s in air. We thus find that oxygen is
a constituent of the air, and by burning something in the air we can
remove the oxygen therefrom, leaving behind for our study the nitrogen,
which constitutes about four-fifths of the air, the oxygen accounting
for nearly all the rest.

The other great product of the burning of a candle is carbonic acid--a
gas formed by the union of the carbon of the candle and the oxygen of
the air. Whenever carbon burns, whether in a candle or in a living
creature, it produces carbonic acid.

_IV.--Combustion and Respiration_

Now I must take you to a very interesting part of our subject--to the
relation between the combustion of a candle and that living kind of
combustion which goes on within us. In every one of us there is a living
process of combustion going on very similar to that of a candle. For it
is not merely true in a poetical sense--the relation of the life of man
to a taper. A candle will burn some four, five, six, or seven hours.
What, then, must be the daily amount of carbon going up into the air in
the way of carbonic acid? What a quantity of carbon must go from each of
us in respiration! A man in twenty-four hours converts as much as seven
ounces of carbon into carbonic acid; a milch cow will convert seventy
ounces, and a horse seventy-nine ounces, solely by the act of
respiration. That is, the horse in twenty-four hours burns seventy-nine
ounces of charcoal, or carbon, in his organs of respiration to supply
his natural warmth in that time.

All the warm-blooded animals get their warmth in this way, by the
conversion of carbon; not in a free state, but in a state of
combination. And what an extraordinary notion this gives us of the
alterations going out in our atmosphere! As much as 5,000,000 pounds of
carbonic acid is formed by respiration in London alone in twenty-four
hours. And where does all this go? Up into the air. If the carbon had
been like lead or iron, which, in burning, produces a solid substance,
what would happen? Combustion would not go on. As charcoal burns, it
becomes a vapour and passes off into the atmosphere, which is the great
vehicle, the great carrier, for conveying it away to other places. Then,
what becomes of it?

Wonderful is it to find that the change produced by respiration, which
seems so injurious to us, for we cannot breathe air twice over, is the
very life and support of plants and vegetables that grow upon the
surface of the earth. It is th