Chapter VI: The Nature of Crystals: Metals

THE use of metals has been one of the great factors in the development of the activities of the human race. The beginning of the story is so far back in the ages that we can only make guesses as to how men first made metal tools and weapons. Perhaps copper was picked up in its native state, and its weight suggested its usefulness in a fight. Copper is too soft to take a cutting edge, and it may not have been very long before it was found that there was an alloy of copper and tin which was far harder and more serviceable than copper alone. Perhaps there was tin in the stones of which the copper smelting furnace was built, perhaps copper and tin occurred together in the same mineral. And so the age of bronze set in. Iron came later, of course. From that time to this there have been workers of metal--important members of the human community. We have but to think of the magnitude of the metal industry in this country alone, to realize how great a part the metals play in the life of the world.

In all these thousands of years a vast body of experience has been gained. Some of it is in books, some of it is still a tradition handed down by the skilled workman to the apprentice. There is even a sense of the nature, or condition, or property of a metal which cannot be put into words, and is only taught by example to such as have the power to understand. Nor is this any trifling matter: the whole movement of trade and the welfare of a nation may rest upon it.

On the other hand, the properties of the metals must depend, in the first place, on the properties of the individual atoms, and, in the second place, on the atomic arrangement, which is in effect the state of crystallization. In the very centers of the metal industries it has been realized of recent years that the scientific observer with his microscope can bring some system into the mass of disordered knowledge, and can improve the quality of the manufacture and the certainty of its production.

Two photographs of aluminium: the surface has been prepared so as to show the different crystals. The crystals scatter the incident light differently because they are set in different ways, and the surfaces exposed by treatment are therefore different in nature, in one figure the scale has been reduced somewhat from the natural size, in the other considerably enlarged.

(From a paper by Carpenter and Elam, read before the Institute of Metals, Sept. 1920.)]

Yet, as I have said already, the microscope can only go to a certain length: it stops far short of the point which we must reach if we are to understand how the atoms are acting so as to give the various materials their specific properties. It can show the existence of the separate crystals in the metal, but not the arrangement of the atoms in the crystals (Pls. XXII, XXIII). In the X-rays we find a new hope; indeed, it is more than a hope. We may be sure that the intimate knowledge which they give us will in the end throw a flood of light upon the inner meaning and purpose of all the complex properties of metals. It may be a long time before the new movement will become great and obvious. The experience of thousands of years has to be caught up with and explained. It is quite otherwise with such a subject as electrical engineering, or wireless telephony, which is a branch of it. Here the whole process is rooted in the work of the physics laboratory, and development has been directed by knowledge and anticipation. The worker in metals has been guided through the ages by trial and error, by experiment with little knowledge to guide it. It is a very slow process; but it has been going on a long time, and its findings command respect. They must be studied very carefully in the new light which the X-rays give us.

Already we begin to find explanations, as we may call them, of some of the properties of the metals. They depend upon the crystalline structure, as we might have expected. Sometimes the crystals are to be seen by the naked eye; sometimes they become obvious when the surface is properly prepared and placed under the microscope. But the easiest and most complete way of discovering them is by means of the X-rays, with their fineness of vision.

The structures of almost all the metal crystals have been determined by the X-rays, and it appears that they are usually very simple. For instance, the atoms of gold, silver, copper, and aluminium are put together like the piles of round shot that used to stand beside the guns of a hundred years ago. It is worth while to look a little carefully into this arrangement, although we are really repeating the comparison (p. 189) between the structures of ice and diamond.

(By courtesy of the University of London Press, Ltd.)

The photograph shows the irregular outline of the crystal grains in a sample of steel, and, in addition, a cross-crossing of lines within each grain, known as “Widmanstätten lines.” These last are due to the fact that the polishing tool has rut across the “atom layers,” in much the same way as polishing cuts across layers of mother of pearl.

(From Belaiew’s “Crystallisation of Metals.”)]

Suppose we put together a number of balls into the triangular arrangement of Fig. 53, and surround them by a triangular guard, as shown, just as balls are packed together for playing pool on the billiard table.

