhaps, be thought
to throw light upon the subject.
The diminution of the quantity of air in which a candle, or brimstone,
has burned out, is various; But I imagine that, at a medium, it may be
about one fifteenth, or one sixteenth of the whole; which is one third
as much as by animal or vegetable substances putrefying in it, by the
calcination of metals, or by any of the other causes of the complete
diminution of air, which will be mentioned hereafter.
I have sometimes thought, that flame disposes the common air to deposit
the fixed air it contains; for if any lime-water be exposed to it, it
immediately becomes turbid. This is the case, when wax candles, tallow
candles, chips of wood, spirit of wine, ether, and every other substance
which I have yet tried, except brimstone, is burned in a close glass
vessel, standing in lime-water. This precipitation of fixed air (if this
be the case) may be owing to something emitted from the burning bodies,
which has a stronger affinity with the other constituent parts of the
atmosphere[3].
If brimstone be burned in the same circumstances, the lime-water
continues transparent, but still there may have been the same
precipitation of the fixed part of the air; but that, uniting with the
lime and the vitriolic acid, it forms a selenetic salt, which is soluble
in water. Having evaporated a quantity of water thus impregnated, by
burning brimstone a great number of times over it, a whitish powder
remained, which had an acid taste; but repeating the experiment with a
quicker evaporation, the powder had no acidity, but was very much like
chalk. The burning of brimstone but once over a quantity of lime-water,
will affect it in such a manner, that breathing into it will not make it
turbid, which otherwise it always presently does.
Dr. Hales supposed, that by burning brimstone repeatedly in the same
quantity of air, the diminution would continue without end. But this I
have frequently tried, and not found to be the case. Indeed, when the
ignition has been imperfect in the first instance, a second firing of
the same substance will increase the effect of the first, &c. but this
progress soon ceases.
In many cases of the diminution of air, the effect is not immediately
apparent, even when it stands in water; for sometimes the bulk of air
will not be much reduced, till it has passed several times through a
quantity of water, which has thereby a better opportunity of absorbing
that part of the air, which had not been perfectly detatched from the
rest. I have sometimes found a very great reduction of a mass of air, in
consequence of passing but once through cold water. If the air has stood
in quicksilver, the diminution is generally inconsiderable, till it has
undergone this operation, there not being any substance exposed to the
air that could absorb any part of it.
I could not find any considerable alteration in the specific gravity of
the air, in which candles, or brimstone, had burned out. I am satisfied,
however, that it is not heavier than common air, which must have been
manifest, if so great a diminution of the quantity had been owing, as
Dr. Hales and others supposed, to the elasticity of the whole mass being
impaired. After making several trials for this purpose, I concluded that
air, thus diminished in bulk, is rather lighter than common air, which
favours the supposition of the fixed, or heavier part of the common air,
having been precipitated.
An animal will live nearly, if not quite as long, in air in which
candles have burned out, as in common air. This fact surprized me very
greatly, having imagined that what is called the _consumption_ of air by
flame, or respiration, to have been of the same nature, and in the same
degree; but I have since found, that this fact has been observed by many
persons, and even so early as by Mr. Boyle. I have also observed, that
air, in which brimstone has burned, is not in the least injurious to
animals, after the fumes, which at first make it very cloudy, have
intirely subsided.
I must, in this place, admonish my reader not to confound the simple
_burning of brimstone_, or of matches (_i. e._ bits of wood dipped in
it) and the burning of brimstone with a burning mirror, or any _foreign
heat_. The effect of the former is nothing more than that of any other
_flame_, or _ignited vapour_, which will not burn, unless the air with
which it is surrounded be in a very pure state, and which is therefore
extinguished when the air begins to be much vitiated. Lighted brimstone,
therefore reduces the air to the same state as lighted wood. But the
focus of a burning mirror thrown for a sufficient time either upon
brimstone, or wood, after it has ceased to burn of its own accord, and
has become _charcoal_, will have a much greater effect: of the same
kind, diminishing the air to its utmost extent, and making it thoroughly
noxious. In fact, as will be seen hereafter, more phlogiston is expelled
from these substances in the latter case than in the former. I never,
indeed, actually carried this experiment so far with brimstone; but from
the diminution of air that I did produce by this means, I concluded
that, by continuing the process some time longer, it would have been
effected.
Having read, in the Memoirs of the Philosophical Society at Turin, vol.
I. p. 41. that air in which candles had burned out was perfectly
restored, so that other candles would burn in it again as well as ever,
after having been exposed to a considerable degree of _cold_, and
likewise after having been compressed in bladders, (for the cold had
been supposed to have produced this effect by nothing but
_condensation_) I repeated those experiments, and did, indeed, find,
that when I compressed the air in _bladders_, as the Count de Saluce,
who made the observation, had done, the experiment succeeded: but having
had sufficient reason to distrust bladders, I compressed the air in a
glass vessel standing in water; and then I found, that this process is
altogether ineffectual for the purpose. I kept the air compressed much
more, and much longer, than the Count had done, but without producing
any alteration in it. I also find, that a greater degree of cold than
that which he applied, and of longer continuance, did by no means
restore this kind of air: for when I had exposed the phials which
contained it a whole night, in which the frost was very intense; and
also when I kept it surrounded with a mixture of snow and salt, I found
it, in all respects, the same as before.
It is also advanced, in the same Memoir, p. 41. that _heat_ only, as the
reverse of _cold_, renders air unfit for candles burning in it. But I
repeated the experiment of the Count for that purpose, without finding
any such effect from it. I also remember that, many years ago, I filled
an exhausted receiver with air, which had passed through a glass tube
made red-hot, and found that a candle would burn in it perfectly well.
