Chapter II: The Nature of Gases

WE have seen that all things are made of about ninety kinds of atoms, and that in them is wrapped up the mystery and the infinite variety of the material world. In each there is a nucleus which is positively charged; round the nucleus are electrons which are units of negative electricity. The positive charge of the nucleus is a multiple of a certain unit charge, equal to the charge on the electron, but of opposite sign. The number of electrons which every atom possesses under normal conditions is an exact balance to the positive charge on the nucleus, so that the atom as a whole is not charged; its positive and negative charges balance. Whether or no the electrons are revolving round the central nucleus like planets round a sun, or whether they possess other more complicated motions are not matters of importance to us for the moment. Something is known of these points, but the whole question is difficult. The only consequences of this strange arrangement of nucleus and electrons which we must consider can be drawn without thinking about the possible motions. One consequence is that the atoms do not encroach on one another’s domains under ordinary circumstances. Each has an outer cloak or shell of electrons; and when two atoms are brought close together there is a resisting force which we may suppose to be due to the mutual repulsion of the two shells. But when two atoms are hurled at each other with sufficient speed the outer defenses may be broken down and the atoms pass through each other. When this happens the atoms may afterward disentangle themselves and pass on their way as if there had been no encounter at all: one or both may have suffered the loss of an electron or two, but the damage is soon made good. It is only when the nucleus of one approaches sufficiently close to the nucleus of the other that there is a change of motion like that due to the meeting of two balls. Changes of this kind are so rare and imply such a closeness of approach that we are bound to think of the nucleus as very small indeed. These penetrations of atomic domains are brought to our notice by the actions of radium and similar substances, as explained in the first chapter, and are of importance to us because they make us realize the empty nature of the atom, and its sun and planet structure. They do not occur in the usual relations of atoms to one another, because the speed is far too small. The domain which the atom occupies to the exclusion of others is about a hundred-millionth of an inch across; it is within this minute space that the nucleus and the electrons perform their relative motions. The light atoms have smaller domains, and the heavier somewhat larger: a factor of three or four will take us from the smallest to the largest.

I have said that all atoms are in motion, and that there is a constant struggle between some form of attractive force which would draw all the atoms together and this motion which would keep them independent. The existence of an attractive force which we here take into account as something very important does not at first seem to be reconcilable with the atomic structure we have just considered, because in this we supposed that the outer shells of electrons would prevent the atoms from coming too close to one another. It is a difficult point, because both views are certainly correct. It is, no doubt, our present ignorance of the nature of these forces that prevents us from arriving at a clear understanding. We have seen how it can happen that when two atoms approach each other at great speeds they go through each other, while at moderate speeds they bound off each other like two billiard balls. We have to go a step further, and see how, at very slow speeds of approach, they may actually stick together. We have all seen those swinging gates which, when their swing is considerable, go to and fro without locking. When the swing has declined, however, the latch suddenly drops into its place, the gate is held, and after a short rattle the motion is all over. We have to explain an effect something like that. When the two atoms meet, the repulsions of their electron shells usually cause them to recoil; but if the motion is small and the atoms spend a longer time in each other’s neighborhood, there is time for something to happen in the internal arrangements of both atoms, like the drop of the gate-latch into its socket, and the atoms are held. It all depends on some structure of the atom which causes a want of uniformity over its surface, so that there is usually a repulsion; but the repulsion will be turned into attraction if the two atoms are allowed time to make the necessary arrangements, or even if at the outset they are presented to each other in the right way. We shall see later several very interesting examples of this effect.

We are going to consider in this chapter the case when the attractive forces between the atoms do not act, whether from want of time, or from feebleness, or from any other reason. A crowd of atoms is, when this is the case, a gas.

Such cases are very numerous. In particular there are certain atoms which furnish notable examples; they are Nos. 2, 10, 18, 36, 54, 86: that is to say, they are those in which the nuclei possess positive charges whose magnitudes are represented by one or other of these numbers, and which normally possess negative electrons to match. These atoms have only the feeblest desire to join up with one another. They do not enter into combination with atoms of other kinds; in other words, they do not form chemical compounds. We may call them the “unsociable” atoms. They take no obvious part in the doings of the world, and their existence was entirely overlooked until a few years ago. It was only when the late Lord Rayleigh was making careful measurements of the weight of nitrogen obtained from various sources, that he noted a small but unmistakable discrepancy between the density of nitrogen as prepared from the break-up of a known compound of nitrogen and the density of what was left of air when every known gas had been abstracted from it. According to the view held at the time of his experiment, the residue should have been pure nitrogen. As a matter of fact, atmospheric air contains a small percentage of one of these “unsociable” atoms or gases; it is No. 18, that which has eighteen units of positive electricity in the nucleus. So Rayleigh’s very careful measurements led to the discovery of the hitherto unknown substance. It was named argon, the “lazy one.” Perhaps the name does not express its chief characteristic; for the atom is as quick in its movements as any other of its own size. The weight of the air in the lecture room of the Royal Institution is about 15 cwt.; it contains about 18 lb. of argon. If the gas had had the least tendency to form any chemical association, such an amount, though relatively small, would have been easily detected by the delicate analytical methods of chemistry.

