Chapter I: The Atoms of Which Things Are Made

NEARLY two thousand years ago, Lucretius, the famous Latin poet, wrote his treatise, _De rerum natura_--concerning the nature of things. He maintained the view that air and earth and water and everything else were composed of innumerable small bodies or corpuscles, individually too small to be seen, and all in rapid motion. He tried to show that these suppositions were enough to explain the properties of material things. He was not himself the originator of all the ideas which he set forth in his poem; he was the writer who would explain the views which were held by a certain school, and which he himself believed to be true. There was a rival set of views, according to which, however closely things were looked into, there would be no evidence of structure: however the water in a bowl, let us say, was subdivided into drops and then again into smaller drops and so on and on, the minutest portion would still be like the original bowl of water in all its properties. On the view of Lucretius, if subdivision were carried out sufficiently, one would come at last to the individual corpuscles or _atoms_, the word atom being taken in its original sense, something which _cannot be cut_.

There is a mighty difference between the two views. On the one, there is nothing to be gained by looking into the structure of substances more closely, for however far we go we come to nothing new. On the other view, the nature of things as we know them will depend on the properties of these atoms of which they are composed, and it will be very interesting and important to find out, if we can, what the atoms are like. The latter view turns out to be far nearer the truth than the former; and for that all may be grateful who love to inquire into the ways of Nature.

Lucretius had no conception, however, of atomic theories as they stand now. He did not realize that the atoms can be divided into so many different kinds, and that all the atoms of one kind are alike. That idea is comparatively new: it was explained with great clearness by John Dalton at the beginning of the nineteenth century. It has rendered possible the great advances that chemistry has made in modern times and all the other sciences which depend on chemistry in any degree. It is easy to see why the newer idea has made everything so much simpler. It is because we have to deal with a limited number of sorts only, not with a vast number of different individuals. We should be in despair if we were compelled to study a multitude of different atoms in the composition of a piece of copper, let us say; but when we discover that there is only one kind of atom in a piece of pure copper, and in the whole world not many different kinds, we may feel full of enthusiasm and hope in pressing forward to the study of their properties, and of the laws of their combinations. For, of course, it is in their combinations that their importance lies. The atoms may be compared to the letters of the alphabet, which can be put together in innumerable ways to form words. So the atoms are combined in equal variety to form what are called molecules. We may even push the analogy a little further and say that the association of words into sentences and passages conveying meanings of every kind is like the combination of molecules of all kinds and in all proportions to form structures and materials that have an infinite variety of appearances and properties and can carry what we speak of as life.

The atomic theory of Lucretius did not contain, therefore, the essential idea which was necessary for further growth and progress. It withered away, and the very atom came to be used in a vague, incorrect fashion as meaning merely something very small: as sometimes in Shakespeare’s plays, for instance. In another and very different application of “atomic” theory Lucretius was strangely successful. He had the idea that disease was disseminated by minute particles. At the time of the Renaissance Fracastoro was inspired by the atomic theory of infection as he read it in the poem of Lucretius; but after his day the secret of bacteriology was again covered up until it was laid bare by Pasteur.[1]

Let us think of Nature as a builder, making all that we see out of atoms of a limited number of kinds; just as the builder of a house constructs it out of so many different kinds of things--bricks, slates, planks, panes of glass, and so on. There are only about ninety sorts of atoms, and of these a considerable number are only used occasionally. It is very wonderful that all the things in the world and in the universe, as far as we know it, are made of so few elements. The universe is so rich in its variety, the earth and all that rests on it and grows on it, the waters of the seas, the air and the clouds, all living things that move in earth or sea or air, our bodies and every different part of our bodies, the sun and moon and the stars, every single thing is made up of these few kinds of atoms. Yes, one might say, that is so: but if the builder is given bricks and mortar and iron girders he will build you an infinite variety of buildings, palaces or cottages or bridges; why may not Nature do something like that? But one has to think that when a builder sets out to make a structure he has a plan which has cost thought to devise, and he gives instructions to his workmen who are to carry out his wishes, and so the structure grows. We see him walking about with his plans in his hand. But the plans of the structures of Nature are locked up in the atoms themselves. They are full of wonder and mystery, because from them alone and from what they contain grows the infinite variety of the world. How they came to be such treasure-houses we are not asking now. We ask ourselves what these atoms are like: we have been asking the question ever since their exceeding importance began to be realized more than a hundred years ago. Have they size and form and other characteristics such as are possessed by bodies with which we are familiar? We must look into these points.

