Popular Science Monthly/Volume 19/September 1881/What Is a Molecule?

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MODERN science declares that every substance consists of an aggregation of extremely small particles, which are called molecules. Thus, if we conceive a drop of water magnified to the size of the earth, each molecule being magnified to the same extent, it would exhibit a structure about as coarse-grained as shot; and these particles represent real masses of matter, which, however, are incapable of further subdivision without decomposition. A lump of sugar, crushed to the finest powder, retains its qualities; dissolved in water, the mass is divided into its molecules, which are still particles of sugar, though they are far too small to be seen by the highest powers of the microscope. The physical subdivision of every body is limited by the dimensions of its molecules; but the chemist can carry the process further. He "decomposes," or breaks up, these molecules into "atoms"; but the parts thus obtained have no longer the qualities of the original substance. Hence the molecule may be considered as the smallest particle of a substance in which its qualities inhere; and every molecule, though physically indivisible, can be broken up chemically into atoms, which are themselves the molecules of other and elementary bodies.

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 pressure, and the proportions of such increase which result from heating a gas. Similarly, Charles's important law, that the volume of a given mass of gas under a constant pressure varies directly as its temperature, follows obviously from the hypothesis.

Priestley was the first to remark that gases diffuse through each other. This fact is familiarly illustrated by the passage of odorous gases through the atmosphere. If a bottle of ether is opened in a room, its vapor diffuses through the air, and its presence is soon recognized by the sense of smell. In this case, the ether-molecules may be figured as issuing from the bottle with great velocity; and, if their course were not interrupted by striking against the molecules of the air, the room would be instantaneously permeated by their odor. But the molecular particles of both air and ether are so inconceivably numerous, that they can not avoid striking one another frequently in their flight. Every time a collision occurs between two molecules, the paths of both are changed; and the course of each is so continually altered that it is a long time in making any great progress from the point at which it set out, notwithstanding its great velocity.

We must next inquire how these velocities are measured, and what is their amount. We have seen that the pressure exerted by a gas is due to what may be appropriately called the molecular bombardment of the walls of its containing vessel; and, knowing this pressure, we can calculate the velocity of the projectiles, if we can ascertain their weight, just as we can estimate the speed of a bullet when its weight and mechanical effect are known. Now, a cubic centimetre of hydrogen at a pressure of one atmosphere weighs about one thousandth part of a gramme; we have, therefore, to find at what rate this mass must move—whether altogether or in separate molecules makes no difference—to produce this pressure on the sides of a cubic centimetre. The result gives six thousand feet per second as the velocity of the molecule of hydrogen, while in other gases the speed is much less.

The question of molecular weights brings us face to face with the chemical aspect of the hypothesis; and we have now to examine the support which is given to it by chemical phenomena, and show how wonderfully these are correlated with the physical proofs. Bearing in mind the distinction between physical and chemical changes, we know that we can make a mixture of finely divided sulphur and iron, for example, in any proportion. But these bodies when heated combine chemically to form a new substance called sulphide of iron; and the two classes of products exhibit great differences, which are indicated by a most remarkable characteristic. Chemical combination, unlike mechanical mixture, always takes place in certain definite proportions. Thus, fifty-six grains of iron combine with exactly thirty-two grains of sulphur; and, if there is any excess of either substance, it remains uncombined. This principle is known as the law of definite combining proportions, and the atomic theory, which, in one shape or another, is as old as philosophy, 'was first applied to its explanation by the English chemist Dalton in 1807. He suggested that the ultimate particles of matter, or atoms between which union is assumed to take place, have a definite weight; in other words, that they are distinct masses of matter. In the combination of the two elements in question, therefore, an atom of iron unites with an atom of sulphur to form a molecule of sulphide of iron; and the union takes place in the proportion by weight of fifty-six to thirty-two, simply because these numbers represent the relative weights of the two sorts of atoms. Now, Dalton may be wrong, and there may be no such things as atoms; but every science postulates fundamental principles, of which the only proof that can be offered is a certain harmony with observed facts; and the chemist assumes the reality of atoms and molecules because they enable him to explain what would otherwise be a chaos of unrelated facts. The combining proportions of substances, then, indicate their relative molecular weights; and, bearing this in mind, we must turn again for a moment to the physical side of the question, to inquire whether, and in what way, the physicist can determine the weight of a molecule.