The atom of helium, the smallest of the series, is identical with the atom expelled by radium and other radioactive substances in the act of disintegration. It has two electrons normally; though as it flies through matter when radium has ejected it its complement of electrons is apt to be torn away for the time. The positive charge on the nucleus is not affected by the flight, so that when the atom comes to the end of it, the deficiency in electrons is quickly repaired: there are always stray electrons to be picked up. Then the atom takes up the quiet and independent existence which is its characteristic. Perhaps most of the helium in the world has at some time been fired off, atom by atom, from radioactive substances. At any rate, it is found in places where such actions must have occurred. Helium is now collected in large quantities in America and Canada, where it is found bubbling up in certain springs. It is used for filling dirigible balloons, for which purpose its main properties make it most suitable. It is light, and its lifting power is almost as great as that of hydrogen, the one-electron atom; the atomic weight increases on the whole with the number of electrons. The lifting power of a gas, we must remember, depends not on the density of the gas but on the difference between the density of the gas and the density of the air. The densities of hydrogen, helium, and air are in the proportion 1:2:14.4; the lifting powers of hydrogen and helium are in the proportion 13.4 to 12.4. But its main virtue is that it is not inflammable. The hydrogen atom is very sociable, and in particular has a violent desire to become associated with oxygen: if hydrogen and oxygen are mixed, it needs but a spark to start the combination, with fire and explosion as the result. A hydrogen-filled balloon is therefore liable to disaster; but helium seeks no change, and there is no danger from fire. The name of the gas is due to its discovery in the sun; a bright line in the sun’s spectrum could not be identified with lines due to any of the known elements on the earth. The name “helium,” or “sun-substance,” was therefore given to the unknown substance to which the line was due. It was at a later date that helium was found to be a member of the series of gases which Lord Rayleigh and Sir William Ramsay were led to examine as the consequence of Rayleigh’s nitrogen experiments.

The ten-electron atom neon, the “new one,” is less common than argon. It has a peculiar property of glowing easily and brightly under the stimulus of electric discharge, and is often used in electric-light bulbs: we have all observed the reddish-orange glow of the neon lamp.

Krypton (36), the “hidden one,” and “xenon” (54), the “stranger,” are very rare. The last of the series, with eighty-six electrons normally, is the heavier fragment of the break-up of the radium atom. Like the rest, it tends to pursue an independent existence, so that radium, when it breaks up, turns into two gases. In some of the Wilson pictures (Plate III a, b) of the tracks of the helium atoms we may see a track that begins in the middle of the chamber: it is due to the break-up, in its turn, of an atom of this heavy gas, for it also is radioactive. In fact, its average length of life is only three and a half days.

We must not suppose that these strange atoms cannot be made to associate together under any circumstances. It is possible to make them join together as a liquid, but only at extremely low temperatures. At ordinary temperatures they are all gases. The liquefaction of helium is one of the achievements of the laboratory of Kamerlingh Onnes at Leiden, where the production of low temperatures has been carried to a very great state of efficiency.

There are certain other atoms--hydrogen, nitrogen, oxygen, and others--which readily form into small companies, or molecules, each of which is almost as free from any desire to associate itself with other molecules of the same kind and, in many cases, of other kinds as the atoms of helium and argon. Two atoms of hydrogen make a very stable and “unsociable” molecule; so do two atoms of nitrogen, or of oxygen. In these cases, the properties of the substance are at ordinary temperatures those of a gas. The liquefaction of hydrogen was accomplished by Sir James Dewar in the laboratories of the Royal Institution, London; the machinery is still there. In the anteroom there is a picture which shows Dewar pouring liquid hydrogen from one of his vacuum flasks into another during the course of a lecture which he was giving. The air consists mainly of a mixture of oxygen and nitrogen molecules. Other well-known molecules which form gases under ordinary circumstances are carbon monoxide (CO), carbon dioxide (CO{2}), methane (CH{4}), and so on. In all these cases when two of these molecules meet each other at speeds which are proper to ordinary temperatures they recoil from the impact, and so maintain an independent existence. What we have now to consider are the consequences we should expect to follow from this condition of independence.

