_Of the effect of the CALCINATION of METALS, and of the EFFLUVIA of
PAINT made with WHITE-LEAD and OIL, on AIR._
Having been led to suspect, from the experiments which I had made with
charcoal, that the diminution of air in that case, and perhaps in other
cases also, was, in some way or other the consequence of its having more
than its usual quantity of phlogiston, it occurred to me, that the
calcination of metals, which are generally supposed to consist of
nothing but a metallic earth united to phlogiston, would tend to
ascertain the fact, and be a kind of _experimentum crucis_ in the case.
Accordingly, I suspended pieces of lead and tin in given quantities of
air, in the same manner as I had before treated the charcoal; and
throwing the focus of a burning mirror or lens upon them, so as to make
them fume copiously. I presently perceived a diminution of the air. In
the first trial that I made, I reduced four ounce measures of air to
three, which is the greatest diminution of common air that I had ever
observed before, and which I account for, by supposing that, in other
cases, there was not only a cause of diminution, but causes of addition
also, either of fixed or inflammable air, or some other permanently
elastic matter, but that the effect of the calcination of metals being
simply the escape of phlogiston, the cause of diminution was alone and
uncontrouled.
The air, which I had thus diminished by calcination of lead, I
transferred into another clean phial, but found that the calcination of
more lead in it (or at least the attempt to make a farther calcination)
had no farther effect upon it. This air also, like that which had been
infected with the fumes of charcoal, was in the highest degree noxious,
made no effervescence with nitrous air, was no farther diminished by the
mixture of iron filings and brimstone, and was not only rendered
innoxious, but also recovered, in a great measure, the other properties
of common air, by washing in water.
It might be suspected that the noxious quality of air in which _lead_
was calcined, might be owing to some fumes peculiar to that metal; but
I found no sensible difference between the properties of this air, and
that in which _tin_ was calcined.
The _water_ over which metals are calcined acquires a yellowish tinge,
and an exceedingly pungent smell and taste, pretty much (as near as I
can recollect, for I did not compare them together) like that over which
brimstone has been frequently burned. Also a thin and whitish pellicle
covered both the surface of the water, and likewise the sides of the
phial in which the calcination was made; insomuch that, without
frequently agitating the water, it grew so opaque by this constantly
accumulating incrustation, that the sun-beams could not be transmitted
through it in a quantity sufficient to produce the calcination.
I imagined, however, that, even when this air was transferred into a
clean phial, the metals were not so easily melted or calcined as they
were in fresh air; for the air being once fully saturated with
phlogiston, may not so readily admit any more, though it be only to
transmit it to the water. I also suspected that metals were not easily
melted or calcined in inflammable, fixed, or nitrous air, or any kind
of diminished air.[8] None of these kinds of air suffered any change by
this operation; nor was there any precipitation of lime, when charcoal
was heated in any of these kinds of air standing in lime-water. This
furnishes another, and I think a pretty decisive proof, that, in the
precipitation of lime by charcoal, the fixed air does not come from the
charcoal, but from the common air. Otherwise it is hard to assign a
reason, why the same degree of heat (or at least a much greater) should
not expel the fixed air from this substance, though surrounded by these
different kinds of air, and why the fixed air might not be transmitted
through them to the lime-water.
Query. May not water impregnated with phlogiston from calcined metals,
or by any other method, be of some use in medicine? The effect of this
impregnation is exceedingly remarkable; but the principle with which it
is impregnated is volatile, and intirely escapes in a day or two, if the
surface of the water be exposed to the common atmosphere.
It should seem that phlogiston is retained more obstinately by charcoal
than it is by lead or tin; for when any given quantity of air is fully
saturated with phlogiston from charcoal, no heat that I have yet applied
has been able to produce any more effect upon it; whereas, in the same
circumstances, lead and tin may still be calcined, at least be made to
emit a copious fume, in which some part of the phlogiston may be set
loose. The air indeed, can take no more; but the water receives it, and
the sides of the phial also receive an addition of incrustation. This is
a white powdery substance, and well deserves to be examined. I shall
endeavour to do it at my leisure.
Lime-water never became turbid by the calcination of metals over it, the
calx immediately seizing the precipitated fixed air, in preference to
the lime in the water; but the colour, smell, and taste of the water was
always changed and the surface of it became covered with a yellow
pellicle, as before.
When this process was made in quicksilver, the air was diminished only
one fifth; and upon water being admitted to it, no more was absorbed;
which is an effect similar to that of a mixture of nitrous and common
air, which was mentioned before.
The preceding experiments on the calcination of metals suggested to me a
method of explaining the cause of the mischief which is known to arise
from fresh _paint_, made with white-lead (which I suppose is an
imperfect calx of lead) and oil.