But first let us realize that in the last twenty-five years or so we have been given, so to speak, new eyes. The discoveries of radioactivity and of X-rays have changed the whole situation: which is indeed the reason for the choice of the subject of these lectures. We can now understand so many things that were dim before; and we see a wonderful new world opening out before us, waiting to be explored. I do not think it is very difficult to reach it or to walk about in it. In fact, the new knowledge, like all sudden revelations of the truth, lights up the ground over which we have been traveling and makes things easy that were difficult before. It is true that the new lines of advance now open lead the way to fresh difficulties: but therein lies the whole interest and spirit of research. We will try to take the first steps into the new country so that we may share in the knowledge that has already come, and comes in faster every day.

We go back to our questions about the atoms. Before the new period set in remarkably accurate answers had already been given to some of them, at least. In this theater of the Royal Institution, Lord Kelvin gave several addresses which dealt with the properties of atoms, and especially with their sizes. By several most ingenious and indirect devices he arrived at conclusions which we are now able to test by accurate methods; and we find that he was remarkably close to the truth. It was, of course, far more difficult to say what was the size of any particular atom than it was to say how much larger one atom was than another. For instance, the sizes of the atoms of potassium and carbon could be roughly compared by taking into account the relative weights of equal volumes of the solid potassium metal and of diamond, which is a form of pure carbon. Potassium is lighter than water, the diamond is three and a half times as heavy. We know from chemical observations that the individual potassium atom is rather more than three times as heavy as the carbon atom. If we suppose that the packing of the atoms in the two cases is the same (as a matter of fact, we now know that it is only approximately so) we must conclude that the atoms in the metal potassium are much larger than the carbon atoms in the diamond, because, though heavier individually, they pack so as to make a lighter material.

To make a reasonable estimate of the actual size of any one atom is a much more difficult matter, but all the four lines of reasoning which Kelvin employed led him to very nearly the same result. “The atoms or molecules of ordinary matter must be something like the 1/10,000,000th or from the 1/10,000,000th to the 1/100,000,000th of a centimeter in diameter.”[2] Our new methods tell us that the diameter of the carbon atom in diamond is 1.54 hundred-millionths of a centimeter, and that of the atom in the metal potassium is 4.50 hundred-millionths. We see that Lord Kelvin’s estimate was wonderfully near the truth, considering the indirect and inexact methods which alone were at his disposal.

In Fig. 1 are shown sections of certain atoms on a scale of fifty millions to one. The inserted figures give in each case the distance, in hundred-millionths of a centimeter, between the centers of two neighboring atoms in the pure substance. For example, the distance between two carbon atoms in the diamond is 1.54 hundred-millionths of a centimeter. In the case of oxygen the diameter has been calculated from the structure of crystals in which oxygen occurs. If the lecture room of the Royal Institution were magnified as much as the atoms of Fig. 1, its height would be greater than the distance from the earth to the moon. We need some such comparison as this to make us realize the excessive smallness of the things of which we are talking.

At the same time, we must keep in mind that they are not negligible because they are small: they are the actual elements of construction of the world and of the universe, and their size has nothing to do with their importance. But their smallness accounts readily for the case with which we all overlook them, and for the difficulty we have in examining them when at last we have realized what they mean to us. The value of the new methods of which I propose to speak lies in the fact that they enable us to deal with them, although they are so small.

We have now answered in a way the question as to the size of the atoms; but when we go further and ask ourselves about the shape we are not so successful.

The chemist, whose science is immediately concerned with the combinations of atoms, has rarely found it necessary to discuss their shapes, and gives them no particular forms in his diagrams. That does not mean that the shapes are unimportant, but rather that the older methods could not define them. There is one sense, however, in which the chemist pays much attention to form. The atoms in a compound are arranged in some fashion or other which is important to the combination. If one could see it and sketch it, one would be obliged to show it in perspective. In the science of organic chemistry especially it is found to be necessary to imagine such arrangements in space. It is not enough to represent them on the flat with no perspective at all; in fact, it is obvious that any flat design must be imperfect in any sort of chemical picture. We are, unfortunately, compelled to use the flat for our drawings; solid models in space are costly to make, while paper and pencil are cheap. It is curious to reflect what a handicap this technical difficulty puts on the proper development of a very important matter. Now, when we come to prescribe the arrangements of the atom to its neighbors, and to say that if one neighbor lies in this direction, another must lie in that, we are, in effect, giving shape to our atoms; at any rate, it is all the shaping that can be done for the present. We cannot do more until we know more about the internal structure of the atom: what its parts are, and how they are disposed to one another.

In the newer work, as we shall see, the arrangement of the atoms is much more closely examined, and for the first time their actual distances apart are measured. We find it absolutely necessary to make models because we do not see with sufficient clearness if we are content to draw on paper. We represent our atoms as round balls, and we find that we are able to represent most of our discoveries in this way. This really means that when an atom has several neighbors of the same kind it is equally distant from them all; and this is actually the case. Nevertheless, there are exceptions, as in the crystal of pure bismuth, where each atom has six neighbors and three of them are closer than the other three. We have to make a ball with three flats on it for use in constructing the bismuth model (Plate I a).