Let us imagine a closed vessel containing a number of atoms or molecules moving about within it--containing a gas, as we should say. They continually meet one another and the walls of the vessel, and behave as a number of billiard balls set in motion on a table would do if their motion were frictionless and therefore perpetual. In fact, it is convenient to use a small billiard table as an illustration, and Messrs. Burroughs and Watts have very kindly given us one for the purpose.

If the balls are in motion they drive the loose cushion before them and they lose some of their energy. If, on the other hand, the cushion is suddenly advanced, the energy of the motion of the balls is increased.]

The balls soon come to rest on the table, because the cushions, as well as the balls, are not perfectly elastic; moreover there are losses by friction as the balls run over the cloth, smooth though it is. Nevertheless, the motion, once started, lasts long enough to give an idea of what must happen if it were maintained indefinitely.

It is natural to ask how the force of gravity would affect the movement of the atoms in our closed vessel. Would it not bring them all to the bottom? Why should the gas fill the upper as well as the lower parts of the vessel? The answer is that gravity certainly has its effect in full, but it is so very small as to be unobservable in our particular case. If we might imagine that all the heat were taken from the gas, and its movements therefore ceased, and if the attractive forces could be ignored, the atoms would, of course, lie about at rest on the bottom of the vessel. If a very little heat were now given to them, we might imagine them to begin dancing up and down, like perfectly elastic balls on a perfectly elastic floor. If the rise were a thousandth of a degree Fahrenheit, they would bounce to a height of about seven inches. With enough heat they would begin to hit the top of the vessel as well; we might suppose them to be so few that they did not hit one another very often. But at ordinary temperatures their movements would be so rapid--something like 6,000 feet a second--that gravity would make little difference in their velocity as they ascended and descended, and there would be, at any moment, as many at the top as at the bottom of the vessel. If they were as numerous as the molecules of the air under ordinary conditions, they would hit one another more often than the walls. In the air, the usual length of path between two successive encounters with other molecules is only about four-millionths of an inch. Since gravity has no obvious effect, the billiard table is all the better an illustration; we might find an analogy to gravity by giving it a slight tilt, but it would not to be worth while doing so.

If the atoms or molecules of a gas are continually hitting the walls, the latter must always experience an outward pressure: we speak, in fact, of the pressure of a gas upon its envelope. The distention of a balloon is due to the bombardment of the covering by the molecules. If we put a loose cushion on our table and make the balls roll about, the cushion is driven backward. If there were twice as many balls as there are, there would be twice as much pressure. This is the well-known gas law that pressure is proportional to density, other things being kept the same. We increase the pressure on the cushions if we make the balls move faster; in the same way, the pressure of a gas rises with the temperature.

Suppose now that I suddenly advance this loose cushion while the balls are moving and striking it. It is obvious that the motion of the balls is increased. In the same way, if one of the walls of the gas vessel is pushed in, as when a piston is forced deeper into a cylinder, the motion of the atoms is increased. In other words, the temperature is raised. We all know how hot a bicycle pump becomes when we use it to force air into a tire. The converse is equally true. If the cushion on our table is withdrawn as the balls strike it, their motion is diminished. If we have played cricket, we know that when we want to catch a ball we must draw our hands back as the ball begins to touch them: the retreating hands destroy the motion of the ball gradually. If we hold them in a fixed position, the ball is sure to jump out again. So also when a lacrosse player catches a ball, he draws his crosse downward when the ball first enters it, and makes the stopping of the ball an operation lasting over two or three feet of its path. A tennis racquet can be used to catch a tennis ball in the same way, but the action must be well timed, because the racquet face is so stiff. In the case of the gas, the corresponding effect is its chilling by expansion. We had an example in the use of Mr. Wilson’s apparatus, where the sudden enlargement of a space full of moist air caused such a chill that the moisture settled as a fog on the tracks of the helium atoms.

=A.= Tuning fork over jar.

=B.= Fog apparatus.

The long glass tube is full of fog.]

The expansion had to be fairly sudden, because if it had been otherwise there would have been time for heat to flow in from outside during the action, and the desired low temperature would not have been reached.

The expansion of great masses of air in the atmosphere is a frequent source of rain and snow. In the constant movement of the winds it may happen that some huge volume of damp air expands into a space where the pressure has been lowered and becomes so cold that the water vapor begins to condense. It is easy to repeat the experiment on a small scale. The glass tube which we see on the table (Plate VI b) contains air which has become charged with moisture which it bubbled through water on its way to fill the tube. The road by which it came is now closed by a tap. At the other end of the tube is a second tap, which at the moment is closed and cuts off the tube from a connection with a vacuum pump. If this second tap is opened, the air in the tube expands, and at once a white mist fills the tube. A beam from the lantern passes down the tube and lights up the mist. We may allow the air to be drawn off by the vacuum pump: and we can repeat the experiment as often as we like. Every time that we fill the tube with damp air which has been filtered from all suspended particles of dirt and smoke, we get the same sort of white mist that we see sometimes in the clean country. But if we allow the air from the room to flow directly into the tube without being filtered, the expansion produces a dense fog, such as London air is ready to produce at any time, as we know only too well.

