that it is in this
elementary state that it exists in wood, coals, and a great variety of
other combustible bodies. --Indeed, Mrs. B., you are very ungenerous;
you are not satisfied with convincing me that my objections are
frivolous, but you oblige me to prove them so myself.

MRS. B.

You must confess, however, that I make ample amends for the detection of
error, when I enable you to discover the truth. You, understand, now,
I hope, that carbonic acid is equally produced by the decomposition of
chalk, or by the combustion of charcoal. These processes are certainly
of a very different nature; in the first case the acid is already
formed, and requires nothing more than heat to restore it to its gaseous
state; whilst, in the latter, the acid is actually made by the process
of combustion.

CAROLINE.

I understand it now perfectly. But I have just been thinking of another
difficulty, which, I hope, you will excuse my not being able to remove
myself. How does the immense quantity of calcareous earth, which is
spread all over the globe, obtain the carbonic acid with which it is
combined?

MRS. B.

The question is, indeed, not very easy to answer; but I conceive that
the general carbonisation of calcareous matter may have been the effect
of a general combustion, occasioned by some revolution of our globe, and
producing an immense supply of carbonic acid, with which the calcareous
matter became impregnated; or that this may have been effected by a
gradual absorption of carbonic acid from the atmosphere. --But this
would lead us to discussions which we cannot indulge in, without
deviating too much from our subject.

EMILY.

How does it happen that we do not perceive the pernicious effects of the
carbonic acid which is floating in the atmosphere?

MRS. B.

Because of the state of very great dilution in which it exists there.
But can you tell me, Emily, what are the sources which keep the
atmosphere constantly supplied with this acid?

EMILY.

I suppose the combustion of wood, coals, and other substances, that
contain carbon.

MRS. B.

And also the breath of animals.

CAROLINE.

The breath of animals! I thought you said that this gas was not at all
respirable, but on the contrary, extremely poisonous.

MRS. B.

So it is; but although animals cannot breathe in carbonic acid gas, yet,
in the process of respiration, they have the power of forming this gas
in their lungs; so that the air which we _expire_,