mbler partly filled with water. Examine in the course of half
an hour. What evidence now exists that the water has passed through the
membrane?

2. Tie over the large end of a "thistle tube" (used by chemists) a thin
animal membrane, such as a piece of the pericardium or a strip of the
membrane from around a sausage. Then fill the bulb and the lower end of
the tube with a concentrated solution of some solid, such as sugar, salt,
or copper sulphate. Suspend in a vessel of water so that the liquid which
it contains is just on a level with the water in the vessel. Examine from
time to time, looking for evidence of a movement in each direction through
the membrane. Why should the movement of the water into the tube be
greater than the movement in the opposite direction? (If the thistle tube
has a very slender stem, it is better to fill the bulb before tying on the
membrane. The opening in the stem may be plugged during the process of
filling.)

[Fig. 32]

Fig. 32--An osmosometer.

NOTE.--With a special piece of apparatus, known as an _osmosometer_, the
principle of osmosis may be more easily illustrated than by the method in
either of the above experiments (Fig. 32). This apparatus may be obtained
from supply houses.

CHAPTER VII - RESPIRATION

Through the movements of the blood and the lymph, materials entering the
body are transported to the cells, and wastes formed at the cells are
carried to the organs which remove them from the body. We are now to
consider the passage of materials from outside the body to the cells and
_vice versa_. One substance which the body constantly needs is oxygen, and
one which it is constantly throwing off is carbon dioxide. Both of these
are constituents of

*The Atmosphere.*--The atmosphere, or air, completely surrounds the earth
as a kind of envelope, and comes in contact with everything upon its
surface. It is composed chiefly of oxygen and nitrogen,(29) but it also
contains a small per cent of other substances, such as water-vapor, carbon
dioxide, and argon. All of the regular constituents of the atmosphere are
gases, and these, as compared with liquids and solids, are very light.
Nevertheless the atmosphere has weight and, on this account, exerts
pressure upon everything on the earth. At the sea level, its pressure is
nearly fifteen pounds to the square inch. The atmosphere forms an
essential part of one's physi