nt a factor
in daily life that an entire chapter will be devoted to it later.

Another constituent of the air with which we are familiar is carbon
dioxide. In pure air, carbon dioxide is present in very small
proportion, being continually taken from the air by plants in the
manufacture of their food.

Various other substances are present in the air in very minute
proportions, but of all the substances in the air, oxygen, nitrogen,
and carbon dioxide are the most important.

CHAPTER VIII

GENERAL PROPERTIES OF GASES

85. Bicycle Tires. We know very well that we cannot put more than a
certain amount of water in a tube, but we know equally well that the
amount of air which can be pumped into a bicycle or automobile tire
depends largely upon our muscular energy. A gallon of water remains a
gallon of water and requires a perfectly definite amount of space, but
air can be compressed and compressed, and made to occupy less and less
space. While it is true that air is easily compressed, it is also true
that air is elastic and capable of very rapid and easy expansion. If a
puncture occurs in a tire, the compressed air escapes very quickly;
that is, the compressed air within the tube has taken the first
opportunity offered for expansion.

[Illustration: FIG. 51.--By squeezing the bulb, air is forced out of
the nozzle.]

The fact that air is elastic has added materially to the comfort of
the world. Transportation by bicycles and automobiles has been greatly
facilitated by the use of air tires. In many hospitals, air mattresses
are used in place of hair, feather, or cotton mattresses, and in this
way the bed is kept fresher and cleaner, and can be moved with less
danger of discomfort to the patient. Every time we squeeze the bulb of
an atomizer, we force compressed or condensed air through the
atomizer, and the condensed air pushes the liquid out of the nozzle
(Fig. 51). Thus we see that in the necessities and conveniences of
life compressed air plays an important part.

86. The Danger of Compression. Air under ordinary atmospheric
conditions exerts a pressure of 15 pounds to the square inch. If, now,
large quantities of air are compressed into a small space, the
pressure exerted becomes correspondingly greater. If too much air is
blown into a toy balloon, the balloon bursts because it cannot support
the great pressure exerted by the compressed air within. What is true
of air is true of all gases. Dangerous boile