Let us now ask ourselves what binds the atoms together into the various combinations and structures. Like our builder, we have got in our materials--the bricks, slates, beams, and so on; we have our various kinds of atoms. If we look round for mortar and nails we find we have none. Nature does not allow the use of any new material as a cement. The atoms cling together of themselves. The chemist tells us that they must be presented to one another under proper conditions, some of which are very odd; but the combination does take place, and there is something in the atoms themselves which maintains it when the conditions are satisfied. The whole of chemistry is concerned with the nature of these conditions and their results.

The atoms seem to cling to one another in some such way as two magnets do, when opposite poles are presented to each other; or two charges of electricity of opposite nature. In fact, there is no doubt that both magnetic and electric attractions are at work. We are not entirely ignorant of their mode of action, but we know much more about the rules of combination--that is to say, about the facts of chemistry--than we do about the details of the attractions. However, we need not trouble ourselves about these matters for the present; we have merely to realize that there are forces drawing atoms together.

We may now ask why, if there are such forces, the atoms do not all join together into one solid mass? Why are there any gases or even liquids? How is it that there are any atoms at all which do not link up with their neighbors? What prevents the earth from falling into the sun and the final solidification of the entire universe?

The earth does not fall into the sun because it is in motion round the sun, or, to be more correct, because the two bodies are moving round each other. It is motion that keeps them apart; and when we look closely into the matter we find that motion plays a part of first importance in all that we see, because it sets itself against the binding forces that would join atoms together in one lump. In a gas, motion has the upper hand; the atoms are moving so fast that they have no time to enter into any sort of combination with each other: occasionally atom must meet atom and, so to speak, each hold out vain hands to the other, but the pace is too great and, in a moment, they are far away from each other again. Even in a liquid where there is more combination and atoms are in contact with each other all the time, the motion is so great that no junction is permanent.

In a solid the relative importance of the attractive forces and the motion undergoes another change: the former now holds sway, so that the atoms and the molecules are locked in their places. Even in the solid, however, the atoms are never perfectly still; at the least they vibrate and quiver about average positions, just as the parts of an iron bridge quiver when a train goes over it. It is difficult to realize that the atoms and molecules of substances which appear to be perfectly at rest--the table, a piece of paper, the water in a glass--are all in motion. Yet many of the older philosophers grasped the fact. For example, Hooke, an English physicist of the seventeenth century, explains by a clear analogy the difference which he supposed to exist between the solid and the liquid form: ascribing it to a movement of the atoms which was greater in the liquid than in the solid state. “First,” he says, “what is the cause of fluidness? This I conceive to be nothing else but a very brisk and vehement agitation of the parts of a body (as I have elsewhere made probable); the parts of a body are thereby made so loose from one another that they easily move any way, and become fluid. That I may explain this a little by a gross similitude, let us suppose a dish of sand set upon some body that is very much agitated, and shaken with some quick and strong vibrating motion, as on a millstone turn’d round upon the under stone very violently whilst it is empty; or on a very stiff drum-head, which is vehemently or very nimbly beaten with the drumsticks. By this means the sand in the dish, which before lay like a dull and unactive body, becomes a perfect fluid; and ye can no sooner make a hole in it with your finger, but it is immediately filled up again, and the upper surface of it leveled.

Nor can ye bury a light body, as a piece of cork under it, but it presently emerges or swims, as ’twere on the top; nor can ye lay a heavier on the top of it, as a piece of lead, but it is immediately buried in sand, and (as ’twere) sinks to the bottom. Nor can ye make a hole in the side of the dish, but the sand shall run out of it to a level. Not an obvious property of a fluid body, as such, but this does imitate; and all this merely caused by the vehement agitation of the containing vessel; for by this means, each sand becomes to have a vibrative or dancing motion, so as no other heavier body can rest on it, unless sustein’d by some other on either side: nor will it suffer any body to be beneath it, unless it be a heavier than itself.”

Hooke’s experiment can be repeated in a somewhat different form. A cylindrical metal box, ten inches wide and three inches deep, is fixed upon a platform which is supported on metal balls so that it moves easily. It is connected through an eccentric joint with a turning table as shown in Plate I B. When the wheel is turned rapidly, the box and the sand which it contains are violently agitated as Hooke prescribes. The details of the mechanism are best understood by reference to the figure. A heavy metal ball placed on top of the sand disappears at once, and light objects, such as ping-pong balls, rise to the surface. A very ludicrous effect is produced if we bury in the sand some of the celluloid figures which cannot be made to lie down because they are heavily weighted at the bottom. The figures slowly rise out of the sand and finally stand erect (Plate I b and Fig. 1 a).

