air could
not get through the walls of your lungs into the blood. Plants would
begin to wither and droop, although they would not die quite as
quickly as animals and fishes and people. But no sap could enter their
roots and none could pass from cell to cell. The plants would be as
little able to breathe through their leaves as we through our lungs.

If gas escaped in the room where you were, you could not smell it even
if you stayed alive long enough to try; the gas would rise to the top
of the room and stay there. All gases and all liquids would stay as
they were, and neither would ever form mixtures.

It would not make so much difference in the dead parts of the world
if diffusion ceased; the rocks, mountains, earth, and sea would not be
changed at all at first. To be sure, the rivers where they flowed into
the oceans would make big spaces of saltless water; and when water
evaporated from the ocean the vapor would push aside the air and stay
in a layer over the ocean, instead of mixing with the air and rising
to great heights. But the real disaster would be to living things. All
of them would be smothered and starved to death as soon as diffusion
ceased.

Here is an experiment that shows how gases diffuse:

EXPERIMENT 85. Take two test tubes with mouths of the same
size so that you can fit them snugly against each other when
you want to. Fill one to the brim with water and hold your
thumb or a piece of cardboard over its mouth while you place
it upside down in a pan of water. Take the free end of a
rubber tube that is attached to a gas pipe and put it into the
test tube a short distance, so that the gas will go up into
the tube, as shown in Figure 149. Now turn on the gas gently.
When all the water has been forced out of the tube and the gas
bubbles begin to come up on the outside, turn off the gas.
Put a piece of cardboard, about an inch or so square, over the
mouth of the tube so that no air can get into it, and take the
tube out of the water, _keeping the mouth down and covered_.
Bring the empty test tube, which of course is full of air,
mouth up under the test tube full of gas, making the mouths of
the two tubes meet with the cardboard between them, as shown
in Figure 150. Now have some one pull the cardboard gently
from between the two test tubes, so that the mouths of
the tubes will be pressed against each other and so that
pra