we find this white powder, produced by the lime-water and the vapour from
the candle, appears to us very much like whitening or chalk, and, when
examined, it does prove to be exactly the same substance as whitening or
chalk. So we are led, or have been led, to observe upon the various
circumstances of this experiment, and to trace this production of chalk to
its various causes, to give us the true knowledge of the nature of this
combustion of the candle to find that this substance, issuing from the
candle, is exactly the same as that substance which would issue from a
retort, if I were to put some chalk into it with a little moisture, and
make it red-hot: you would then find that exactly the same substance would
issue from it as from the candle.
But we have a better means of getting this substance, and in greater
quantity, so as to ascertain what its general characters are. We find this
substance in very great abundance in a multitude of cases where you would
least expect it. All limestones contain a great deal of this gas which
issues from the candle, and which we call _carbonic acid_. All chalks, all
shells, all corals contain a great quantity of this curious air. We find
it fixed in these stones; for which reason Dr. Black called it fixed
air finding it in these fixed things like marble and chalk. He called it
fixed air, because it lost its quality of air, and assumed the condition
of a solid body. We can easily get this air from marble. Here is a jar
containing a little muriatic acid, and here is a taper which, if I put it
into that jar, will shew only the presence of common air. There is, you
see, pure air down to the bottom; the jar is full of it Here is a
substance marble[17], a very beautiful and superior marble and if I put
these pieces of marble into the jar, a great boiling apparently goes on.
That, however, is not steam it is a gas that is rising up; and if I now
search the jar by a candle, I shall have exactly the same effect produced
upon the taper as I had from the air which issued from the end of the
chimney over the burning candle. It is exactly the same action, and caused
by the very same substance that issued from the candle; and in this way we
can get carbonic acid in great abundance we have already nearly filled
the jar. We also find that this gas is not merely contained in marble.
Here is a vessel in which I have put some common whitening chalk, which
has been washed in water and deprived of its coarser particles, and so
supplied to the plasterer as whitening. Here is a large jar containing
this whitening and water, and I have here some strong sulphuric acid,
which is the acid you might have to use if you were to make these
experiments (only, in using this acid with limestone, the body that is
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