We know now that the motion of the atoms of a body is really its heat: that the faster they move or vibrate the hotter the body becomes. Whenever we warm our hands by the fire, we allow the energy radiated by the fire to quicken up the movements of the atoms of which the hands are composed. When we cool any substance we check those movements. If we could still them altogether we should lower the temperature to a point beyond which it would be impossible to go, the absolute zero, as it is usually called, 273 degrees centigrade below zero.

As I have said already, we have found two new allies, radioactivity and X-ray, in our attempt to see the very minute atom. They have increased the fineness of our vision some ten thousand times. The microscope had done its best for us; but the smallest thing which it could show us was composed of billions of atoms. No improvement could be made in the microscope lenses: technique had reached its highest. The difficulty was really due to the fact that light is a wave motion and light waves cannot show us the details of objects unless the objects are much larger in every way than the length of the wave. We wanted a new light of very short wave length. It came in the form of the X-rays. At the same time radioactivity came to show us what a single atom could do by itself if it were given a tremendous speed. We can now see the single atom, indirectly, no doubt, but quite usefully, and also observe something which it does: the X-rays help us with the former and radioactivity with the latter. I hope to explain to you how both these agents are adding to our knowledge, and I shall take radioactivity first.

The atom of radium might be roughly represented in size by one of the larger balls that lie before you. It is one of the heaviest and largest of the atoms; a number of them together form a substance which is a metal like iron or gold. It is, of itself, in no obvious way peculiar as long as it continues to be an atom of radium, but, for some reason, which no one understands, there comes a moment when it bursts. A small portion is hurled away like the shot from a gun, and the remainder recoils like the gun itself. The remainder is not radium any more, it is a smaller atom, having entirely different properties. The radium has turned into a new substance. As a matter of fact, the new substance is a gas, while the projectile turns out to be an atom whose weight is low down in the series of atomic weights, the lowest but one in fact; it is called helium. No one knows what brings about the explosion, nor does anyone know a way of hastening it, or of hindering it. The radium atom is just as likely to explode at any given moment if it is in a furnace as if it is immersed in liquid air. Indeed, its independence of its surroundings in respect to its time of explosion is shown in a much stronger light by the fact that combination with other atoms makes no change. Combination, or molecule-forming, is, no doubt, concerned with the outside arrangements of the atoms, but the bursting of the atom comes from inside.

The old alchemist tried to find a means of converting one atom into another, preferably lead into gold. In the action of radium there is a transmutation, to use an old word, of the kind of which the alchemist dreamt. But it is not exactly what he strove for, in two ways. In the first place, it cannot be controlled by human will--which is extraordinary, because there are not so many things of which this can be said. Even when an operation is quite beyond our power to understand, we can often decide whether or no it shall happen. We cannot understand how a seed germinates, much less make one that will do so; but we can lock up seeds in a drawer and prevent them from germinating as long as we like. But the radium explosion does not wait on anything which we do.

In the second place, the transmutation does not end in gold: it ends rather in lead. The gas which consists of atoms of radium that have shot off one atom of helium is very short-lived: the average life of each of its atoms is a little less than four days, in contrast to the average life of the radium atom, which is about two thousand years. The second explosion “transmutes” the gas atom into a new substance called Radium A, and on the occasion another helium atom is shot away. There is a further succession of explosions, at very varying average intervals, and the final product is actually lead, not gold. The gas was called the “radium emanation” by Rutherford, who discovered it.

The whole operation is very wonderful, but I want to call attention to what happens to the projectile when it has left the gun. The velocity with which it starts is so great that one could never have thought any particle of matter could have possessed it. When Huygens argued with Newton on the subject of the nature of light, he condemned Newton’s idea that light consisted of a flight of corpuscles, on the ground that material particles could not possibly travel as fast as light had just been found to move. It is curious that we now find atoms moving with speeds comparable with, a tenth or twentieth of, that which then seemed impossible. There are even certain particles, called electrons, also emitted by radioactive substances, which travel, in some cases, very nearly as fast as light. It is also curious that the second argument of Huygens was equally unfortunate in view of the observed phenomena of radioactivity. He said that it would be impossible, on Newton’s theory, for two people to look into each other’s eyes because the particles would meet each other and fall to the ground. We shall presently see that this argument also is set at naught by the facts of radioactivity.