We lay on these another layer, forming a triangle a little smaller than the first, and again other layers until the triangular pyramid is finished (Plate XXIV b). Obviously there can be no closer method of packing round balls together. Now if we look into the arrangement of the layers one above the other, we find that the balls in any layer are exactly over the balls in the next layer but _two_. In the absence of a model this effect may be realized by the help of Fig. 54. The crosses represent the centers of the balls in a certain layer, the circles the centers in the next layer, and the black spots the centers in the third. The centers in the fourth will be over the crosses, in the fifth over the circles, and in the sixth over the spots, and so on.

When balls are arranged in this way, it is possible to cut cubes out of the assembly, as in Plate XXV a. It is always a surprise when this fact is first realized, but it is well to understand the cause of it, because so many crystals are made up of atoms piled together in this way, and they so often grow as cubes or in some way show their close connection with the cubic form.

Now if, when we have laid down two layers and come to the arrangement of the third, we place it so that each ball is exactly over a ball in the first layer, which arrangement is the only alternative to the one we chose before, we have another way of packing the balls which is as compact as the other. In this case, the balls in any layer are exactly over the balls in the _next layer but one_, and Fig. 54 a will be replaced by Fig. 54 b, and the arrangement of Plate XXV a by that of XXV b. This arrangement will not stand up now without containing walls, if we are to have a reasonable number of balls in the model; we must pin them together in some way. When we look down on this model from above, we see six-sided tunnels running through it, and we do not sec any arrangement of this kind when we look in any other direction.

A. Small groups of shot are in close packing, and there are irregular gaps between the groups.

B. A pyramid, built by the super-position of layers like that of Fig. 53.]

The model has a single axis in the vertical direction, and round that axis the arrangement is such that a crystal built on this plan would naturally form hexagonal columns.

In the case of the cube there are four ways of thinking of the arrangement of the layers; there is a layer perpendicular to each diagonal of the cube, and, as a cube has four diagonals, there are four sets of layers. This does not mean that the atoms in any one layer are specially tied together in that layer; merely that one can sort out the atoms of the crystal into this kind of layer in four different ways.

Now it turns out that these layers are of very great importance in respect to the properties of the metal crystals built on the close-packed cubic plan. Gold, silver, copper, aluminium, and other metals like them can be drawn into wires, rolled into sheets, and beaten into various shapes. They are, as we say, ductile, and their ductility is one of the characteristics that make them so useful. They can be bent and pulled into all sorts of convenient forms. It seems possible to make a metal flow like treacle. Gold can be hammered into leaves so thin that the metal in a sovereign will cover a large field; the others can be beaten nearly as thin.

A. Cubic packing. B. Hexagonal packing.

(From Pope’s “Modern Aspects of the Molecular Theory.”)

A shows how balls are packed together to form a cube. It is exactly the same packing as in Plate XXIX B. The close-packed layers of Fig. 53 are horizontal in Plate XXIV B, and in A they are perpendicular to a diagonal of the cube.

B shows the other form of close packing. Each horizontal layer is a close-packed layer of Fig. 53.]

Cups and vessels of all sorts, chains and ornaments, and innumerable useful things are made by taking advantage of this singular property of ductility. The first thing that we should like the X-ray to explain for us more clearly, if we can make them do so, is that feature in their structure which accounts for this most valuable property. We should also like to understand the inner meaning of the hardening and other changes that are due to “cold-working,” as it is called--that is to say, to hammering or straining the metal when it is cold. And what is annealing, the softening and relief from strain that heat brings about? Why are all these things so obvious in the case of a metal, while they do not appear in, for instance, diamond or rock salt or quartz?

Already we begin to see some little way into these difficult questions; and in particular we have found out something about the way in which the metal yields to a pull or any other strain, and have learned that it has to do with the layers of which I have spoken.

A metal is rarely one whole crystal: it is, in general, an assemblage of crystals, pointing in all directions. Sometimes these crystals can be seen easily; sometimes the microscope is required to show them. Very often they are too small even for the microscope, and the X-rays alone can make them clear.

If we put a number of shot on a tray and let them all run together by tilting the tray slightly (Plate XXIV a), we observe that there is a tendency for the shot to arrange themselves like the balls in Fig. 53. It will not often happen that all the shot will form one arrangement: there will be groups, each properly arranged in itself, but not correctly aligned with its neighbors. In just the same way there will be local arrangements among the atoms of a metal--in other words, there will be crystallization in groups, larger or smaller, the connection between the groups being somewhat irregular. We may observe at once that the connection between group and group is not necessarily any weaker than the connection between the atoms in any one group. Why this is so, it is difficult to say. We need not be surprised at it, because the ties between atoms are complicated things, imperfectly known to us, and we cannot predict accurately what will happen in every case.

