place. The blood-vessels that convey the pure blood from the heart,
divide into myriads of little branches that terminate in capillary
vessels like those lining the air-cells of the lungs. The blood meanders
through these minute capillaries, depositing the oxygen taken from the
lungs and the food of the stomach, and receiving in return the decayed
matter, which is chiefly carbonic acid.

This carbonic acid is formed by the union of oxygen with _carbon_ or
_charcoal_, which forms a large portion of the body. Watery vapor is
also formed in the capillaries by the union of oxygen with the hydrogen
contained in the food and drink that nourish the body.

During this process in the capillaries, the bright red blood of the
arteries changes to the purple blood of the veins, which is carried
back to the heart, to be sent to the lungs as before described. A
portion of the oxygen received in the lungs unites with the dissolved
food sent from the stomach into the blood, and no food can nourish the
body till it has received a proper supply of oxygen in the lungs. At
every breath a half-pint of blood receives its needed oxygen in the
lungs, and at the same time gives out an equal amount of carbonic acid
and water.

Now, this carbonic acid, if received into the lungs, undiluted by
sufficient air, is a fatal poison, causing certain death. When it is
mixed with only a small portion of air, it is a slow poison, which
imperceptibly undermines the constitution.

We now can understand how it is that all who live in houses where the
breathing of inmates has deprived the air of oxygen, and loaded it
with carbonic acid, may truly be said to be poisoned and starved;