Matter consists of molecules—Nature of molecules—Laws of their action in gases—Law of Avogadro—Molecules composed of atoms—Proved by composition of water—Combinations of atoms—Elementary substances—Qualities of matter depend on atoms—Dimensions and velocities of molecules and atoms—These are ascertained facts, not theories. If in building a house that is to stand when the rains fall and the winds blow, it is requisite to go down to the solid rock for a foundation, so much the more is it necessary in building up a theory to begin at the beginning and give it a solid groundwork. Nine-tenths of the fallacies current in the world arise from the haste with which people rush to conclusions on insufficient premises. Take, for instance, any of the political questions of the day, such as the Irish question: how many of those who express confident opinions, and get angry and excited on one side or the other, could answer any of the preliminary questions which are the indispensable conditions of any rational judgment? How many marks would they get for an examination paper which asked what was the population of Ireland; what proportion of that population was agricultural; what proportion of that agricultural population consisted of holders of small tenements; what was the scale of rents compared with that for small holdings in other countries;[10] how much of that rent was levied on them for their own improvements; and other similar questions which lie at the root of the matter? In how many cases would it be found that the whole superstructure of their confident and passionate theories about the Irish difficulty was based on no more solid foundation than their like or dislike of a particular statesman or of a particular party?
I propose therefore to begin at the beginning, and, taking the simplest case, that of dead or inorganic matter, show how the material universe is built up by the operation of the all-pervading law of polarity. What does matter consist of? Of molecules, and molecules are made up of atoms, and these are held together or parted, and built up into the various forms of the material universe, primarily by polar forces.
Let me endeavour to make this intelligible to the intelligent but unscientific reader. Suppose the Pyramid of Cheops shown for the first time to a giant whose eye was on such a scale that he could just discern it as a separate object. He might make all sorts of ingenious conjectures as to its nature, but if microscopes had been invented in Giant-land and he looked through one, he would find that it was built up, layer by layer, on a regular plan and in determinate lines and angles, by molecules, or what seemed to him almost infinitely small masses, of squared stone. For pyramid write crystal, and we may see by the human sense, aided by human instruments and human reason, a similar structure built up in the same way by minute particles. Or again, divide and subdivide our iron filings until we reach the limit of possible mechanical division discernible by the microscope; each one remains essentially a[11] bar of iron, as capable of being magnetised, and showing the same qualities and behaviour under chemical tests as the original bar of iron from which the filings were taken. This carries us a long way down towards the infinitely small, for mechanical division and microscopic visibility can be carried down to magnitudes which are of the order of 1/100000th of an inch.
But this is only the first step; to understand our molecules we must ascertain whether they are infinitely divisible, and whether they are continuous, expanding by being spread out thinner and thinner like gold-beater’s skin: or are they separate bodies with intervals between them, like little planets forming one solar system and revolving in space by fixed laws. Ancient science guessed at the former solution and embodied it in the maxim ‘that nature abhors a vacuum’: modern science proves the latter.
In the first place bodies combine only in fixed proportions, which is a necessary consequence if they consist of definite indivisible particles, but inconceivable if the substance of each is indefinitely divisible. Thus water is formed in one way and one only: by uniting one volume or molecule of oxygen with two of hydrogen, and any excess of one or the other is left out and remains uncombined. But if the molecules could be divided into halves, quarters, and so on indefinitely, there can be no reason why their union should take place always in this one proportion and this only.
A still more conclusive proof is furnished by the behaviour of substances which exist in the form of gases. If a jar is filled with one gas, a second and third gas can be poured into it as readily as into a vacuum, the result being that the pressure on the sides[12] of the jar is exactly equal to the sum of the separate pressures of each separate gas. This evidently means that the first gas does not occupy the whole space, but that its particles are like a battalion of soldiers in loose skirmishing order, with such intervals between each unit that a second and third battalion can be marched in and placed on the same ground, without disturbing the formation, and with the result only of increasing the intensity of the fire.
Now gas is matter as much as solids or liquids, and in the familiar instance of water we see that it is merely a question of more or less heat whether the same matter exists as ice, water, or steam. The number and nature of the molecules is not changed, only in the one case they are close to one another and solidly linked together; in the other, further removed and free to move about one another, though still held together as a mass by their mutual attractions; and in the third, still further apart, so that their mutual attraction is lost and they dart about, each with its own proper motion, bombarding the surface which contains them, and by the resultant of their impacts producing pressure.
In this latter and simpler form of gas the following laws are found to prevail universally for all substances. Under like conditions volumes vary directly as the temperature and inversely as the pressure. That is to say, the pressure which contains them remaining the same, equal volumes of air, steam, or any other substance in the state of gas, expand into twice the volume if the temperature is doubled, three times if it is tripled, and so on; contracting in the same way if the temperature is lowered. If on the other hand the temperature remains constant, the volume is reduced to one[13] half or one third, if the pressure is doubled or tripled. From these laws the further grand generalisation has been arrived at, that all substances existing in the form of gas contain the same number of molecules in the same volume.
