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