It is said that when gold is at a high temperature a fracture cuts through the crystals, but when the gold is cold it goes round them; and this will illustrate the complexity of the effect.

A block of one of these metals may reasonably be expected, therefore, to consist of a mass of crystals, large and small; and this is exactly what the X-rays show to be the case, even when the microscope fails because the crystals are too small for it to see.

Now when we take a single crystal and try to bend it or distort it, we find always that it gives finally through a slip along a plane: all that is on one side of the plane slipping with respect to all that is on the other. These planes are the planes we spoke of before--those that contain atoms arranged as in Fig. 53. A single metal crystal does not give way exactly in the direction in which it is pulled. If we had two blocks of glass, let us say, held together by grease as in Fig. 55, and pulled them, they would give along the plane between them.

Naturally they will slide over one another on this plane rather than themselves be torn to pieces. In the case of a metal there is not merely one plane, but many planes, and many of them will be planes of sliding either together or one after another. We might represent the crystal by a set of lines as in Fig. 56 (_a_), which, if pulled in the direction of the arrows, would yield as in Fig. 56 (_b_). Often when a single metallic crystal has been stretched, we can see the marks on its surface which show the lines along which slip has taken place.

Professor Carpenter and Miss Elam have shown recently some beautiful examples of this kind of effect in the case of large crystals of aluminium. An ordinary piece of the metal consists of a multitude of crystals pointing in all ways, as we have already understood to be the case. By a somewhat complicated process of heating and stretching, the many small crystals can all be made to line up and form a small number of large crystals, just as we might imagine that by shaking or tapping the tray of shot shown in Plate XXIV a in some way, to be found out by experience, we could get all the minor regular groupings merged into one large one.

The pieces chosen for experiment were of a form often used for pieces to be tested for their resistance to pull; the form is shown in Plate XXVI; the original length of each piece is eight inches. The broad ends are intended to be gripped by the jaws of the machine that is to stretch the piece; the narrower part is that which is to give way, and to show by the way in which it does so, and the pull that is exerted, the capacity of the metal to resist the forces that would strain it.

When one of the test pieces so treated is put into the testing machine and pulled, it gives way in a curious fashion, which differs for different specimens (Plate XXVI, 1 to 4). Sometimes the width of the piece remains the same, and it thins out gradually as the test piece lengthens under the pull: it may grow longer by several inches before it gives way. Sometimes the thickness remains the same, and the piece shrinks sideways, developing a waist which finally is the place of breaking. At other times, again, there are more curious changes still. These pieces have been examined by the X-rays, and it turns out that the nature of the yield depends entirely on the way in which the large crystals are set toward the line of pull. The metal gives way along the plane of slip. If, for example, the crystal is so set--and there is no telling during the heat and strain treatment how the forming crystal will lie--that the layers of which we have spoken are as in Fig. 56, then stretching will make the piece draw in sideways.

Sometimes the direction in which the crystal gives way depends on a more complicated use of two sets of slip planes alternately. When the crystal might slip on more than one set of planes, it is apt to choose the one which is more nearly perpendicular to the line of pull. We can imagine that this is so because a slip means a riding of one set of atoms over another, and the motion would be helped by a force tending to pull one layer away from the other. If we had a solid body made up of a row of balls like the top layer in Fig. 57, and it rested on a similar row like the bottom layer, it might be easier to drag the top layer over the bottom if the line of pull were along _P_ rather than along _Q_.

Since (see Fig. 55) the pull always tends to bring the plane of slip more nearly into its own line, there arises a sort of seesaw action: the crystal slips along one set of planes until the set comes too nearly into line with the pull, and then along another. In the end the line of pull bisects the angle between the two sets. The balance is often shown in the shape of the broken ends; in Plate XXVI, 3, for example, there is a kind of knife-edge at the point of rupture. The two sides of the edge are parallel to two different sets of slip planes, and are equally inclined to the line of the pull which finally tore the metal in two.