Water, alcohol, and ether expand when heated, like other forms of matter, but they do so very unequally. Their vapors, on the other hand, are expanded by heat at exactly the same rate under like conditions. The theory supposes that the molecules which are close together in the liquids become widely separated when these are converted into vapors; and the action of the particles on each other becomes less and less as they are driven farther apart by heat, until at last it is inappreciated. When the molecules of the vapors in question are thus freed from other influences, it is found that heat acts in an exactly similar manner upon each of them; and this is found to be true of all gaseous bodies. The obvious explanation in the case before us is, that there are the same number of particles within a given space in the vapors of all three liquids. This is the law of Avogadro, which is formulated as follows: "Equal volumes of all substances, when in the form of gas, contain the same number of molecules"; and we shall see how simply this conception is applied for the purpose of determining the molecular weights of all bodies which are capable of being vaporized. It will be understood that we are still dealing, as in the case of chemical combination, with relative weights only. We have no means of ascertaining the absolute weight of a molecule of any substance; but we can state with perfect accuracy what relation these weights bear to one another. For this purpose, the molecule of hydrogen, which is the lightest body known to science, has been selected as the unit. Calling the weight of a litre of hydrogen one, we find by the balance that a litre of oxygen weighs sixteen; and as, by Avogadro's law, both litres contain the same number of molecules, the molecule of oxygen is sixteen times heavier than that of hydrogen. The molecular weight of any substance, therefore, which can be brought into the gaseous condition, is found by simply determining experimentally the specific gravity of its vapor relatively to hydrogen.

In this way the physicist ascertains the molecular weights of all easily vaporizable bodies, and these are found to be in uniform and exact agreement with those which the chemist deduces from the law of combining proportions. The molecular hypothesis is thus brought to a crucial test; and two entirely independent lines of inquiry agree in giving it support of such a character as compels conviction. The law of gravitation and the undulatory theory of light do not command more cogent circumstantial evidence than this.

We have now briefly reviewed the fields from which the certain data of molecular science are gathered. We have weighed the molecules of gases, and measured their velocity with a high degree of precision. But there are other points, such as the relative size of the molecules of various substances, and the number of their collisions per second, about which something is known, though not accurately.

With regard to the absolute diameter of a molecule and their number in a given space, everything at present is only probable conjecture. Still, it may be interesting to state the views which are held on these questions by such investigators as Sir William Thomson and the late Professor Clerk-Maxwell; but we give these without attempting to indicate the character of the speculations on which their conclusions rest.

Summing up, then, both the known and unknown, we may say that the molecular weights and velocities of many substances are accurately known. It is also conjectured that collisions take place among the molecules of hydrogen at the rate of seventeen million-million million per second; and in oxygen they are less than half that number. The diameter of the hydrogen molecule may be such that two million of them in a row would measure a millimetre. Lastly, it is conjectured that a million-million-million-million hydrogen molecules would weigh about four grammes; while nineteen million-million-million would be contained in a cubic centimetre. Figures like these convey no meaning to the mind, and they are introduced here only to show the character and present state of the research.

A few concluding words must indicate the tremendous energy residing in the forces by which the molecules of matter are bound together. The molecules of water, for example, can not be separated from each other without changing the liquid into a gas, or, in other words, converting the water into steam; and this can only be accomplished by heat. The force required is enormous; but, since the determination, by Joule, of the mechanical equivalent of heat, we are able not only to measure this force, but also to express it in terms of our mechanical standard. It has been found that, in order to pull apart the molecules of one pound of water, it is necessary to exert a mechanical power which would raise eight tons to the height of one hundred feet. Such is the energy with which the molecules of bodies grasp each other; such is the strength of the solder which binds the universe together.—Chambers's Journal.