No one has ever seen or handled a single molecule, and molecular science therefore deals with things invisible and imperceptible by our senses. We can not magnify a drop of water sufficiently to see its structure; and the theory that matter is built up of molecules depends, like the philosophy of every science, on its competence to explain observed facts. These are of two kinds—namely, physical and chemical. A physical change in the condition of a body is illustrated by dissolving a lump of sugar in water. The sugar disappears, but remains present in the water, from which it may be recovered by evaporation. But if we burn the lump, we effect a chemical change in its condition. The sugar again disappears, and in its place we get two other substances—namely, carbon and water.
Similarly, water is converted by boiling into the invisible vapor, steam; but the change in its condition is physical only, for the steam condenses to water on being cooled. If, however, we pass water through a red-hot iron tube, it disappears, and is replaced by the two gases, oxygen and hydrogen. In the latter case, the liquid suffers a chemical change, or, as we say, is "decomposed" into its constituent elements. Those changes, therefore, which bodies undergo without alteration of substance are called physical; while those which are accompanied by alteration of substance are called chemical.
Turning our attention first to the physical side of the question, let us inquire how far some of the fundamental laws of science are illustrated by the molecular hypothesis. Among the most important of these is the law of Boyle, which declares that the pressure of gases is proportional to their density. The theory under review is based at present on the phenomena of gases, and considers these as aggregations of molecules in constant motion. Their movements are supposed to take place in straight lines, the molecules hurrying to and fro across the containing vessel, striking its sides, or coming into contact with their neighbors, and rebounding after every collision, like a swarm of bees in a hive flying hither and thither in all directions.
We know that air, or any gas, confined in a vessel, presses against its sides, and against the surface of any body placed within it. This pressure is due to the impact of the flying molecules; and the constant succession of their strokes is, according to this theory, the sole cause of what is called the pressure of air and other gases. As each molecule strikes the side of the vessel the same number of times, and with an impulse of the same magnitude, the pressure in a vessel of given size must be proportionate to the number of molecules—that is, to the quantity of gas in it; and this is a complete explanation of Boyle's law. Let us next suppose that the velocity of the molecules is increased. Then each molecule will strike the side of the containing vessel not only more times per second, but with greater force. Now, an increase in the velocity of the molecules corresponds in theory to a rise of temperature; and in this way we can explain the increase of