In the case of the aluminium, the yield is so easy that a sheet of some thickness, when composed of a single crystal, can be bent quite easily by one’s fingers. An ordinary piece of aluminium sheet is quite stiff, however, and the explanation of the difference is that, when there are crystals pointing in all directions, there are some ready to take and bear the strain, no matter from what direction it comes. The strength of a chain is that of its weakest link, and the weak part of a crystal is its slip plane. This is a point of extraordinary importance in the manufacture of metal, though it is often linked up with so many others that its special effect is difficult to sort out from the rest. Many factors go to the design of steel, let us say, for some given purpose; but one of them is certainly the degree of fineness of the crystal grains of which it is composed. Fineness and uniformity of size both contribute to the toughness of steel and its quality generally.

These photographs an due to Professor Carpenter and Miss Elam (_Proceedings of the Royal Society, A._, 100, p. 346). They show the yielding of aluminium under strain, the part finally giving way being a single large crystal in the narrower portion of the test piece. In (1) and (2) the slip planes are disposed--to the observer--like the layers of Fig. 56. The test piece has contracted sideways, and finally developed a waist, as the picture shows. There is no thinning from back to front. In (3) and (4) the reverse has taken place before the break: the width has remained the same, but the material has grown thinner (this cannot be seen in the photograph).]

When the single crystal of aluminium gives way along a layer, we may suppose there is a moment when the one set of atoms is riding over the other, followed by a drop into place again: _A_ goes through _B_ into _C_ (Fig. 57). When the latter case is reached, the close-packed arrangement is resumed. The metal is still a crystal. Now, as we know, the regular crystalline arrangement is the natural one, and so the substance slips easily from one natural arrangement to another, adjusting itself to the pull or other strain by doing so. No doubt this is one of the causes, and a very important cause, of ductility.

But why does the metal often become harder when it is beaten? And what happens to it when it is annealed? Perhaps we are guided toward an answer by considering what happens to gold when it is beaten into leaf and subsequently heated. Gold leaf is very thin, as we have seen. It is even transparent, but it absorbs part of the spectrum of light that passes through it, allowing a greenish light to filter through. It is yellow when viewed by reflected light, as we know. It is very curious that when it has been heated to a dull red heat it becomes permanently transparent, and white by reflected light. Faraday was very interested in this fact; he suggested as a partial explanation that the thin layer of gold broke up, the metal gathering itself together in little heaps, and that the light went through the holes that were left. Sir George Beilby has made many experiments, and added considerably to the information we have in regard to the behavior of this and other substances when heated in the same way. If there are holes in the heated leaf, they are exceedingly small, he says, beyond the power of the microscope to see. Now the X-rays have something to say on this point. When gold leaf is examined by their aid, it is found that it consists of masses of cubic crystals of gold all lying with faces parallel to the leaf. They are not necessarily cubes, of course. They consist, like ordinary salt, which is cubic, of rectangular blocks of all sizes. They must be exceedingly thin blocks, and no doubt their thickness is far less than their width or length. When the leaf has been heated, the blocks are piled up anyhow, perhaps gathered together to some extent in heaps, as Faraday supposed, even if they are too small to be seen by the microscope; and perhaps this is the reason why gold and silver leaf become transparent when heated. Why gold should be green when looked through is a mystery. But we do see that the beating of the gold has spread out the crystal blocks so that they all lie with one face in the leaf surface, and that heat has destroyed this amount of regularity of arrangement. When the heated gold leaf is pressed with a body having a smooth, hard surface, such as an agate, it goes back to the other condition, as we might expect, since the pressure would force the blocks once more into the flat. In both cases the metal is crystalline, but there is more arrangement in the usual than in the annealed form of the gold leaf. The same effect is found with silver. To show it in the case of copper it would be necessary to carry out the experiment under such circumstances that the air could not act on the metal. When copper is heated in the open, a film of copper oxide quickly forms all over it, an action which also can be followed by the X-rays. We often see this tarnish form slowly on copper even when no heat is applied. But it is easy to show by X-rays that in copper foil there is the same arrangement of the crystal block as in the case of the gold leaf. On the other hand, a block of ordinary copper shows no such arrangement; the crystals are arranged anyhow.

