compose the carbon dioxide is
supplied by the sunlight (Chapter XII).

[Fig. 57]

Fig. 57--*Under surface* of a geranium leaf showing breathing pores, highly
magnified (O.H.).

*Summary.*--Oxygen, by uniting with materials at the cells, keeps up a
condition of chemical activity (oxidation) in the body. This supplies heat
and the other forms of bodily energy. Entering as a free element, oxygen
leaves the body as a part of the waste compounds which it helps to form.
The free oxygen is transported from the lungs to the cells by means of the
hemoglobin of the red corpuscles, while the combined oxygen in carbon
dioxide and other compounds from the cells is carried mainly by the
plasma. The limited supply of free oxygen in the body at any time makes
necessary its continuous introduction into the body.

*Exercises.*--1. Describe the properties of oxygen. How does it unite with
other elements? How does it support combustion?

2. State the purpose of oxygen in the body. What properties enable it to
fulfill this purpose?

3. What is the proof that oxygen does not remain permanently in the body?
How does the oxygen entering the body differ from the same oxygen as it
leaves the body?

4. What is the necessity for the _continuous_ introduction of oxygen into
the body, while food is introduced only at intervals?

5. How are the red corpuscles able to take up and give off oxygen? How is
the plasma able to take up and give off carbon dioxide?

6. If thirty cubic inches of air pass from the lungs at each expiration
and 4.5 per cent of this is carbon dioxide, calculate the number of cubic
feet of the gas expelled in twenty-four hours, estimating the number of
respirations at eighteen per minute.

7. What is the weight of this volume of carbon dioxide, if one cubic foot
weigh 1.79 ounces?

8. What portion of this weight is oxygen and what carbon, the ratio by
weight of carbon to oxygen in carbon dioxide being twelve to thirty-two?

9. What is the final disposition of carbon dioxide in the atmosphere?

PRACTICAL WORK

*To show the Difference between Free Oxygen and Oxygen in
Combination.*--Examine some crystals of potassium chlorate (KClO3). They
contain oxygen _in combination_ with potassium and chlorine. Place a few
of these in a small test tube and heat strongly in a gas or alcohol flame.
The crystals first melt, and the liquid which they form soon app