The velocity with which the helium atom begins its flight is something like 10,000 miles in a second. In less than a minute it could get to the moon and back again if the speed were maintained, but the curious thing is that for all the speed and energy with which it starts it never gets far when it has to pass through anything material. Even if it is allowed to finish its course in the air, its speed has fallen to something of quite ordinary value after it has traversed a course of two or three inches in length. The course is, in general, perfectly straight, as we shall presently see in an actual experiment, and this is the very important point which we must consider with particular care. At first sight one does not realize how remarkable it is that its path should be straight: one thinks of a bullet fired through a block of wood, let us say, and making a cylindrical hole, or of the bullet in its straight course through the air. But the comparison is unfair. The bullet is a mass of lead enormously heavier than any molecule which it meets, and it brushes the air aside. But the helium atom is lighter and smaller than the atoms of nitrogen or oxygen of which the atmosphere is mainly composed, and we must think of some more truthful comparison. Suppose that a number of billiard balls are lying on a billiard table, and let them represent air molecules. If they are in movement the picture will be more correct, but the point does not really matter. Now let us drive a ball across the table, aiming at a point on the opposite cushion, and watch what happens as the ball tries to get through the crowd that lies on the table, which crowd may or may not be in movement. It hits one of the balls and is turned to one side; it hits several in succession, and soon loses all trace of its original direction of movement. Shall we now drive it with all the force we can, and see whether it keeps any more nearly to the straight path? We try, and find that there is no improvement at all. The straight path cannot be obtained by any increase of speed, however great.

This picture or model is much more faithful than that of the moving bullet, and shows more clearly the remarkable nature of the radium effect. A helium atom must encounter a very large number of air molecules if it proceeds on a straight-line path, and if the atoms are of the size we have supposed them to be. In fact, the molecules lie far more thickly on the path than we can represent by the billiard-table model. It is possible to calculate how many air molecules, some oxygen, some nitrogen, would be pierced by a straight line three inches long drawn suddenly at any moment in the air, and the result is to be expressed in hundreds of thousands. How can the helium atom charge straight through this crowd, every member of which is heavier than itself? It does so, however, and we have to find some explanation.

Perhaps it might be thought that the straightness of the path is only apparent, and that if we could look into it in sufficient detail we should see that it was made up of innumerable zigzags made in going round the molecules met with. But a moment’s reflection shows that the idea is absurd: the atom would need to possess the intelligence of a living being to give it the power of recovering a line once lost. If there were a cake shop on the opposite side of a crowded street, and if we gave a boy sixpence and directed him to the shop, he would no doubt pursue a path which was effectively straight, though it would be broken up by the need of dodging the various people and vehicles which the boy met with. But one cannot imagine an atom of helium doing anything of the sort.

There is only one way of explaining the marvel of the straight path: we must suppose that the helium atom _goes through_ the molecules it meets, and that somehow it is enabled to do so by the fact that it is moving at such an unusual speed. It is a very startling idea.

PLATE II.

Shimizu-Wilson ray track apparatus.

The apparatus which shows the tracks of the helium atom shot out by radium. The chamber as sketched in Fig. 2 is to be seen on the upper left of the figure. The disc to the left of it is a screen, in which is a hole. The light from a lantern--not shown--shines through the hole and lights up the fog tracks. A second screen revolves, and lets the radium rays, _i.e._ the helium atom, shine into the chamber just before the expansion is made. On the right is the driving machinery.]

However, no other suggests itself; and, as a matter of fact, it turns out that we can explain many other things by its aid. Consequently, we feel sure that we are on the right track.

The piston _PP_ is dropped suddenly from the position indicated by the dotted lines to the position indicated by the full lines; so that the air in the chamber is suddenly chilled by expansion and fog settles on the tracks of the helium atoms shot out by the radium at _R_.]

It is time now that we should see this effect with our own eyes: the conclusion at which we have arrived is so new and so full of meaning that we would like to have an experimental demonstration if possible, and convince ourselves of the reality of these straight-line paths. We owe to Mr. C. T. R. Wilson a beautiful piece of apparatus which gives us a vivid picture of what happens, and we will make use of it at once. The experiment is, in my opinion, one of the most wonderful in the world of science. We are going to see the actual tracks of separate helium atoms, each of which begins its course at a speed of ten thousand miles a second and yet completes it after traversing about three inches of air. But we must first enter upon some explanation of how the apparatus works; for there are ingenious devices in it.

There is a cylindrical box of brass, with a glass top and a base which can be raised or lowered so as to alter the depth of the box. There is a machinery of wheels, cranks and levers by which the bottom of the box can be suddenly dropped at convenient intervals. Whenever this happens, the air or other gas which the box contains is chilled by the sudden expansion. We shall study effects of this kind more carefully in the next chapter. At the side of the box, in its interior, a minute speck of radium is mounted on a suitable holder. Every moment some of its atoms break up and expel atoms of helium, of which a certain number are shot straight into the box. The diameter of the box is big enough to allow the atoms to finish their courses in the air within. The average life of radium is so long that even if the apparatus held together for two thousand years, half of the radium speck would still be left. Yet each second, ten, twenty or a hundred atoms disappear in the expulsion of the helium atoms. Perhaps in no better way can it be shown how many atoms are concentrated in a small compass.

