tendency to expand, but that they can be compressed by the application
of force. This observation has led scientists to suppose that
substances are composed of very minute particles called molecules,
separated by small spaces called pores; and that when a gas is
condensed, the pores become smaller, and that when a gas expands, the
pores become larger.

The fact that certain substances are soluble, like sugar in water,
shows that the molecules of sugar find a lodging place in the spaces
or pores between the molecules of water, in much the same way that
pebbles find lodgment in the chinks of the coal in a coal scuttle. An
indefinite quantity of sugar cannot be dissolved in a given quantity
of liquid, because after a certain amount of sugar has been dissolved
all the pores become filled, and there is no available molecular
space. The remainder of the sugar settles at the bottom of the vessel,
and cannot be dissolved by any amount of stirring.

If a piece of potassium permanganate about the size of a grain of sand
is put into a quart of water, the solid disappears and the water
becomes a deep rich red. The solid evidently has dissolved and has
broken up into minute particles which are too small to be seen, but
which have scattered themselves and lodged in the pores of the water,
thus giving the water its rich color.

There is no visible proof of the existence of molecules and molecular
spaces, because not only are our eyes unable to see them directly, but
even the most powerful microscope cannot make them visible to us. They
are so small that if one thousand of them were laid side by side, they
would make a speck too small to be seen by the eye and too small to be
visible under the most powerful microscope.