There are other properties of a gas which the billiard table will help us to understand. Let us mix with the ordinary billiard balls a number of light ping-pong balls, and set the whole lot in motion. We see at once that in the general movement the ping-pong balls acquire greater velocities than the others. Just so if a gas contains two kinds of atom, one heavy and one light, the latter, in the constant interchange of motions, acquires a higher average speed than the former. When hydrogen is mixed with oxygen, the hydrogen molecules actually move four times as fast as the oxygen molecules on the average. A calculation, into which we do not enter, tells us that atoms which mix with one another all possess the same average energy, the lighter making up for their deficiency in weight by an excess of velocity. Even if the gases are not mixed, but are contained in separate vessels, the same rule holds, provided that the gases are at the same temperature. Although the atoms of the two gases cannot interchange and balance their energies directly, they do so in effect by way of the various kinds of matter which reach from one to the other, by the walls of the vessels, by the table on which perhaps they both stand, and by the atmosphere. In fact, the average motion of the atom is fixed by the temperature.

We can easily find an illustration of this effect. Sound is a movement which is handed on from atom to atom in a gas through which the sound is passing, just as a chain of workers pass buckets of water to a fire. The quicker the workers move their hands and arms, the quicker the water moves. Just so sound travels faster the greater the velocities of the atoms; or, what comes to the same thing, the lighter the atoms. An organ pipe blown with coal gas gives a higher note than when it is blown with air, because the molecules of the lighter gas move more quickly and the vibrations of the pipe are more frequent. A simple experiment will help to make this clear. On the table is a glass jar, into which water has been poured until the air column which is left responds loudly to the motion of a tuning-fork held over it (Plate VI a). The air waves pass up and down the jar in exact time with the oscillations of the fork; the natural period of the jar is the same as the note of the fork, as I can tell by blowing gently across the top of the jar and so drawing a whispering note from it. I now introduce gas into the jar through an india-rubber tube, and the response fades away. Movements now pass up and down the jar more quickly, and the natural period of the jar is no longer that of the fork. If I now pour out some of the water in the jar and begin again with a filling of air, there is no response to the fork until I put in a certain amount of gas. Then the note swells out as soon as the mixture is such that the timing of the gas movement in the jar agrees with the periodicity of the tuning-fork.

Let us imagine, again, that a very small hole is made in the walls of a vessel which contains a gas. Every time that an atom, or molecule, as the case may be, strikes the hole it passes out and never comes back. It is clear that a light gas will leak away more quickly than a heavy gas, because its atoms are moving about in the vessel at a greater rate, and a larger number will strike the hole every second. This effect is frequently employed to separate two gases, when other means are ineffective. For example, Rayleigh and Ramsay used it to separate argon from nitrogen, the mixture of the two being the residue of the atmospheric air when all other gases had been removed. The mixture was made to flow along a series of clay tobacco-pipe stems, and the nitrogen leaked out through the pores of the pipes more quickly than the argon. The argon atom is forty times as heavy as the hydrogen atom, and the nitrogen molecule twenty-eight times; the nitrogen therefore leaks away more quickly through the porous clay walls of the pipe stem, and the gas that issues from the other end of the pipe system is richer in argon than the mixture which entered it. The process of diffusion of one gas into another is really the same in character, because the gaps between atoms or molecules of one gas are to be compared with the pores in the clay pipe. Diffusion is a very slow action, in spite of the fact that the atoms are moving so quickly, the reason being that encounters are so numerous. One is apt to think that one gas diffuses into another quickly: on such evidence as that, if a gas tap is left open, the smell of the gas is quickly perceived all over the room. This dispersion is, however, due to convection currents rather than to diffusion, streams of the house gas running in concentrated form through the air. The effect is beautifully seen in the mounting of smoke from a cigarette (Plate VII). If the cigarette is laid down on the ash-tray a fine stream of blue smoke arises in a waving pencil, which becomes spread and bent and twisted into delicate spirals and curved surfaces. As the stream mingles finally with the air, it gives an example of convection. Diffusion between the smoke-laden air stream and the pure air is taking place also all the time; but the process is so slow that the edges of the clouds remain sharply defined for a long time. So also when a room is warmed by hot air, the distribution of the heat is due to convection currents, streams of hot air percolating through the cold.

Cigarette smoke.

