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.
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.