Also, rarefaction by the air-pump does not injure air in the least
degree.
Though this experiment failed, I have been so happy, as by accident to
have hit upon a method of restoring air, which has been injured by the
burning of candles, and to have discovered at least one of the
restoratives which nature employs for this purpose. It is _vegetation_.
This restoration of vitiated air, I conjecture, is effected by plants
imbibing the phlogistic matter with which it is overloaded by the
burning of inflammable bodies. But whether there be any foundation for
this conjecture or not, the fact is, I think, indisputable. I shall
introduce the account of my experiments on this subject, by reciting
some of the observations which I made on the growing of plants in
confined air, which led to this discovery.
One might have imagined that, since common air is necessary to
vegetable, as well as to animal life, both plants and animals had
affected it in the same manner; and I own I had that expectation, when I
first put a sprig of mint into a glass jar, standing inverted in a
vessel of water: but when it had continued growing there for some
months, I found that the air would neither extinguish a candle, nor was
it at all inconvenient to a mouse, which I put into it.
The plant was not affected any otherwise than was the necessary
consequence of its confined situation; for plants growing in several
other kinds of air, were all affected in the very same manner. Every
succession of leaves was more diminished in size than the preceding,
till, at length, they came to be no bigger than the heads of pretty
small pins. The root decayed, and the stalk also, beginning from the
root; and yet the plant continued to grow upwards, drawing its
nourishment through a black and rotten stem. In the third or fourth set
of leaves, long and white hairy filaments grew from the insertion of
each leaf and sometimes from the body of the stem, shooting out as far
as the vessel in which it grew would permit, which, in my experiments,
was about two inches. In this manner a sprig of mint lived, the old
plant decaying, and new ones shooting up in its place, but less and less
continually, all the summer season.
In repeating this experiment, care must be taken to draw away all the
dead leaves from about the plant, lest they should putrefy, and affect
the air. I have found that a fresh cabbage leaf, put under a glass
vessel filled with common air, for the space of one night only, has so
affected the air, that a candle would not burn in it the next morning,
and yet the leaf had not acquired any smell of putrefaction.
Finding that candles would burn very well in air in which plants had
grown a long time, and having had some reason to think, that there was
something attending vegetation, which restored air that had been injured
by respiration, I thought it was possible that the same process might
also restore the air that had been injured by the burning of candles.
Accordingly, on the 17th of August 1771, I put a sprig of mint into a
quantity of air, in which a wax candle had burned out, and found that,
on the 27th of the same month, another candle burned perfectly well in
it. This experiment I repeated, without the least variation in the
event, not less than eight or ten times in the remainder of the summer.
Several times I divided the quantity of air in which the candle had
burned out, into two parts, and putting the plant into one of them, left
the other in the same exposure, contained, also, in a glass vessel
immersed in water, but without any plant; and never failed to find, that
a candle would burn in the former, but not in the latter.
I generally found that five or six days were sufficient to restore this
air, when the plant was in its vigour; whereas I have kept this kind of
air in glass vessels, immersed in water many months, without being able
to perceive that the least alteration had been made in it. I have also
tried a great variety of experiments upon it, as by condensing,
rarefying, exposing to the light and heat, &c. and throwing into it the
effluvia of many different substances, but without any effect.
Experiments made in the year 1772, abundantly confirmed my conclusion
concerning the restoration of air, in which candles had burned out by
plants growing in it. The first of these experiments was made in the
month of May; and they were frequently repeated in that and the two
following months, without a single failure.
For this purpose I used the flames of different substances, though I
generally used wax or tallow candles. On the 24th of June the experiment
succeeded perfectly well with air in which spirit of wine had burned
out, and on the 27th of the same month it succeeded equally well with
air in which brimstone matches had burned out, an effect of which I had
despaired the preceding year.
This restoration of air, I found, depended upon the _vegetating state_
of the plant; for though I kept a great number of the fresh leaves of
mint in a small quantity of air in which candles had burned out, and
changed them frequently, for a long space of time, I could perceive no
melioration in the state of the air.
This remarkable effect does not depend upon any thing peculiar to
_mint_, which was the plant that I always made use of till July 1772;
for on the 16th of that month, I found a quantity of this kind of air to
be perfectly restored by sprigs of _balm_, which had grown in it from
the 7th of the same month.
That this restoration of air was not owing to any _aromatic effluvia_ of
these two plants, not only appeared by the _essential oil of mint_
having no sensible effect of this kind; but from the equally complete
restoration of this vitiated air by the plant called _groundsel_, which
is usually ranked among the weeds, and has an offensive smell. This was
the result of an experiment made the 16th of July, when the plant had
been growing in the burned air from the 8th of the same month. Besides,
the plant which I have found to be the most effectual of any that I have
tried for this purpose is _spinach_, which is of quick growth, but will
seldom thrive long in water. One jar of burned air was perfectly
restored by this plant in four days, and another in two days. This last
was observed on the 22d of July.
In general, this effect may be presumed to have taken place in much less
time than I have mentioned; because I never chose to make a trial of
the air, till I was pretty sure, from preceding observations, that the
event which I had expected must have taken place, if it would succeed at
all; lest, returning back that part of the air on which I made the
trial, and which would thereby necessarily receive a small mixture of
common air, the experiment might not be judged to be quite fair; though
I myself might be sufficiently satisfied with respect to the allowance
that was to be made for that small imperfection.
FOOTNOTES:
[3] The supposition, mentioned in this and other passages of the first
part of this publication, viz. that the diminution of common air, by
this and other processes is, in part at least, owing to the
precipitation of the fixed air from it, the reader will find confirmed
by the experiments and observations in the second part.