or reject from the
lungs, always contains a certain proportion of carbonic acid, which is
much greater than that which is commonly found in the atmosphere.

CAROLINE.

But what is it that renders carbonic acid such a deadly poison?

MRS. B.

The manner in which this gas destroys life, seems to be merely by
preventing the access of respirable air; for carbonic acid gas, unless
very much diluted with common air, does not penetrate into the lungs, as
the windpipe actually contracts and refuses it admittance. --But we must
dismiss this subject at present, as we shall have an opportunity of
treating of respiration much more fully, when we come to the chemical
functions of animals.

EMILY.

Is carbonic acid as destructive to the life of vegetables as it is to
that of animals?

MRS. B.

If a vegetable be completely immersed in it, I believe it generally
proves fatal to it; but mixed in certain proportions with atmospherical
air, it is, on the contrary, very favourable to vegetation.

You remember, I suppose, our mentioning the mineral waters, both natural
and artificial, which contain carbonic acid gas?

CAROLINE.

You mean the Seltzer water?

MRS. B.

That is one of those which are the most used; there are, however,
a variety of others into which carbonic acid enters as an ingredient:
all these waters are usually distinguished by the name of _acidulous_ or
_gaseous mineral waters_.

The class of salts called _carbonats_ is the most numerous in nature; we
must pass over them in a very cursory manner, as the subject is far too
extensive for us to enter on it in detail. The state of carbonat is the
natural state of a vast number of minerals, and particularly of the
alkalies and alkaline earths, as they have so great an attraction for
the carbonic acid, that they are almost always found combined with it;
and you may recollect that it is only by separating them from this acid,
that they acquire that causticity and those striking qualities which I
have formerly described. All marbles, chalks, shells, calcareous spars,
and lime-stones of every description, are neutral salts, in which
_lime_, their common basis, has lost all its characteristic properties.

EMILY.

But if all these various substances are formed by the union of lime with
carbonic acid, whence arises their diversity of form and appearance?

MRS. B.

Both from the different proportions of their component parts, and from a
variety of foreign ing