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.
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