The hardening of these metals by cold working is, therefore, due in some way to the fact that they are put into a state of strain by the rearrangement of the crystals which the X-rays show; annealing is the release of this strain and the destruction of the arrangement. As to why this is so, we are still very ignorant: we can simply be satisfied that we have made one step forward.

It is worth noting that, in general, when a metal has been thrown into a state of strain in this way, it is more readily subject to the action of chemicals, as we might expect. It is not so well settled into what we may call a comfortable condition.

We ought now to go on to the consideration of other peculiarities possessed by metals, since we may expect them all to be due to more or less the same causes and we must study them all together. Two of their most remarkable properties lie in their powers of conducting heat and electricity. We all know how quickly heat spreads through a metal: we might be inclined to say that a metal could be identified by its possession of that property. We all know, too, how metals, especially copper, are used as conductors of electric current.

Going back to our first consideration of the nature of the atoms, and of the differences between the various atoms, we find at once a feature which on the whole seems capable of giving us a satisfactory explanation of their conducting powers; no doubt, too, it has much to do with their crystalline structure and their ductility. The atoms of the metals always have one or more electrons which are lightly held. For instance, sodium has eleven electrons; two of these are very close to the nucleus, eight more form a very strongly held system round the first two. The odd electron belongs to an outer system altogether, which becomes filled up as we go from sodium to magnesium with two in the outermost system, aluminium with three, and so on. This odd electron is not held tightly. When it is stripped off for any reason, the atom is outwardly reduced to the form of the unsociable atom “neon,” except that as a whole it carries a positive electric charge due to the want of balance on loss of an electron. An aluminium crystal is an assemblage of spheres like neon, all in close packing, as explained, and all the odd electrons are more or less free to move about in the structure. It must be said, however, that this picture is doubtless much too crude to be the whole truth; there must be much more in the design of which as yet we know nothing. Yet it must be right to a certain extent. We see at once why metals are conductors of electricity: it is because the electrons, the fundamental charges of negative electricity, can move about so easily. When a current of electricity runs along a metal wire, it is the electrons that make the flowing stream. It is curious that they must move, being negatively charged, in the opposite direction to that in which the so-called current of electricity is always imagined to flow. It was always a matter of words, this talk of a flowing current of electricity. It is quite a new discovery that anything moves at all, and we need not be surprised that the real direction of flow is opposite to that which had been supposed.

So we must think of the battery or the dynamo, not as manufacturing electricity, but as sending round a circuit a stream of the electrons that are already there and are more or less free to move.

(By courtesy of the University of London Press, Ltd.)

Damascus blades.

(From Belaiew’s “Crystallisation of Metals.”)]

Just so the engine in a factory makes a leather belt continually travel round a certain circuit; but the engine does not manufacture leather.

When a metal is heated, the contained electrons dance more quickly to and fro, and may break away into the open. Electrons are pouring in a continuous stream from the hot wire in the “valve” of wireless telephony, and the outpouring is necessary to the action of the valve.

The electrons do not move so easily in a metal when it is hot as when it is cold. Here again it is easy to imagine how this may be. We can see that the electrons will have more difficulty in threading their way among the atoms of the metal if through heat the latter are moving to and fro and getting in their road. It is much more difficult to explain the strange fact, discovered at Leiden by Kamerlingh Onnes, that some substances when their temperature has been lowered to a certain very low point--a point which differs for different metals--offer no resistance at all to the movement of the electrons, so that a current once started will keep on running for days before it finally fades away, the metal being kept continuously at this extremely low temperature.

The electrons must to some extent contribute to the capacity of a metal for conveying heat as well as electricity, because the electrons at the hot end of a metal bar must pass on some of their excessive energy to the electrons at the cold.

Thus the presence of electrons in the metal, able to move with some freedom among the atoms of the structure, gives a very good reason why the metal conducts both heat and electricity. Of course it is only a rough picture that we have drawn; many details require to be filled in, and no doubt many really important facts have been left out altogether because of our ignorance.

Let us turn back to the question of the ductility of metals, and consider whether the presence of the electrons helps us here also. We now see our atoms as spheres, all of them charged with positive electricity and packed closely; and we may perhaps be right in thinking that the electrons hold them together like a cement.

(By courtesy of the University of London Press, Ltd.)