The air in the chamber is kept damp, consequently the chill due to expansion tends to produce a fog. Fog when it has to settle prefers to deposit itself on a solid nucleus of some sort, rather than to form independent drops in the air. The small particles of dust, if there are any, are made use of, which is the reason why fogs so readily form in a dirty atmosphere. But of all things moisture prefers to settle on those atoms through which the helium atom has passed. The reason is that the atom is temporarily damaged by the transit: a small portion has generally been chipped away. The portion removed is what we now call an “electron”; it is charged with negative electricity, and the atom which has lost it is correspondingly charged with positive electricity. The electron set free settles on some neighboring atom, sooner or later; and in consequence there are two charged atoms, one positive and one negative, where previously there were no charged atoms at all. The charged atoms have a great attraction for moisture, and the fog forms on them in preference to anything else. If, therefore, a helium atom has just made its straight road through the gas, and has left behind it numbers of charged atoms on its track, and if, at that moment, the sudden expansion causes a chill, fog settles along the track. A bright light is made to illuminate the chamber, so that the fog tracks are visible as bright straight lines, showing against the blackened background of the bottom of the cylindrical chamber. They last a few seconds and then the fog particles slowly disperse. If the helium atom completes its track just before the fog is formed, the line is sharp and clear; because the charged atoms have not had time to wander from the track. But if the track is made some time before the expansion, the line of fog is more diffuse. It is to be remembered that the helium atoms are being shot out all the time, day and night; but it is only when an expansion is made that tracks are made visible.[3]

If we watch the successive expansions, we see that the tracks, though quite straight over large parts of their course, do undergo at times sudden sharp deflection, especially when they are nearing the end. This remarkable effect turns out to be most important, and we must refer to it presently.

Let us now try to picture to ourselves in what way we must modify our first conception of the atom so that we can explain the effects we now see. The atoms must be so constituted that when they meet one another in the ordinary way, as, for example, when molecules of oxygen collide in the atmosphere, they behave as if each had a domain of its own into which no other might enter. Or, when they are pressed together, as in a solid, they occupy as a whole an amount of space which is sufficient to make room for them all. But when one atom--the helium atom is our chief example--is hurled against others with sufficient speed, the one atom goes through the other, as if the defenses round the domains had been broken down. We find a satisfactory explanation when we imagine each atom to be like a solar system in miniature. There is to be a nucleus, corresponding to the sun, and round the nucleus there are to be satellites or planets, which we call electrons. The nucleus is charged with positive electricity; each electron is charged with negative electricity, and all electrons are alike. The positive charge on the nucleus is just enough to balance the united negative charges of the electrons. The electrons are supposed to be in movement, just as the planets are revolving round the sun, but the movements are no doubt complicated, and their nature need not for the moment concern us at all.

Instead, therefore, of a round hard ball of a certain size, which was our first rough picture of an atom, we have something like a solar system in miniature. We can at once see how one atom of this kind can pass through another, just as we might imagine one solar system passing through another, without injury to either provided that no one body of one system made a direct hit on a body of the other and that the motion was quick enough. The latter condition is necessary because if one solar system stayed too long inside or in the neighborhood of another there would certainly be very serious disturbances of the courses of the planets.

But then, we may ask, how can an atom, if this be its nature, have the power of keeping another outside its own domain? How can it appropriate any portion of space to itself, and prevent the intrusion of another atom when the speed at which they meet is low? The explanation becomes clear when we consider the special arrangement of the positive and negative charges. Every atom is surrounded by a shell or cloak of electrons; and, when two atoms collide, it is their shells which first come close together. Since like charges of electricity repel one another, the two atoms will experience a force which tends to keep them apart: in other words, they will resist encroachment on their own domains. This is, no doubt, a very rough picture of what actually happens, and as a matter of fact it is difficult to explain the strength of the resisting forces on such a simple hypothesis. Still, it is on the right lines, no doubt. When the two atoms approach each other at a high speed, the system of electrons and nucleus of one atom slip through those of the other. A model will help to illustrate the point.

