THE DANCE OF THE MOLECULES

Otago Witness, Issue 3831, 16 August 1927, Page 77

THE DANCE OF THE MOLECULES

By Professor J. Arthur Thompson, in John o’ London’s Weekly. Everyone knows that dampness in the air means the presence of much watervapour, such as we see forming visible particles when we breathe heavily on a frosty morning. When this water vapour is blown against the cold rocks on the mountain side it becomes liquid water, and trickles down to form one of the innumerable runlets that feed the streams. But if great cold sets in the liquid water becomes a solid icicle. Thus we know wate; as solid, as liquid, and as gas, for the distinction between vapour and gas is not important for the moment. What is tme of water is true of many other substances, that they occur in three states —gaseous, liquid, and solid. Even the air may be made into a liquid, and carbonic acid gas into a solid like snow. When a strong beam of light enters a darkened room we see a great crowd of motes or dust-particles dancing as if they were alive. In the strict sense we do not really see the particles, for they are too small to be seen ; we see what might be called minute haloes round the invisible particles. Some of the rays of the sunbeam rebound from the surface of the particle, and form a little disc of light. It is interesting to watch the dancing motes, partly because we had no suspicion that there was so much dust in the air of the room, partly because we are made to think of the draughts and differences of temperatures which make the particles move, and partly because the motes give us a rough picture of the incessant movements among the molecules of any and every gas. The molecules of a gas are in rapid perpetual motion in straight lines, and one does not get very far before it collides with another. On an average a molecule travels about 20 times its own length between two collisions, but that means only a millionth of a centimetre. On the other hand, the molecules of a gas are travelling with great velocity, at the rate of about a quarter of a mile in a second; that is to say, rather faster than s-und. In this distance (Quarter of a mile) and in this time (a second) a molecule suffers five thousand million collisions. It might be thought that this would slow them down, but it is part of the theory of gases • —a theory that works well—that the molecules rebound with no loss of speed and with no loss of heat or any other form of energy. A toy balloon is filled with gas and its wall stands out tense and firm. We say that this is due to the pressure of the gas, which just means the continual im pact of the flying molecules. They are continually and steadily bombarding the ■walls of the balloon, and if the temperature is raised, the molecules travel more rapidly, and there are more impacts ner second. Getting into a very warm room, the toy balloon may suddenly burst, which means that the bombardment of molecules is too much for the walls to stand. If one fumbles in lighting a gas-jet, and allows some of the coal-gas to escape into th« room, the molecules spread themselves out as far as they can go. Soon they will be found in every corner of the room, but so few and far between that we cannot smell the gas. In this case the gas is in a very diffuse state, differing from the gas in a dense state in hav ing far fewer mutual collisions between its molecules; but the molecules of the coal-gas in the room will be colliding, of course, with the molecules of the mixture of gases that forms the air. But now let us think of the molecucs not rushing apart, but coming much closer to one another When the volume of a strong vessel containing gas is reduced, by forcing in the lid, or in other ways, the flying molecules will be crowded, and there will be more impacts on the walls. As we say in three words, the pressure increases. If we can continue the compression, the molecules become so crowded that they are attracted to one another, as a stone to the earth, and they tend to cluster in little groups. Thev form a close-packed film on the walls of the vessel. In short, the gas condenses into a liquid. In practice, this is made easier by cooling the vessel to a very low temperature, for this slows down the movements of the molecules.

The molecules of a liquid are still rushing to and fro, but they are much more crowded and have much less freedom of movement. They are so near one another that it is difficult to get them nearer without great pressure or great cold, or both combined. Yet everyone knows how those on the surface of water tend to break free or evaporate into the air, while it is also true that those on the surface may hold so tightly together that they form a “ skin ” on which a dry needle may be floated. When molecules are forced to come together still more closely than in the liquid state, the result, is a solid. They may form a higgledy-piggledy crowd, so dense that individual shifting about is hardly possible. This is the case in glass, and no one can bend a window pane to any appreciable extent. But the crowded molecules may assume an orderly arrangement, like a regiment, not like a mob ; and this spells crystal. Just as some liquids are much more fluid than others, which means that the molecules have different degrees of free play, much more in water than in syrup, so there are varieties of solidity. The lump of cobbler’s wax is much more plastic than the lump of lead. In no case, however, should we think of the molecules of a solid as quite motionless. For, while they cannot travel or collide, they are in a state of vibration, within a narrow range, and each kind of solid has its own favourite frequency of vibration.

What we have said is only a beginning, for we must also think of the colloidal state of matter, of ordinary solutions, and of matter in the form of delicate films. Enough for the time, however; and it is great gain if we are intellectually humble enough to think of gaseous, liquid, and solid states of matter as three degrees in the freedom of movement among the constituent molecules. In a gas there is a wild dance with incessant collisions; in a liquid there is a surging to and fro, but much less freedom; in a solid there is hardly more than the almost imperceptible individual swaying of the men in a regiment.