The long needle-shaped bodies are cementite crystals forming part of the general mass of steel. (From Belaiew’s “Crystallisation of Metals.”)]

But the most important point is that the atoms are not tied together by sharing electrons as in the diamond: they must rather repel one another than otherwise, being all charged with positive electricity. They are free to roll or slide over one another, because they are not attached to one another at definite points, as often occurs in other crystals. These things seem helpful when we consider the slipping of one plane over another.

So far we have been considering the crystalline structure and properties of a few of the metals in the pure state. Now in practice we meet with alloys far more often than with pure metals, and that for the reason that alloys have properties of their own of the greatest value. Alloys are, in fact, extraordinarily interesting in their immense variety and in the wide range of their usefulness. New forms are constantly being discovered. No matter what peculiar virtue may be required for some special purpose, an alloy of some sort is forthcoming, which satisfies the demand more or less completely. We must try to see some reason for these things, if we can, in what we have recently discovered. Of course, we know so little as yet, and there is so much to explain, that in a few years’ time we may think very little of our present attempts, but we must make a beginning.

It very often happens that the addition to a metal of quite a small quantity of a second metal, or even a non-metal, causes a notable improvement in hardness. Pure metals are generally very soft, because their slip planes are so ready to give. The first of all the great alloys was bronze, a mixture of copper and tin, which is far harder than either metal alone. The mixture of copper and zinc produces the serviceable brass, of which there are varying qualities, depending on the proportions of the mixture. Steel is formed by the addition of a small percentage of carbon to pure iron. There are alloys of copper and aluminium, which are very tough and do not corrode, but are difficult to work in the shaping machine. There is an alloy of copper and nickel, which does not corrode and is easily molded; it is used for the coverings of bullets. German silver is a white, ductile alloy, non-corroding, which is used in the manufacture of such articles as spoons and forks, which are afterward coated with silver in the process known as electroplating. An alloy of nickel with chromium stands very great heat, and is used for the wiring of electric furnaces.

(By courtesy of the University of London Press, Ltd.)

(By courtesy of Sir George Beilby.)

A. Section magnified a thousand times and showing the cementite needles in process of being broken up and rounded off.

(From Belaiew’s “Crystallisation of Metals.”)

B. The dark band is a scratch made by a very fine needle in a polished piece of speculum (mirror) metal, highly magnified. The fine vertical scratches are made by emery powder in polishing. Many small particles have been torn up and deposited in the trough made by the needle.

(From Beilby’s “Aggregation and Flow of Solids.”)]

Chromium, cobalt, and tungsten combined in definite proportions make stellite, an extraordinarily hard, non-corroding substance; some of the standard weights at the National Physical Laboratory are being made of it. There are alloys for the making of bells, very soft alloys for type metal, and a great variety of solders. There is the aluminium bronze, which is used for cheap jewelry and consists of aluminium with a small percentage of copper. And so on to a long list, if it were necessary to make one.

Let us take one of the simpler cases--for example, that in which the addition of a small quantity of aluminium to copper produces an increase in hardness. The X-rays show that the structure of the copper crystals remains the same, except that here and there an aluminium atom takes the place of a copper atom. Now the weakness of the crystal lies, as we have seen, in the fact that one part slides on another part along a certain plane. This plane is now no longer even: there is a scattering of aluminium atoms in it, and we can readily suppose that the slipping has become more difficult, and that we have here the cause of hardening.[5] There is a remarkable effect which makes us think we are right in supposing so. The atoms of aluminium must strain the structure of the copper crystal, because the copper will not take up more than a certain number. If an alloy is made containing more than about 10 per cent of aluminium, the X-rays tell us that the copper crystals are broken up altogether, and a new structure is formed.[6] The aluminium atoms must be distorting the copper crystal, and this fits in very well with the fact that it hardens the copper. On the other hand, when nickel is added to copper the atoms of the former replace the atoms of the latter to any extent: evidently they can slip into the places of the copper atoms without straining the copper crystals at all. And in this case there is no hardening effect, which is just what we should expect. It is only when we push in atoms which really strain the copper crystal and make its planes uneven that the hardening is brought about. We have jammed the sliding planes.

