end of the year either exactly the same or different one way or
the other by at most a few pounds. What, it may be asked, has become
of the enormous amount of oxygen thus introduced into the human
system in the course of one year? We can answer this question
satisfactorily. No part of the oxygen remains in the body, but is
given out again, combined with carbon and hydrogen. The carbon and
hydrogen of certain parts of the animal body combine with the oxygen
introduced through the lungs and skin, and pass off in the forms of
carbonic acid and vapour of water. At every expiration and every
moment of life, a certain amount of its elements are separated from
the animal organism, having entered into combination with the oxygen
of the atmosphere.

In order to obtain a basis for the approximate calculation, we may
assume, with Lavoisier and Seguin, that an adult man absorbs into
his system 32 1/2 ounces of oxygen daily,--that is, 46,037 cubic
inches = 15,661 grains, French weight; and further, that the weight
of the whole mass of his blood is 24 pounds, of which 80 per cent.
is water. Now, from the known composition of the blood, we know that
in order to convert its whole amount of carbon and hydrogen into
carbonic acid and water, 64.102 grains of oxygen are required. This
quantity will be taken into the system in four days and five hours.
Whether the oxygen enters into combination directly with the
elements of the blood, or with the carbon and hydrogen of other
parts of the body, it follows inevitably--the weight of the body
remaining unchanged and in a normal condition--that as much of these
elements as will suffice to supply 24 pounds of blood, must be taken
into the system in four days and five hours; and this necessary
amount is furnished by the food.

We have not, however, remained satisfied with mere approximation: we
have determined accurately, in certain cases, the quantity of carbon
taken daily in the food, and of that which passes out of the body in
the faeces and urine combined--that is, uncombined with oxygen; and
from these investigations it appears that an adult man taking
moderate exercise consumes 13.9 ounces of carbon, which pass off
through the skin and lungs as carbonic acid gas. [1]

It requires 37 ounces of oxygen to convert 13 9/10 of carbon into
carbonic acid. Again; according to the analysis of Boussingault,
(Annales de Chim. et de Phys., lxx. i. p.136), a horse consumes 79
1/10 ounces of carbon in