To verify my hypothesis, I first put a small pot full of this kind of
paint, and afterwards (which answered much better, by exposing a greater
surface of the paint) I daubed several pieces of paper with it, and put
them under a receiver, and observed, that in about twenty-four hours,
the air was diminished between one fifth and one fourth, for I did not
measure it very exactly. This air also was, as I expected to find, in
the highest degree noxious; it did not effervesce with nitrous air, it
was no farther diminished by a mixture of iron filings and brimstone,
and was made wholesome by agitation in water deprived of all air.
I think it appears pretty evident, from the preceding experiments on the
calcination of metals that air is, some way or other, diminished in
consequence of being highly charged with phlogiston; and that agitation
in water restores it, by imbibing a great part of the phlogistic
matter.
That water has a considerable affinity with phlogiston, is evident from
the strong impregnation which it receives from it. May not plants also
restore air diminished by putrefaction by absorbing part of the
phlogiston with which it is loaded? The greater part of a dry plant, as
well as of a dry animal substance, consists of inflammable air, or
something that is capable of being converted into inflammable air; and
it seems to be as probable that this phlogistic matter may have been
imbibed by the roots and leaves of plants, and afterwards incorporated
into their substance, as that it is altogether produced by the power of
vegetation. May not this phlogistic matter be even the most essential
part of the food and support of both vegetable and animal bodies?
In the experiments with metals, the diminution of air seems to be the
consequence of nothing but a saturation with phlogiston; and in all the
other cases of the diminution of air, I do not see but that it may be
effected by the same means. When a vegetable or animal substance is
dissolved by putrefaction, the escape of the phlogistic matter (which,
together with all its other constituent parts, is then let loose from
it) may be the circumstance that produces the diminution of the air in
which it putrefies. It is highly improbable that what remains after an
animal body has been thoroughly dissolved by putrefaction, should yield
so great a quantity of inflammable air, as the dried animal substance
would have done. Of this I have not made an actual trial, though I have
often thought of doing it, and still intend to do it; but I think there
can be no doubt of the result.
Again, iron, by its fermentation with brimstone and water, is evidently
reduced to a calx, so that phlogiston must have escaped from it.
Phlogiston also must evidently be set loose by the ignition of charcoal,
and is not improbably the matter which flies off from paint, composed of
white-lead and oil. Lastly, since spirit of nitre is known to have a
very remarkable affinity with phlogiston, it is far from being
improbable that nitrous air may also produce the same effect by the same
means.
To this hypothesis it may be objected, that, if diminished air be air
saturated with phlogiston, it ought to be inflammable. But this by no
means follows; since its inflammability may depend upon some particular
_mode of combination_, or degree of affinity, with which we are not
acquainted. Besides, inflammable air seems to consist of some other
principle, or to have some other constituent part, besides phlogiston
and common air, as is probable from that remarkable deposit, which, as I
have observed, is made by inflammable air, both from iron and zinc.
It is not improbable, however, but that a greater degree of heat may
inflame that air which extinguishes a common candle, if it could be
conveniently applied. Air that is inflammable, I observe, extinguishes
red-hot wood; and indeed inflammable substances can only be those which,
in a certain degree of heat, have a less affinity with the phlogiston
they contain, than the air, or some other contiguous substance, has with
it; so that the phlogiston only quits one substance, with which it was
before combined, and enters another, with which it may be combined in a
very different manner. This substance, however, whether it be air or any
thing else, being now fully saturated with phlogiston, and not being
able to take any more, in the same circumstances, must necessarily
extinguish fire, and put a stop to the ignition of all other bodies,
that is, to the farther escape of phlogiston from them.
That plants restore noxious air, by imbibing the phlogiston with which
it is loaded, is very agreeable to the conjectures of Dr. Franklin,
made many years ago, and expressed in the following extract from the
last edition of his Letters, p. 346.
"I have been inclined to think that the fluid _fire_, as well as the
fluid _air_, is attracted by plants in their growth, and becomes
consolidated with the other materials of which they are formed, and
makes a great part of their substance; that, when they come to be
digested, and to suffer in the vessels a kind of fermentation, part of
the fire, as well as part of the air, recovers its fluid active state
again, and diffuses itself in the body, digesting and separating it;
that the fire so re-produced, by digestion and separation, continually
leaving the body, its place is supplied by fresh quantities, arising
from the continual separation; that whatever quickens the motion of the
fluids in an animal, quickens the separation, and re-produces more of
the fire, as exercise; that all the fire emitted by wood, and other
combustibles, when burning, existed in them before in a solid state,
being only discovered when separating; that some fossils, as sulphur,
sea-coal, &c. contain a great deal of solid fire; and that, in short,
what escapes and is dissipated in the burning of bodies, besides water
and earth, is generally the air and fire, that before made parts of the
solid."
FOOTNOTES:
[8] I conclude from the experiments of M. Lavoisier, which were made
with a much better burning lens than I had an opportunity of making use
of, that there was no _real calcination_ of the metals, though they were
made to _fume_ in inflammable or nitrous air; because he was not able to
produce more than a slight degree of calcination in any given quantity
of common air.