A. The smoke of the cigarette is rising in a straight column. The air all round is being dragged up with it, though the photograph cannot show it: there is a rising stream of which the smoke marks the centre.

B. and C. Here the cigarette has been suddenly moved just before the photograph was taken. The streams of rising air that first met at the cigarette point and went up side by side are rolling and twisting over one another, preserving their identity. They do not mix, except very slowly by means of the diffusion of particles from one stream to another, though the particles are in motion, otherwise the smoke would be a sharpless mass. Meeting streams of air become mixed by convection rather than diffusion.]

It is not due to the molecules of the hot air making their way individually through the molecules of the cold; that goes on, but its progress is slow. Convection is more effective than conduction.

Movements of bodies of hot gas in a cold are, of course, governed by the laws of gravity: the lighter body, if it keeps together, tends to rise in the heavier. The smoke of the cigarette rises because the air over the glowing end is warmed and made light; colder currents run in from all round and, meeting one another, rise together round the thin sheet of smoke. The thin sheet is their joint boundary; if the air is still and the currents are steady the upright column grows long; but a slight movement of the cigarette upsets the even flow, and the column breaks into beautiful curves. The action of a chimney in the formation of a draught is, no doubt, known to everyone; but it may be interesting to look again at an old experiment of Faraday’s. A little spirit on some tow is lighted and held over the mouth of the shorter limb of the bent tube in the figure. By blowing for a moment the flame is made to go down the short limb and up the long one; once started on the downward direction, it does not change when the blowing stops. The flues of grates in hospital wards are often built on this plan, the draught going under the floor. The movement is, of course, due to the fact that the hot air in the long limb is lighter than a corresponding volume of the air outside.

The reverse happens sometimes in the house when the chimney is colder than the air outside, and a down draught brings a sooty smell into the room.

Something nearer to a conduction process takes place when the gas in a vessel is heated through the walls. The molecules when they come to the wall in the course of their movement receive impulses from the vibrations of the solid material, somewhat in the same way as the ball in the figure receives a violent knock from the vibrating prong of the tuning-fork.

When Sir James Dewar designed the “vacuum flask” to hold his liquid air, he made a double-walled glass vessel and extracted the air from between the walls. He left no molecules to pick up energy from the outer wall and carry it to the inner.

The pith ball is hurled violently from the vibrating fork.]

No heat could be conveyed to the liquid air by conduction or by convection. Heat can also travel by radiation through the ether; but this can be stopped by silvering the glass surfaces inside the double walls. When this is done the isolation of the air is almost perfect.

The entire independence of the atoms or molecules of a gas gives it perfect divisibility.

Observe the tube at the bottom (now scaled off) through which the air has been drawn from between the inner and outer glass of each flask.]

When we cut a solid body with a knife we have to exert a force to tear the molecules from one another; but such binding forces in a gas are negligible. If anything is moving through the air it experiences a resistance only because it is necessary to set some of the air in motion, and this requires the expenditure of energy. Gases are light, of course, and the energy required to move them is correspondingly small. The lightness of the air and the ease with which we pass through it make it easy to forget both how great is the pressure of the air at the surface of the earth and how weighty is the air in any large space such as this room. The air exerts a pressure of about a ton on every square foot of our bodies; that we do not collapse under it is due to the fact that any air within our bodies is at practically the same pressure as the air outside. The little rubber figure in the illustration (Plate VIII a) collapses utterly if its air content is withdrawn. The thin tin vessel shown in Plate VIII b has at first contained some water which has been raised to the boil so that the steam has driven out all the air. The opening by which the steam is issuing is closed, and some cold water is then poured over the vessel. The steam within condenses, and the pressure falls to almost nothing. The tin vessel then crumples up under the pressure from outside. Perhaps the magnitude of air pressure is brought home to us in an even more striking way when we consider that an iron bar, one square inch in section and nearly five feet long, when resting erect on the table, exerts of its own weight no more pressure on the square inch of the table with which it is in contact than the air does.

If we take into sufficient account, therefore, the weight of the air, it is not surprising that it takes a great force to set it in swift motion, or that, when moving rapidly, it can exert great pressure on any body which stands in its road. We all are familiar with the pressure of the wind, and know what havoc a gale may cause. So also the revolving airplane screw drives back a mass of air at a high speed, and the large force of reaction gives the necessary speed to the plane. Again, when it is on the wing, the plane, like a bird, is continually tending to fall and to carry with it masses of air beneath and around it. But it takes force to set such air masses in motion, and the reaction gives the uplift to the plane. If the plane had no forward motion it would quickly create a downward movement in the air below it, and fall with it; but it is always riding on to new masses of air which have not begun to fall. A simple little experiment illustrates the point.

