different in its kind. It is the combustion of the carbon by the contact
of oxygen which the body has supplied to it. If I set this into action at
once, you will see combustion produced. Just what occurs in my
lungs--taking in oxygen from another source, namely, the atmosphere--takes
place here by a more rapid process.

You will be astonished when I tell you what this curious play of carbon
amounts to. 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! What a wonderful change of carbon must take place under these
circumstances of combustion or 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 on in our atmosphere. As much as
5,000,000 pounds, or 548 tons, 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 the lead which I shewed you, or the
iron which, in burning, produces a solid substance, what would happen?
Combustion could 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