In A a piece of speculum metal, after being rubbed with fine emery, has been polished with rouged leather. The metal has been dragged over the emery scratches; there is a reminiscence of butter spread on bread. In B the polishing with rouge has been carried further; the emery scratches have disappeared, but the outlines of the grains in the metal begin to appear.

(From Sir George Beilby’s “Aggregation and Flow of Solids,” by courtesy of the author.)]

In the case of steel the action is of the same kind, but here the carbon atoms that are the cause of the hardening do not replace the iron atoms, but are forced into the empty spaces between them. We can easily see that this may distort the iron crystal, and as before prevent the movement along a plane of slip. Once again there is a limit to the amount of the alloying substance: only a small percentage of carbon can be introduced into the iron without breaking up its ordinary simple structure.

The problems of iron and steel contain, however, many more complications than this. We have only to ask what happens when more carbon is put in than the iron structure can carry, and we find we have a new problem. Among other things, a new crystal appears, formed of molecules, each containing three atoms of iron and one of carbon; it is known as cementite. The new crystals are very hard and unyielding, and in form are like needles (Plate XXVIII). Their presence hardens the iron very greatly and makes it difficult to work. A beautiful example of its effect on steel is to be found in the old swords that once made their way from India through Damascus into Europe. Damascus steel was greatly valued for the excellence of its qualities. Fine specimens are to be seen in the Wallace Collection; they show the characteristic wavy pattern (Plate XXVII) which has always been looked on as evidence of genuineness. When examined under the microscope the lines of the pattern are seen to consist of multitudes of dots, forming a sort of Milky Way in the steel. These dots are the tiny crystals of cementite. As Colonel Belaiew tells us, the steel when it was first made was most difficult to work. The smith, with his little furnace, would heat the steel red hot, but after he had struck but a few blows and made a slight impression on the steel, the momentary softening had gone. The hardness due to the cementite crystals had only been removed for an instant. More heating, a few more blows, and slowly the steel became less rigid. In fact, the cementite crystals were changing their form. They were becoming less like needles, gathering themselves together into more rounded shapes, and as they did so the steel became more pliable (Plate XXIX a). At last the fine Damascus steel was reached, so strong and yet so elastic.

In A the metal has been etched with acid: the “flowed” parts have been readily attacked and removed, the grains now show up very clearly. In B polishing has begun again.

(From Sir George Beilby’s “Aggregation and Flow of Solids,” by courtesy of the author.)]

It is very likely that much of the keen edge that these swords would take was due to the presence of the very hard particles embodied and held in the softer iron. The edge would be like a saw with extremely fine teeth. In the trial of skill between Saladin and King Richard which Walter Scott describes in _The Talisman_, the former threw a gossamer veil into the air and severed it by drawing his scimitar across it, a fine test of keenness and of skill. Richard, on the other hand, used his sword like an ax, and clove in two an iron bar, the mace of one of his knights. This also was a test requiring great qualities in the steel, but on the part of the man the skill lay more in the power to strike a terrific blow than in delicacy of touch.

Grinding, sharpening, and polishing are really very interesting operations. When we put a knife on the grindstone we let the hard crystals in the stone cut minute furrows in the steel, actually removing the material. This is one stage of the sharpening process. But the polishing on the oil stone or the strop is a different thing altogether. Here we actually make the steel to flow, smoothing down the furrow; sometimes, as Sir George Beilby has shown, actually drawing a skin of metal over the deeper hollows. The metal seems to remain crystalline all the time; the X-rays show readily the crystals in a razor blade. Probably the action is the same as that which took place in the gold leaves when they were heated. The oil that we use helps in the smoothing process. The metal is strained by the flow; in time the strain tends to come undone, and heat especially can take away the keenness of the edge (Plates XXIX b, XXX, XXXI).

An alloy is generally a much worse conductor of electricity than a pure metal. It may well be that when the stranger atoms are forced into the structure of the pure metal, and the planes of atoms are made uneven, the electrons are more hampered in their passage through the metal. More energy is required to force them along, and the metal becomes hotter through the passage of the current than if it were pure. In the case of a pure metal, as I have already said, the resistance to the movement of the electrons becomes greater if the temperature is raised. We can imagine that the electrons find it harder to get past the atoms when the latter are more active: heat makes them move to and fro more quickly about their proper positions.