A. The india-rubber figure collapses when connected to the vacuum pump.

B. The water in the tin was boiling vigorously when the Bunsen burner was removed and the stop-cock closed. When cold water was poured over it, it collapsed.]

A piece of paper of any convenient size, say three inches by one, is launched as the figure shows; it turns over and over and reaches the ground along a sloping path. The direction of the turning is related to that of the sliding, in the same way as that of a ball running down the tinder side of an inclined plane.

The explanation is that the leading edge of the paper runs on to new air which has not begun to fall, whereas the following half of the paper is on air which has started to move downward because the leading edge was lately resting on it. So the following half falls and the leading half does not, which makes the paper turn, as the illustration shows, until the paper begins to move forward again. But now the edge which was following becomes the leading edge, and vice versa.

The paper turns over and over, its very simple shape being the cause of the motion. Now, a bird or an airplane when gliding moves in a steady, stately fashion, and the design of the wing is in reality anything but simple, as builders of airplanes have found. The exact form of the plane--it is not indeed a plane at all--and especially of the leading edge, is full of subtle importance.

A bird’s wings are used not only for gliding, but also for flapping, and there is a beautiful mechanism which adapts them for their special purpose. The wing is, in fact, a set of valves, which open when the wing rises and close when it descends; so that there is less pressure on the wing in the up stroke than in the down. The action is something like that of the webbed foot of a duck, which opens out and exerts a greater pressure on the water as the foot is kicked back than when it is being drawn forward; but the mode of action is quite different. The rib of the feather does not always lie in the center, but is often well to one side, and a row of feathers is so arranged that they overlap and turn somewhat on their ribs. When the wing is lifted they open like a louvered window and the air passes through; when the wing is forced down they close, pressing tight against one another.

On the down stroke the gull’s wing has turned over, showing the under side, thus giving a shove forward.]

The two drawings of Fig. 9 a are adapted from Otto Lilienthal’s _Birdflight_, p. 101. They are sections of the condor’s wing. As Lilienthal says, “Every observer of the flight of storks knows that one is able to periodically see through the wings.” Even the countless parts of each feather partake in this valve action. It is clear that with such a mechanism the mere flapping of the wings must give an uplift apart from all other characteristics of the motion. The forward thrust is due to the bending of the wing about its stiff leading edge, as may be seen from the two drawings of the wings of a gull in flight (_Birdflight_, p. 96). They were made in the sunshine: as the wing rises, the hinder parts are turned down and show the bright upper surface; as the wing descends, it twists so as to show the darker under surface. It must also be true that even if the wings are held outstretched without motion there will be an uplift if the air is full of little motions, swirls, and quiverings. The wonderful gliding of birds that travel for miles without a movement of the wings or any apparent effort may conceivably be connected with this effect; it is said that it does not take place when the air is perfectly still.

A very pretty example of the laws of the dynamics of the air is to be found in the swerve of a spinning ball; it is likely to interest most of my readers. We see it and make use of it in nearly every game, though perhaps the golf ball shows it most, because its speed is greatest. Suppose that the golfer “slices” his ball: instead of pursuing a straight course in the direction in which it appeared to be struck, it curls away to the right. The ball is then spinning; the front of the ball is going from left to right of the golfer as he gazes after it, the back of the ball is going the other way. It is clear that he has not hit the ball truly, but has drawn the head of the club across the ball; perhaps he pulled in his arms at the moment of striking and did not follow through properly. As the ball moves forward there is a dense cushion of air in front of it which has not had time to get away. If the ball is spinning as supposed, the left-hand side (the golfer being the observer) is spinning forward in the direction of flight, the right-hand side is moving the opposite way, so far as spin goes. The consequence is that through friction the air on the left-hand side is carried forward more than on the right, and the cushion of air in front of the ball is denser on the left than on the right. Consequently the ball swerves to the right.

The long carry of a golf ball is always due to spin of the proper kind: the stroke must be so made that the ball is turning about a horizontal axis, the lowest part of the ball moving, so far as it is due to the spin, in the forward direction. This makes the ball tend to rise as it flies. Sometimes we see it actually take a curved path which is convex to the ground. If it were not for this action the ball would not go half the distance that it does.

The shading shows where air is piled up by the ball as it spins and moves forward, and this accumulation makes the ball shear off to the right.]

If there were no air at all, we may observe, in addition, the carry of the ball would be two or three times greater than the normal, because its resistance to high-speed ball is so great. We should drive a ball much farther if we could avoid the resistance from the air; but as we cannot do that, we take advantage of what may be done by giving the ball a spin.