Plate IV a shows a set of bar magnets mounted on spiral springs and standing erect. The top of the inside magnet is a north pole, and the tops of the magnets of the outside ring are south poles. The model represents roughly the central nucleus surrounded by a ring of electrons. In the model everything is in one plane; in the atom it is not so, but the point is not important. A single magnet is suspended by a long thread from a point vertically over the “nucleus” magnet. Its lower end is a south pole and the length of the thread is such that the swinging magnet just clears the fixed magnets. Observe now that if we pull the swinging magnet to one side (_S_ in Fig. 3, _a_), but not too far, it moves toward the fixed set and is unable to enter in. It seems to knock at the door at one place after another, but always recoils. Just so would any electron beat in vain against the outer defenses of an atom, if it did not beat hard enough. We can easily imagine that if the single swinging magnet were replaced by a system of magnets, like our stationary set, the same result would follow.

=A.= Bar magnets on spiral strings.

=B.= Models of atoms with electrons.]

Here we have a picture representing our atoms, as we now think of them, beating against one another and recoiling; each occupies a certain domain of space and prevents the intrusion of any other atom.

But if the swinging magnet is drawn sufficiently far to one side so that it acquires a greater speed than before by the time it reaches the stationary set, its momentum will carry it through. If the speed is very great, it shows no appreciable change in its motion due to its passing (Fig. 3, _c_); if the speed is rather less, it often suffers in going through (Fig. 3, _b_). It comes out less vigorous than when it went in; often it has changed the direction of motion also, and it has obviously left energy behind, for the magnets of the stationary set are left quivering. This happens no matter which pole of the swinging magnet is the lower, and clearly the same effect would be shown if the single swinging magnet were replaced by a more complicated set of nucleus and attendant satellites.

The behavior of the model helps us to anticipate what we should find when atoms of our new design come across one another. If they approach at a moderate speed, they may rebound from one another; at a high speed they go through one another, and the higher the speed, the greater the chance of a passage without any obvious result. But there is always the chance that the nucleus of the moving atom may go so near to the nucleus of the atom through which it is passing that it experiences a perceptible deflection. The smaller the nuclei are, the less likely it is that this will happen.

You will have guessed already that you have actually seen such deflections as these in the kinematograph picture, such as also are shown in Plate III, a to d. The tracks of the helium atom are quite straight in the main, but there are decided breaks in the straight lines, usually not more than one or two in each track. They are found mainly toward the finish. This is what might be expected, since the motion will then be slower. Several of them appear in Plate III a; a very good example of this kind of track is reproduced on a large scale in Plate III c. The upper track shows a slight but sharp deflection at a little distance from the end of its course, and a more pronounced deflection farther on. Nearly every track shows some deviations at the very end. Thus the new conception of atomic structure explains all the effects in a satisfactory way.

It is strange to think of an atom as being empty as a solar system; not a round, hard and absolutely impenetrable body, but a combination of nucleus and electrons which occupies a certain space somewhat as an army occupies a country. The bodies of the soldiers do not fill the country from boundary to boundary; but enemy soldiers may not enter, nevertheless.

These very characteristic pictures are the fruit of much watching and photographing. Breaks are found at every expansion, but it may be necessary to wait for a really good one. A very fine picture is shown in Plate III d. It is due to Mr. P. Blackett. In this case, helium was used instead of air. The nucleus of the flying helium atom, in traversing a helium atom belonging to the gas, has made an almost direct hit on the nucleus of the stationary atom: it has cannoned off it, as a billiard player would say. Both atoms now move with not unequal speeds, and both make fog tracks, as the figure shows. In Plate III c if we look carefully, we see that there is a minute spur on the last bend of the track already mentioned, which means that in this case an atom of oxygen or nitrogen has deflected the helium atom and has recoiled in consequence. Its track is very short, because it is much heavier than the atom which struck it, and, therefore, the velocity given to it has been comparatively small.

There is a certain curious feature to be found in some of the photographs which may well be explained. In some of the tracks there are gaps, as if the fog settling had failed. This is indeed the actual fact: there is no moisture to settle, because a helium atom has gone that way some very short time before and has used up the moisture in the neighborhood. In Plate III b several tracks due to radium emanation are shown. They seem to start from anywhere in the chamber because the atoms of the emanation have wandered about the chamber before blowing up.

The next question that arises is as to the number of electron satellites which each atom possesses. Here we come to a very beautiful and remarkable feature of the new discoveries. It is not necessary to explain in full how it was discovered; we will be content with describing it.

In the atom as we now have it the nucleus is charged with positive electricity, the amount of the charge being just enough to neutralize the negative charges on the attendant electrons. All electrons, as we have already seen, are alike. We find that atoms differ in the number of attendants which they can maintain, and that the statement of that number describes the atom completely so far as its attitude toward other atoms is concerned. For instance, the atom of carbon can hold six electrons; the positive charge on the nucleus is the counterpart of six standard negative charges. Every atom which can retain six electrons is a carbon atom: no other definition of the carbon atom is required. Just so the “seven-electron” atom is nitrogen, the “eight-electron” is oxygen, and so on. All numbers are found in nature, with very few exceptions, from the “one-electron” atom—hydrogen—up to the “ninety-two-electron” atom—uranium. The missing numbers will probably be found some day; more or less accidentally, it may well be.