This, which is known as the Law of Avogadro, from the Italian chemist by whom it was first discovered, is the fundamental law of modern chemistry, and the key to all certain and scientific knowledge of the constitution of matter and of the domain of the infinitely small, just as much as the law of gravity is to action of matter in the mass, and the resulting conditions and motions of mechanics and astronomy.
This conclusion obviously follows from it, that difference of weight in different substances arises not from one having more molecules in the same volume than another, but from the molecules themselves being heavier. If we weigh a gallon or litre of hydrogen gas, which is the lightest known substance, and then weighing an equal volume of oxygen gas find that it is sixteen times heavier, we know for certain that the molecule or ultimate particle of oxygen is sixteen times heavier than that of hydrogen.
It is evident that in this way the molecules of all simple substances which can exist in the form of pure gas can be weighed, and their weight expressed in terms of the unit which is generally adopted, that of the molecule of the lightest known substance, hydrogen. But science, not content with this achievement, wants to know not the relative weight only, but the absolute dimensions, qualities, and motions of these little bodies; and whether, although they cannot be divided further by mechanical means, and while retaining the qualities[14] of the substances they build up, they are really ultimate and indivisible particles or themselves composites.
Chemistry and electricity give a ready answer to this latter question. Molecules are composites of still smaller bodies, and to get back to the ultimate particle we must go to atoms. All chemical changes resolve themselves into the breaking up of molecules and rearrangement of their constituent atoms. If the opposite poles of a voltaic battery are inserted in a vessel containing water, molecules of water are broken up, bubbles of gas rise at each pole, and if these are collected, the gas at the positive pole is found to be oxygen, and that at the negative pole hydrogen. Nothing has been added or taken away, for the weight of the two gases evolved exactly equals that of the water which has disappeared. But the molecules of the water have been broken up, and their constituents reappear in totally different forms, for nothing can well be more unlike water than each of the two gases of which it is composed. That it is composed of them can be verified by the reverse experiment of mixing the two gases together in the same proportion of two volumes of hydrogen to one of oxygen as was produced by the decomposition of water, passing an electric spark through the vessel containing the mixture, when with a loud explosion the gases reunite, and water is formed in precisely the same quantity as produced the volumes of gas by its decomposition. Can the ultimate particles of these gases be further subdivided; can they, like those of water, be broken up and reappear in new forms? No; there is no known process by which an atom of oxygen can be made anything but oxygen, or an atom of hydrogen anything but hydrogen.
[15]
The only thing which is compound in the composition of oxygen is that its molecules consist of two atoms linked together. This appears from the fact that while the weight of oxygen, and therefore that of its molecules, is sixteen times greater than that of an equal volume of hydrogen, and therefore of hydrogen molecules, it combines with it in the proportion not of sixteen, but of eight to one. If, therefore, the molecule were identical with the atom of oxygen, we must admit that the atom could be halved, which is contrary to its definition as the ultimate indivisible particle of the substance oxygen. But if the oxygen molecule consists of two linked atoms, O—O, and the hydrogen molecule equally of two, H—H, as can be proved by other considerations, everything is explained by assuming that the molecule of water consists of two atoms of hydrogen linked to one of oxygen, or H?O, and that when this molecule is broken up by electricity, its constituents resolve themselves into atoms, which recombine so as to form twice as many molecules of hydrogen, H—H, as of oxygen, O,—i.e. into two volumes of hydrogen gas to one of oxygen.
Taking the single hydrogen atom as the unit of weight as being the lightest known ponderable body, and calling this weight a microcrith, or standard of the smallest of this order of excessively small weights, this is equivalent to saying that the weight of an oxygen atom is equal to 16 microcriths, and as water is composed of one such atom plus two of hydrogen, the weight of its molecule ought to be 16 + 2 = 18, which is in fact the exact ratio in which the weight of a volume of steam, or water in the form of gas, is heavier than an equal volume of hydrogen.
[16]
This key unlocks the whole secret of the chemical changes and combinations by which matter assumes all the various forms known to us in the universe.
Thus oxygen enters into a great variety of combinations forming different substances, but always in the proportion which is either 16, or some multiple of 16, such as 32, 48, 64. That is, either 1, 2, 3, or 4 atoms of oxygen unite with other atoms to form the molecules from which these other substances are made.
One atom of oxygen weighing 16 microcriths combines, as we have seen, with two atoms of hydrogen weighing 2, to form a molecule of water weighing 18 mc. In like manner one atom of oxygen, 16 mc., combines with one of carbon, which weighs 12 mc., to form a molecule of carbonic oxide weighing 28 mc.; and two of oxygen, 32 mc., with one of carbon, 12 mc., to form a molecule of carbonic dioxide weighing 44 mc.
