We cannot see molecules or molecular pores, but the phenomena of
compression and expansion, solubility and other equally convincing
facts, have led us to conclude that all substances are composed of
very minute particles or molecules separated by spaces called pores.

95. Journeys Made by Molecules. If a gas jet is turned on and not
lighted, an odor of gas soon becomes perceptible, not only throughout
the room, but in adjacent halls and even in distant rooms. An uncorked
bottle of cologne scents an entire room, the odor of a rose or violet
permeates the atmosphere near and far. These simple everyday
occurrences seem to show that the molecules of a gas must be in a
state of continual and rapid motion. In the case of the cologne, some
molecules must have escaped from the liquid by the process of
evaporation and traveled through the air to the nose. We know that the
molecules of a liquid are in motion and are continually passing into
the air because in time the vessel becomes empty. The only way in
which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really
saying that the molecules are in motion.

From these phenomena and others it is reasonably clear that substances
are composed of molecules, and that molecules are not inert, quiet
particles, but that they are in incessant motion, moving rapidly
hither and thither, sometimes traveling far, sometimes near. Even the
log of wood which lies heavy and motionless on our woodpile is made
up of countless billions of molecules each in rapid incessant motion.
The molecules of solid bodies cannot escape so readily as those of
liquids and gases, and do not travel far. The log lies year after year
in an apparently motionless condition, but if one's eyes were keen