circulate the blood through the lung-cells.
The heart has two sides, with a partition
between them that keeps the blood on the
right side separate from the blood on the left;
both sides being hollow, mind. The blood on
the right side of the heart comes there from
all over the body, by a couple of large veins,
dark, before it goes to the lungs. From the
right side of the heart, it goes on to the
lungs, dark still, through an artery. It comes
back to the left side of the heart from the
lungs, bright scarlet, through four veins.
Then it goes all over the rest of the body
from the left side of the heart, through an
artery that branches into smaller arteries,
all carrying bright scarlet blood. So the
arteries and veins of the lungs on one hand,
and of the rest of the body on the other, do
exactly opposite work, you understand."

"I hope so."

"Now," continued Harry, " it requires a
strong magnifying glass to see the lung-cells
plainly, they are so small. But you can fancy
them as big as you please. Picture any one
of them to yourself of the size of an orange,
say, for convenience in thinking about it;
that one cell, with whatever takes place in
it, will be a specimen of the rest. Then you
have to imagine an artery carrying blood of
one colour into it, and a vein taking away
blood of another colour from it, and the
blood changing its colour in the cell."

"Aye, but what makes the blood change
its colour?"

"Recollect, uncle, you have a little branch
from the windpipe opening into the cell which
lets in the air. Then the blood and the air are
brought together, and the blood alters in
colour. The reason, I suppose you would guess,
is that it is somehow altered by the air."

"No very unreasonable conjecture, I should
think," said Mr. Bagges.

"Well; if the air alters the blood, most
likely, we should think, it gives something to
the blood. So first let us see what is the
difference between the air we breathe in, and
the air we breathe out. You know that
neither we nor animals can keep breathing
the same air over and over again. You
don't want me to remind you of the Black
Hole of Calcutta, to convince you of that;
and I dare say you will believe what I tell
you, without waiting till I can catch a mouse
and shut it up in an air-tight jar, and show
you how soon the unlucky creature will get
uncomfortable, and begin to gasp, and that it
will by-and-by die. But if we were to try
this experiment—not having the fear of the
Society for the Prevention of Cruelty to
Animals, nor the fear of doing wrong, before
our eyes—we should find that the poor mouse,
before he died, had changed the air of his
prison considerably. But it would be just as
satisfactory, and much more humane, if you
or I were to breathe in and out of a silk bag
or a bladder till we could stand it no longer,
and then collect the air which we had been
breathing in and out. We should find that
a jar of such air would put out a candle. If
we shook some lime-water up with it, the
lime-water would turn milky. In short,
uncle, we should find that a great part of the
air was carbonic acid, and the rest mostly
nitrogen. The air we inhale is nitrogen
and oxygen; the air we exhale has lost
most of its oxygen, and consists of little more
than nitrogen and carbonic acid. Together
with this, we breathe out the vapour of water,
as I said before. Therefore in breathing,
we give off exactly what a candle does in
burning, only not so fast, after the rate.
The carbonic acid we breathe out, shows that
carbon is consumed within our bodies. The
watery vapour of the breath is a proof that
hydrogen is so too. We take in oxygen with
the air, and the oxygen unites with carbon,
and makes carbonic acid, and with hydrogen,
forms water."

"Then don't the hydrogen and carbon
combine with the oxygen—that is, burn —in the
lungs, and isn't the chest the fireplace, after
all? " asked Mr. Bagges.

' Not altogether, according to those who
are supposed to know better. They are of
opinion, that some of the oxygen unites with
the carbon and hydrogen of the blood in the
lungs; but that most of it is merely absorbed
by the blood, and dissolved in it in the first
instance."

"Oxygen absorbed by the blood? That
seems odd," remarked Mr. Bagges. " How
can that be ?"

"We only know the fact that there are some
things that will absorb gases—suck them in—
make them disappear. Charcoal will, for
instance. It is thought that the iron which
the blood contains gives it the curious
property of absorbing oxygen. Well; the oxygen
going into the blood makes it change from
dark to bright scarlet; and then this blood
containing oxygen is conveyed all over the
system by the arteries, and yields up the
oxygen to combine with hydrogen and carbon
as it goes along. The carbon and hydrogen
are part of the substance of the body. The
bright scarlet blood mixes oxygen with them,
which burns them, in fact; that is, makes
them into carbonic acid and water. Of course,
the body would soon be consumed if this were
all that the blood does. But while it mixes
oxygen with the old substance of the body, to
burn it up, it lays down fresh material to
replace the loss. So our bodies are continually
changing throughout, though they seem to
us always the same; but then, you know, a
river appears the same from year's end to
year's end, although the water in it is different
every day."

"Eh, then," said Mr. Bagges, " if the body
is always on the change in this way, we must
have had several bodies in the course of our
lives, by the time we are old."

"Yes, uncle; therefore, how foolish it is to
spend money upon funerals. What becomes