The same applies to all elementary substances. Thus hydrogen, two atoms of which combine with one of oxygen to form water, combines one atom to one with chlorine to form the molecule of hydrochloric acid, which weighs 36·5 mc., being the united weights of one atom of chlorine, 35·5 mc., and one of hydrogen, 1 mc. These, with hundreds of similar instances, are the results not of theories as to molecules and atoms, but of actual facts, ascertained by innumerable experiments made independently by careful observers over long periods of years, many of them dating back to the labours of the alchemists of the middle ages in pursuit of gold. The atomic theory is the child and not the parent of the facts, and is indeed nothing but the summary of the vast variety of experiments which led up to it, as Newton’s law of gravity is of the facts[17] known to us with regard to the attractions and motions of matter in the mass. But as Newton’s law enables us to predict new facts, to calculate eclipses and the return of comets beforehand, and to compile nautical almanacs; so the new chemistry, based on the atomic theory, affords the same conclusive proof of its truth by enabling us in many cases to predict phenomena which are subsequently verified by experiment, and to infer beforehand what combinations are possible, and what will be their nature.
The actual existence, therefore, of molecules and atoms is as well-ascertained a fact, as that of cwts. and lbs., or of planets and stars, of solar systems and nebul?.
The researches of chemists have succeeded in discovering about 70 substances, of which the same may be said as of the oxygen and hydrogen into which water is decomposed, viz. that they cannot be decomposed by any known process, and must therefore be considered as ultimate and elementary. Their atoms differ widely in size and weight: that of mercury, for instance, being 200 times heavier than that of hydrogen, and the weights varying from 1 mc. for the hydrogen atom, up to 240 for that of uranium. When we call them elementary substances, we merely mean that we know no means of decomposing them. It is possible that all of them may be compounds which we cannot take to pieces of some substratum of uniform matter, and it is remarkable that the weight of nearly all of these elementary atoms is some simple multiple of that of hydrogen, pointing to their being all combinations of one common substratum of matter; but this is merely conjecture, and in the present state of our knowledge we must assume these 66 or 71 ultimate particles[18] or atoms to be the indivisible units out of which all the complicated puzzle of the material universe is put together. They are not all equally important to us. Of the 71 elementary substances enumerated in chemical treatises, 5 are doubtful, and 30 to 35 of the remainder are either known only to chemists in minute quantities, or exist in nature in small quantities, having no very material bearing upon man’s relation to matter. The most important are oxygen, hydrogen, nitrogen, and carbon. Oxygen diluted by nitrogen gives us the air we breathe, combined with hydrogen the water we drink, and with metals and other primitive bases the solid earth on which we tread. Carbon again is the great basis of organised matter and life, to which it leads up by a variety of complex combinations with oxygen, hydrogen, and nitrogen.
The qualities and relations of elementary atoms afford a subject of great interest, but of such vast extent that those who wish to understand it must be referred to professed works on modern chemistry. For the present purpose it is sufficient to say that the following conclusions are firmly established.
All the various forms of matter are composed of combinations of primitive atoms which form molecules, the molecules being neither more nor less than very small pieces of ordinary matter.
The qualities of this matter, or, what is the same thing, of its molecules, depend partly on the qualities of the atoms, which are something quite distinct from those of the molecules, and partly on their mode of aggregation into molecules, affecting the form, size, stability, and other attributes of the molecule.
All matter, down to the smallest atom, has definite[19] weight and is indestructible. No man by taking thought can add the millionth of a milligramme to the weight of any substance, or make it either more or less than the sum of the weights of its component factors, any more than he can add a cubit to his stature. When Shelley sang of the cloud,
I change, but I cannot die,
he enunciated a scientific axiom of the first importance. Creation, in the sense of making something out of nothing, is a thing absolutely unknown and unknowable to us. If we say we make a ship or a steam-engine, we simply mean that we transform existing matter and existing energies into new combinations, which give results convenient for our purpose. So if we talk of making a world, our idea really is that if our powers and knowledge were indefinitely increased we might be able, given the atoms and energies with their laws of existence, to put them together so as to produce the desired results. But how the atoms and their inherent laws got there is a question as to which knowledge, or even conceivability, is impossible, for it altogether transcends human experience.
Before finally taking leave of atoms it may be well to state shortly that science, not content with having proved their existence and weighed them in terms of the lightest element, the hydrogen atom, has attempted, not without success, to solve the more difficult problem of their real dimensions, intervals, and velocities. This problem has been attacked by Clausius, Sir W. Thomson, Clerk Maxwell, and others, from various sides: from a comparison with the wave-lengths of light; with the tenuity of the thinnest films of soap-bubbles just before[20] they burst, and when they are presumably reduced to a single layer of molecules; and from the kinetic theory of gases, involving the dimensions, paths, and velocities of elastic bodies, constantly colliding, and by their impacts producing the resulting pressure on the confining surface. All these methods involve such refined mathematical calculations that it is impossible to explain them popularly, but they all lead to nearly identical results, which involve figures so marvellous as to be almost incomprehensible. For instance, a cubic centimetre of air is calculated to contain 21 trillions of molecules—i.e. 21 times the cube of a million, or 21 followed by 18 ciphers; the average distance between each molecule equals 95 millionths of a millimetre, which is about 25 times smaller than the smallest magnitude visible under a microscope; the average velocity of each molecule is 447 metres per second; and the average number of impacts received by each molecule in a second is 4,700 millions.