We may use models as a rough illustration of the point. The nucleus (Plate IV b) is represented by a white ball of solid rubber, the electrons by smaller balls forming the heads of pins which are stuck into the center ball. The pins may be of different lengths (p. 84).

It is strange that the immense variety in nature can be resolved into a series of numbers. It was at one time thought that the various sorts of atoms owed this variety to something more than that; it is a great surprise to find such a simple kind of difference between atom and atom. The unchanging feature of any particular sort of atom is the positive charge of electricity on the nucleus. It is in consequence of this that the proper number of electrons gather round. We may expect that they will arrange themselves in some fashion; we shall see later that they certainly do so. The sort of arrangement they take in each case, and the nature of the forces put into it, are very difficult questions, most of which we may well put to one side for the present, contenting ourselves with one or two simple aspects of the problem.

In the first place, it is interesting to watch the assembling of the little vertical magnets floating in the glass tank (Plate V a). They are buoyed up by ping-pong balls, painted black, and, so that we may see them easily, they carry white ping-pong balls at top. The magnets are all the same way up, so that naturally they repel one another and cluster round the edge of the basin. But there is an electromagnet underneath the bowl, which, when made active, draws the small magnets together. The arrangement in which they settle finally is governed partly by the pull toward the center and partly by the mutual repulsions. Something of this kind must take place in the atom, but we must not push the analogy too closely, because the forces may be quite unlike those which are exerted in the model. We must content ourselves with observing that when there are only a few magnets afloat they group themselves in a ring; but when the number is increased they arrange themselves in concentric rings. A pretty effect is produced by putting in each additional magnet at the edge of the basin and watching it float away in a stately fashion to take its proper place.

A similar division into concentric shells or groups is found in the arrangement of the electrons round the central nucleus of the atom. We will consider this more carefully in the next chapter. The experiment does not prove that there ought to be such an arrangement, but certainly suggests it.

We may now see more clearly what happens when the helium atom injures the atoms through which it passes and renders them attractive to the particles of moisture that form the fog. It is possible, in fact, for an atom to be deprived of one of its attendant electrons. Having lost one, it resists more strongly the loss of a second, still more of a third. As the helium atom goes on its way, it strips one atom after another of an attendant, and the electron set free goes off on a course of its own. But its separate life is very short-lived: it is soon attached to another atom.

=A.= Floating magnets.

When the number of floating magnets is small, they form into a single ring, but when the number is increased, they form concentric rings.

=B.= Crystals in tube containing emanation.

From Prof. F. Soddy’s “Interpretation of the Radium” (John Murray), by the kind permission of author and publisher.]

The atom that has lost an electron is now positively charged; the gainer is negatively charged. The two atoms would make things even again if they came sufficiently close together, and as they move about in the gas the negatives and the positives do in the end give and take electrons, and the whole gas is neutral once more.

There is a beautiful experiment with which we may end this chapter. When the helium atoms strike certain substances they excite a phosphorescent glow. It is really, when we look into it closely, a set of minute flashes due to the impacts of the separate atoms; under a microscope the effect is as when we drop pebbles into a phosphorescent sea. The glass vessel (Plate V b) contains crystals that phosphoresce under the stimulus of the swift-moving helium atoms; one is kunzite, another zinc sulphide, another willemite. In another tube is a quantity of radium emanation: the gas which, you will remember, is the immediate descendant of radium itself. When it is released and is allowed to pass into the tubes containing the crystals the latter glow in brilliant colors. In the figure the crystals have been made to photograph themselves by their own phosphorescence.

The radium action has, we see, given us a remarkable insight into the structure of the atom, for which there is a general reason to be given. The student of science has long been familiar with the existence of various atoms and with their properties; he has never seen one, nor the effects of one. He has handled atoms in crowds only. When the chemist causes elements to form compounds, or analyzes compounds into elements, he deals with enormous numbers of atoms in any operation big enough to see. But in this radioactive effect we observe the action of one atom at a time, and here lies the secret of the advance. The speed of the helium projectile, a hundred thousand times the speed with which the atoms move ordinarily when they form part of a gas, gives the individual atom the power of making itself felt. When we look at the fog tracks, we see the actions of separate atoms; we see something which would have filled the early defenders of the atomic theory with astonishment and pleasure. One atom of helium passes through one atom of oxygen, let us say, and comes out on the other side, and both may bear evidences of the encounter. Effectively we use such evidence to help us to determine the nature of the atoms. The helium atom is like a spy that has gone into a foreign country and has come out again with a tale to tell.