nary gas flame, it does not melt, but if placed in a
flame of burning hydrogen, it melts very quickly.

74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour
sulphuric acid, or hydrochloric acid, on zinc shavings and to collect
in a bottle the gas which is given off. This gas proves to be
colorless, tasteless, and odorless. (See Laboratory Manual.)

CHAPTER VII

AIR

75. The Instability of the Air. We are usually not conscious of the
air around us, but sometimes we realize that the air is heavy, while
at other times we feel the bracing effect of the atmosphere. We live
in an ocean of air as truly as fish inhabit an ocean of water. If you
have ever been at the seashore you know that the ocean is never still
for a second; sometimes the waves surge back and forth in angry fury,
at other times the waves glide gently in to the shore and the surface
is as smooth as glass; but we know that there is perpetual motion of
the water even when the ocean is in its gentlest moods. Generally our
atmosphere is quiet, and we are utterly unconscious of it; at other
times we are painfully aware of it, because of its furious winds. Then
again we are oppressed by it because of the vast quantity of vapor
which it holds in the form of fog, or mist. The atmosphere around us
is as restless and varying as is the water of the sea. The air at the
top of a high tower is very different from the air at the base of the
tower. Not only does the atmosphere vary greatly at different
altitudes, but it varies at the same place from time to time, at one
period being heavy and raw, at another being fresh and invigorating.

Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.

76. The Character of the Air. The atmosphere which envelops us at
all times extends more than fifty miles above us, its height being far
greater than the greatest depths of the sea. This atmosphere varies
from place to place; at the sea level it is heavy, on the mountain top
less heavy, and far above the earth it is so light that it does not
contain enough oxygen to permit man to live. Figure 40 illustrates by
a pile of pillows how the pressure of the air varies from level to
level.

[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]

Sea level is a low portion of the earth's surface, hence at