The flight of a Rugby football, like that of fast-spinning golf ball, is often curved upward, when the kick has gone rather under the ball and though, of course, it is most obvious when the kick is against the wind, yet I think it can be seen in still air. In tennis the player often draws his racquet over the top of the ball, giving it the opposite kind of spin, the top moving forward faster than the bottom. This makes the ball duck, so that, though hit very hard, it keeps in the court after passing the net.

Fig. 11.--Regular flight of golf ball: a pocket of condensed air, shown by the shading in the drawing, keeps the ball up.]

On the right, section of racquet and ball before the stroke, arrow showing direction of motion of racquet.]

Heavy balls swerve less than light balls going at the same speed; yet we all know the swerve that can be given to a cricket ball, and the swerve that the pitcher can give in baseball, is a marvelous spectacle. Of all ways of studying the effect, the simplest, perhaps is by the use of toy balloons which nowadays are toughly made and will stand much knocking about. It is easy, by striking with the hand, or a racquet if preferred, to give any sort of spin that is desired and to observe all sort of swerves and soaring and “dooks.”

The various examples of the properties of gases which we have been considering are all to be explained, as we have seen, on the hypothesis that some kinds of atoms have very little tendency to associate with other atoms, whether of the same or of other kinds: I have spoken of them as the “unsociable atoms.” Other atoms, again, such as hydrogen or oxygen, though very sociable individually, tend to form more or less unsociable molecules. Thus the air consists of a mixture of unsociable atoms and molecules: there are molecules of oxygen each consisting of two atoms, and molecules of nitrogen analogously constituted, a few molecules of carbon dioxide, each consisting of one atom of carbon and two of oxygen, a certain number of single atoms of argon, and probably small percentages of other gases. All of them form gases because of the lack of tendency to associate; the independence which they possess in consequence, together with their motion, furnishes a ready understanding of their behavior.

We now ask ourselves whether we see any way of connecting the properties of these atoms with the general idea of atomic structure which was put forward in the last chapter. How is the sun and planet conception to be connected with these tendencies to associate, or not to associate, with the formation of molecules having similar tendencies, and so on? To answer these questions fully would be to give an account of chemistry so far as is known, and that is clearly beyond our intentions. But there are certain simple rules which, though they cannot be explained, and though they are often broken in appearance, provide a most useful thread on which to string our facts. Let us go back to our unsociable atoms, Nos. 2, 10, 18, 36, 54, 86. The first thing that strikes is that there are curious connections between these numbers. If we write down the successive differences we have 2, 8, 8, 18, 18, 32. The numbers 2, 8, 18, 32 are twice the squares of 1, 2, 3, and 4. We have already seen that the difference between the various kinds of atoms is simply one of number. I have not attempted to explain the experimental and theoretical proofs of the numbers of electrons on the various atoms: they are complicated, while the result is simple and sufficient for our purpose. Since the number of the electrons on the atom, or rather the number which expresses the positive charge on its nucleus, is in itself of such unique importance, we cannot but think that there must be something underlying the curious numerical differences we have just observed. The probability is greatly increased when we consider the question from another point of view.

Chemists have long discovered and pointed out that remarkable analogies exist between the properties of different kinds of atoms. For our purpose it will be convenient to express their discoveries in terms of the numerical relations. We write down some of them in the following way. We take first the eight atoms, in order of number, which begin with helium; and put under them the next eight beginning with neon. We thus have, continuing the arrangement up to No. 20 (see Plate IV b for rough models):

Helium. Lithium. Beryllium. Boron. Carbon. Nitrogen. Oxygen. Fluorine.

2 3 4 5 6 7 8 9

Neon. Sodium. Magnesium. Aluminium. Silicon. Phosphorus. Sulphur.

10 11 12 13 14 15 16

Chlorine. Argon. Potassium. Calcium; and so on.

17 18 19 20

We have, in fact, written down a portion of the “periodic table.” It is so set out that the atoms helium, neon, and argon, which so closely resemble one another in their main property of unsociability, are in the same column. It then appears that lithium, sodium, and potassium, which also closely resemble one another in their properties, are in the next column, and that the same remarkable classification runs across the page. The mutual resemblances of the substances in the same column are manifested in innumerable ways: they form one of the great features of chemistry. The very name “periodic table” was adopted as a description of the fact.

Now it is reasonable to suppose that the properties of an atom as manifested by its relation to any other may well be determined by some arrangement of its electrons, and especially of those which are most on the surface and are first presented to the other atom. Thus lithium, sodium, and potassium probably behave alike, because they have all the same external presentment of electrons; so with carbon and silicon, with fluorine and chlorine, and so on.

