essels, by which it is conveyed into the left ventricle of the
heart, whence it is propelled to all the different parts of the body by
a large artery, which gradually ramifies into millions of small arteries
through the whole frame. From the extremities of these little
ramifications the blood is transmitted to the veins, which bring it back
to the heart and lungs, to go round again and again in the manner we
have just described. You see, therefore, that the blood actually
undergoes two circulations; the one, through the lungs, by which it is
converted into pure arterial blood; the other, or general circulation,
by which nourishment is conveyed to every part of the body; and these
are both equally indispensable to the support of animal life.

EMILY.

But whence proceeds the carbon with which the blood is impregnated when
it comes into the lungs?

MRS. B.

Carbon exists in a greater proportion in blood than in organised animal
matter. The blood, therefore, after supplying its various secretions,
becomes loaded with an excess of carbon, which is carried off by
respiration; and the formation of new chyle from the food affords a
constant supply of carbonaceous matter.

CAROLINE.

I wonder what quantity of carbon may be expelled from the blood by
respiration in the course of 24 hours?

MRS. B.

It appears by the experiments of Messrs. Allen and Pepys that about
40,000 cubic inches of carbonic acid gas are emitted from the lungs of a
healthy person, daily; which is equivalent to _eleven ounces_ of solid
carbon every 24 hours.

EMILY.

What an immense quantity! And pray how much of carbonic acid gas do we
expel from our lungs at each expiration?

MRS. B.

The quantity of air which we take into our lungs at each inspiration, is
about 40 cubic inches, which contain a little less than 10 cubic inches
of oxygen; and of those 10 inches, one-eighth is converted into carbonic
acid gas on passing once through the lungs*, a change which is
sufficient to prevent air which has only been breathed once from
suffering a taper to burn in it.

[Footnote *: The bulk of carbonic acid gas formed by respiration,
is exactly the same as that of the oxygen gas which disappears.]

CAROLINE.

Pray, how does the air come in contact with the blood in the lungs?

MRS. B.

I cannot answer this question without entering into an explanation of
the nature and structure of the lungs. You recollect that the venous
blood, on bein