to rise half an inch in the tube. Put the
membranous bag down into the pink, milky water, supporting the
tube by passing it through a square cardboard and clamping
it with a spring clothespin as shown in Figure 151. Every few
minutes look to see what is happening. Does any of the red
ink pass through the membrane? Does any of the cornstarch pass
through?

This is an example of diffusion through a membrane. The process is
called _osmosis_, and the pressure that forces the liquid up the tube
is called _osmotic pressure_. It is by this sort of diffusion that
chicks which are being incubated get air, and that growing plants get
food. It is in this way that the cells of our body secure food and
oxygen and get rid of their wastes. There are no little holes in our
blood vessels to let the air get into them from our lungs. The air
simply diffuses through the thin walls of the blood vessels. There
are no holes from the intestinal tract into the blood vessels. Yet the
dissolved food diffuses right through the intestinal wall and through
the walls of the blood vessels. And later on, when it reaches the body
cells that need nourishment, the dissolved food diffuses out through
the walls of the blood vessels again and through the cell walls into
the cells. Waste is taken out of the cells into the blood and passes
from the blood into the lungs and kidneys by this same process
of diffusion. So you can readily see why everything would die if
diffusion stopped.

[Illustration: FIG. 151. Pouring the syrup into the "osmosis tube."]

_APPLICATION 65._ Explain how the roots of a plant can take in
water and food when there are no holes from the outside of
the root to the inside; how bees can smell flowers for a
considerable distance.

INFERENCE EXERCISE

Explain the following:

401. A shell in the bottom of a teakettle gathers most of
the scale around it and so keeps the scale from caking at the
bottom of the kettle.

402. There is oxygen dissolved in water. When the water comes
in contact with the fine blood vessels in a fish's gills, some
of this oxygen passes through the walls of the blood vessels
into the blood. Explain how it does so.

403. Asphalt becomes soft in summer.

404. When the trolley comes off the wire the car soon stops.

405. You cannot see stars in the daytime on earth, yet you
could see them in the daytime on the airless