Such considerations have led to the following hypothesis. Let the two electrons of helium be arranged as a pair symmetrically placed on either side of the helium nucleus. Let every succeeding atom have the same arrangement, and, in addition, a further arrangement of electrons on an outer shell. Thus lithium has two, like helium, and one as a contribution to a new outside grouping. Beryllium has two in the outside group, boron three, carbon four, nitrogen five, oxygen six, and fluorine seven. We will suppose that the list of additions to this list closes with neon, and that in all atoms of higher number the inside group of two and the just completed group of eight are retained, the extra electrons taking their place in new groups. Thus, sodium, like lithium, has one in its outermost group; magnesium has two, like beryllium; and so on. Chlorine, like fluorine, has all but completed an outer shell of eight; while argon, like neon, has completed it. With potassium still another group begins; calcium has two in this newest group, and so on. The last group is not complete until it contains eighteen electrons; so chemical evidence tells us. But we need not pursue this question further, especially as it becomes more complicated.

Arguing in this way, we understand why the members of the same columns should be alike in their properties. We then ask what the particular properties of an atom have to do with the particular number of electrons there are in its outer shell, this number being the same in all members of the same column. To this question also we can find some sort of answer which we can best state in the following way. From a general consideration of the vast accumulation of chemical knowledge regarding the tendencies of atoms to form combinations, and under proper circumstances to dissolve existing combinations and form new ones, certain rules appear which are directly connected with the numbers of the electrons in the outermost groups. In the first place, there is always a tendency to fill up the vacant places of an uncompleted group. Thus if chlorine had one more electron in its external group, that group would be completed in the sense that no more additions are made to it as we pass from atom to atom in the succeeding portions of the table. Consequently chlorine is on the search, so to speak, for the electron which it lacks, and may exert great powers in dragging it away from other atoms which are not holding on to it with sufficient energy. It is true that the atom’s electricities thus become unevenly balanced: the extra electron gives it a negative charge. But in spite of that there is some force, we do not understand its origin, which works for the completion of the external shell of eight electrons. It is, in fact, this power that chlorine possesses of dragging to itself an electron from other atoms, and upsetting their combination in order to get it, which makes the substance so actively poisonous. In the same way, sulphur has two gaps to fill up, and its behavior is largely governed by that fact.

On the other hand, lithium, sodium, potassium have in each case a group just in process of formation: there is so far only one electron in it. The hold upon this electron is feeble, and when a chlorine atom demands it, the electron changes hands at once. The result of the transfer is that the external group of each atom is now a completed group: the chlorine is like argon externally, and, if sodium be the other atom, it is now like neon. Both the atoms are now charged with electricity: the chlorine is negative because it has one electron in excess of its proper number, while the sodium atom is positive because it has one too few to make the balance between the positive charge on the nucleus and the negative charges of the electrons that are left. In consequence, there is an electric attraction between the two atoms: they have now formed a molecule of ordinary salt. Sodium is a soft white metal. As we shall see later, the distinguishing characteristic of the metals is their possession of one or two electrons which can be easily torn from them. In this combination the white metal and the poisonous gas have joined to make the transparent crystalline salt. It is a violent change of character; but it will not surprise us if we remember that the arrangement of the electrons on the outside of the molecule must be quite different from the arrangement on either of the atoms before they become partners, and that the character of the atom or the molecule depends on this arrangement.

There are innumerable examples of this kind of combination. As one involving rather more complication we may take calcium fluoride, which as a crystal goes under the name of fluorspar. Here two atoms of fluorine, each lacking one electron (see the table p. 82), join in an attack upon calcium, which has two electrons in its external group, and each of them takes one electron into its own system. The molecule therefore contains three atoms. Or, again, in alumina, which in crystalline form makes ruby or sapphire, we have two atoms of aluminium, each forced to give up the three electrons in its external group for the benefit of three oxygen atoms, each of which takes two.

Besides this give-and-take arrangement there is another method by which atoms seek to complete their external groups: they may share electrons with one another, each being capable, apparently, of counting them in its own structure, just as two houses may have the same party wall. Thus two hydrogen atoms, each contributing one electron, combine so as to possess a group of two, as helium does, and thus the hydrogen molecule is formed. Two atoms of oxygen enter into combination, and form the oxygen molecule in which each oxygen atom is surrounded by eight electrons, of which four are held in common by both atoms. In the diamond, as we shall see, each carbon atom is surrounded by four other carbon atoms, with each of which it shares two electrons. So each atom is provided with an external shell of eight electrons, none of which it has entirely to itself. This kind of combination is generally very strong, and molecules so formed hold together well. Moreover, many molecules formed in this way are, so to speak, satisfied with their own company: there is little tendency to associate with other molecules. They tend to form gases. But on the whole the most permanent gases are those which have naturally the completed external shell—helium, neon, argon, and the rest. They show, most fully developed, the gaseous properties which we have been considering as the result of the weakness of tendencies to associate and the undisputed sway of movement.