A. The resistance of the pure metal coil in the same circuit as the shining lamp has been reduced by surrounding it with a freezing mixture.

B. The iron wire is stretched by a hanging weight; its expansion and contraction are magnified by the lever arrangement. The wire is heated by passing an electric current along it.]

But heat does not make so much difference in the case of alloys, because the passage of the electrons is already so difficult that heat does not make much change. We can show this by a simple experiment:

A battery sends a current round a circuit which has two branches, as in the figure (Plate XXXII a). One of them contains a coil of copper wire, M, and a lamp, L{1}, the other a coil of an alloy such as German silver, for example, and a lamp, L{2}. The coils are so adjusted in respect to the resistance which they offer to the passage of the electric current through them that the lamps both burn dimly. A vessel containing liquid air is brought up so as to include the coil, M, and the lamp, L{1}, at once burns brightly. The cooling of the copper wire has lowered its resistance to the passage of electrons, and more current flows through the lamp. But when the alloy is, in its turn, immersed in liquid air, no change is made.

Sometimes metals crystallize in more than one way. Iron furnishes one of the simplest and most striking examples. At ordinary temperatures the iron atoms are arranged so that each atom has eight neighbors. The latter are at the corners of a tiny cube, of which the former atom occupies the center. This is not the closest form of packing, as will readily be found on trial. The packing of the pile of shot of which I spoke before gives the closest packing, and in that each shot has twelve neighbors, six touching it round an equator, and three more round a line of latitude in each hemisphere: or, as we may put it, six in its own layer and three in each of the next layers. It is the packing of gold, silver, copper, and aluminium. It is very curious that when iron is heated to a cherry red the atoms change their arrangement and pack in the tightest form, that of the pile of shot. The effect is easiest to see when an iron wire is heated beyond this point and allowed to cool. When it comes to the critical temperature, the atoms suddenly adopt the looser packing and the wire stretches a little: the increase of length is easily observed by the use of some magnifying device. It is very curious, too, that when the old form changes to the new, some energy is set free and the iron suddenly brightens up again. The stretching and brightening have long been matters of observation, but it is only quite recently that we have discovered that the packing of the atoms into two different crystalline forms is at the bottom of what we have seen (Plate XXXII b).

These very few instances of the relation between the properties of a metal and its crystal structure are drawn from an immense subject, most of it still waiting exploration with our new helpers, the X-rays. We cannot say beforehand what will be found out. We can be very sure, however, that the better we understand our materials the better use we can make of them.

FOOTNOTES:

[Footnote 1: See “The Legacy of Rome” (Oxford University Press), p. 270--an article by Dr. Singer.]

[Footnote 2: From a Friday Evening Discourse before the Royal Institution of Great Britain, March 4th, 1881.]

[Footnote 3: For lecture purposes the working of the apparatus is illustrated by a kinematograph film which has been made for the purpose. It shows a series of successive expansions, each forming a new set of lines like those shown in Plate III.]

[Footnote 4: C. V. Boys, “Soap Bubbles and the Forces Which Mold Them.”]

[Footnote 5: Rosenhain, “The Inner Structure of Alloys,” Institute of Metals, May 2, 1923.]

[Footnote 6: Jette. Phragmen, and Westgren, Institute of Metals, March, 1921.]

NOTE

AFTER trials of many ways of making models of atomic structure and of many substances I find that two have real merits:--

Balls representing the atoms may be made of hard dentists’ wax, which softens in boiling water and can then be pressed into proper shape in metal molds made for the purpose, just as we used to remake our golf balls in the old days. The spherical mold is made in two halves; and it is convenient to mount them in the lathe, one on the head and one on the back center. Small balls harden at once, and can be made very quickly: larger balls must be left a little while in the mold. The hard wax can be drilled without becoming softened and deformed by the heat generated in drilling. The models made of the wax are very finished in appearance, and will stand all ordinary temperatures. The wax is rather costly.

Gramophone needles make good connectors, the balls, wax or wood, being drilled to receive them. The holes should be drilled true and in correct position. Convenient little contrivances can be made to be used for this purpose on the lathe.

THE END

=TRANSCRIBER’S NOTES=

A list of figures has been included to enhance readability.

Parentheses were applied to chemical compounds to prevent confusion with italicized text in the transcribed version.