(_d_) Tip the jar containing the carbon dioxide over the mouth of a
tumbler, as in pouring water, though not far enough to spill the acid, and
then insert a burning splinter in the tumbler. Account for the result.
Inference as to the weight of carbon dioxide.

[Fig. 59]

Fig. 59--*Simple apparatus* for illustrating passage of oxygen through the
body.

(_e_) Review experiments (page 101) showing the presence of carbon dioxide
in the breath.

*To illustrate the General Movement of Oxygen through the Body.*--Into a
glass tube, six inches in length and open at both ends, place several
small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means
of a stopper, a smaller glass tube which is bent at right angles and which
is made to pass through a close-fitting stopper to the bottom of a small
bottle. Another small tube is fitted into a second hole in this stopper,
but terminating near the top of the bottle, and to this is connected a
rubber tube about eighteen inches in length. The arrangement is now such
that by sucking air from the top of the bottle, it is made to enter at the
distant end of the tube containing the charcoal. After filling the bottle
one third full of limewater, heat the tube containing the charcoal until
it begins to glow. Then suck the air through the apparatus (as in smoking,
without drawing it into the lungs), observing what happens both in the
tube and in the bottle. What are the proofs that the oxygen, in passing
through the tube, unites with the carbon, forms carbon dioxide, and
liberates energy? Compare the changes which the oxygen undergoes while
passing through the tube with the changes which it undergoes in passing
through the body.

CHAPTER IX - FOODS AND THE THEORY OF DIGESTION

The body is constantly in need of new material. Oxidation, as shown in the
preceding chapter, rapidly destroys substances at the cells, and these
have to be replaced. Upon this renewal depends the supply of energy.
Moreover, there is found to be an actual breaking down of the living
material, or protoplasm, in the body. While this does not destroy the
cells, as is sometimes erroneously stated, it reduces the quantity of the
protoplasm and makes necessary a process of repair, or rebuilding, of the
tissues. This also requires new material. Finally, substances, such as
water and common salt, are required for the aid which they re