Science : Yesterday and To-day

The following article is the first of a series not mainly intended to convey knowledge of particular conclusions that are being reached in various sciences—this will onlybe incidental—but rather to give some conception of the new modes of thought .and changes of method that are being developed with the extension of scientific knowledge, in a manner which is comprehensible and interesting to the lay reader. Next week Professor James Rice will write on Physics.

Crystallography : Old and New

BY SIR WILLIAM BRAGG.

NEARLY twenty years ago the study of crystals entered into a new phase. Whereas older methods had perform been content to deduce results from external form, a newly discovered method now made it possible to examine the details of internal structure. Lines of inquiry were thus opened up which were not only of great interest in themselves, but were also guides to the knowledge of Nature's principles of construction of all solid bodies. The change may be compared with that which came over Egyptology when the hieroglyphS were first interpreted. The language of crystali was discovered.

The fascination of crystals, apart from their desirability as rare gems, is derived no doubt from the regularity of 'their form. The faces can be perfectly smooth and meet in well-defined angles. If the crystal is transparent, as is so often the ease, the play of light reflected from the faces or refracted within the body of the crystal is the cause of a brilliancy which is most attractive. The effect is intensified if, by reason of its nature, the crystal has 'great powers of refraction, like the diamond which seems to shine from a dark corner because it diverts to the eye rays of light which have come front an unsuspected source ; or if some trace of a foreign substance gives colour, as to the ruby and the sapphire.

But the crystallographer long ago found a new and less obvious attractiveness. In the first place, the faces were inclined to each other at angles which were characteristic of the crystal, and were absolutely identical in different specimens of the one kind of crystal. However the crystal had grown, from whatever matrix it had separated itself, the angles were always the same. This suggested a marvellous regularity in the compilation of the atoms and molecules of which the crystal was composed. Evidently, the atoms ranged themselves according to a pattern which was rigidly followed, no matter what external forces were in action. This argued the possession by the atoms and the molecules of forms and forces characteristic of them and extremely constant.

The conception of regularity served also to explain a second remarkable observation. The various mutual inclinations of the faces were related to one another by simple numerical laws. The nature and significance of these laws is a little difficult to describe in words. Per- haps the reader will quickly grasp the point if he imagines himself placed in front of a sheet of paper covered over with a regularly repeated pattern, the unit of pattern being small, so that there are many repetitions of it upon the sheet. He is now to take scissors in hand and cut out of the paper a piece bounded by straight lines, the only

• condition being that each straight line must cut through several units of pattern, cutting each of them in exactly the same way—i.e., at corresponding points. It will be clear that the possible angles these lines make with one another are limited in number, unless we go to extremes and imagine the sheet to be infinite in extent, which would not be truly representative of the natural condition. Also there are connexions between the various angles made by the bounding lines with each other.

If the principle of this analogy is extended to the three dimensional cryStal, it will be clear that the dis- covery of definite relations between the mutual inclina-

tions of the various faces supports the view that a crystal has a pattern like the sheet of the analogy ; it must contain a regularly arranged system of atoms and molecules. Some small unit of pattern is constantly repeated in all directions in space. That unit is formed of a definite number of atoms, arranged in a definite way. There are millions of different kinds of crystals, but only some ninety atoms, of which only 'a few are much used, so that the difference between crystal and crystal is pre-eminently a matter of arrangement, and not merely of composition. Crystals differ from each Other not only in forni but in their reactions to mechanical forces and to all- kinds of agencies, optical, thermal, electrical and so on. Just as a weaver makes an infinite number of different fabrics, having widely varying pro. perties, from a limited number of fibres, so Nature forms the solid bodies of the world from atoms of only a few different kinds. The secret of the versatility is in each case the use of a pattern, and the infinite possibilities of its variation. The hardness of the diamond and the soft slipperiness of wax, the strength of steel and the yielding weakness of lead, the actions of a magnet and the strange optical properties of quartz, all such are closely connected with the ordered arrangement of the crystalline state.

The older crystalkigraphy advanced so far as to show that pattern and arrangement were fundamental facts : and there it halted at a most tantalizing point. The Pattern no doubt existed, but the unit was far too small 'to be seen even by the most powerful microscope.

The new advance began when Lane showed in 1912 that the X-rays could detect the arrangement in the crystal. The X-rays succeeded where light had failed. The success of the X-rays depends in the first place on a very im- portant natural principle, which can be explained in the following way.

The act of seeing a given body requires that some source shall radiate " light," which " light " we can justifiably and conveniently picture to ourselves as ether waves. The radiation falls upon the body and is thereby scattered and modified. Some of the scattered modified radiation reaches the eye : and the latter is so made that it can, with the brain's aid, perceive this radiation and judge, from what has happened to it, various facts about the scattering body. This is called " vision "or " seeing."

In order that this process may be effective the wave- length of the radiation must not be larger than the body to be perceived, simply because a small body cannot do much to a big wave. A swell may be turned aside, that is to say scattered and modified in 'form, by a rock, but not by a pebble on the beach. The reason why the eye cannot perceive the unit of pattern in the crystal is that the dimensions of the unit are far smaller than the wave- length of the shortest light wave that the eye can see. The failure cannot be amended by any improvement in microscopic technique because it arises from the nature of light itself.

It is a curious reflection that our eyes are not fashioned to perceive very small objects which may be of great importance to us. We can see the lion in the path, but not always the microbe, not even with the microscope's aid : and the microbe may be more deadly than the lion. Our eyes give us useful information about the size, form and position of, say, a piece of steel, but no optical means can tell us the regularities in the arrangement of the atoms in the steel, and yet those may determine the steel's usefulness. We have serviceable eyes, but they do fail us when certain kinds of information are urgently desired.

Now the wave-length of the X-ray is many thousands of times' less than that of light ; that is to say, than those ether waves which our -eyes- are • able to detect,

Even among the visible ether waves some are longer than the others. Red waves, being longer than blue, are less scattered, which is the reason • why red flares are used to penetrate fog. The blue waves are more fitted to detect fine particles than the red, for which reason they are largely employed in microscopic analysis. But the X-rays are in a different category altogether. We cannot see them, of course ; but we can detect them in Various convenient ways—for example, by their action on a photographic plate. And they are actually fine enough to " see " the pattern in the crystal. By their aid we can detect the details of Nature's structures, and so we enter the new fields of research.

Just two more points may be discussed very briefly. Though the X-rays are of a proper fineness for perceiving crystalline structure, they would not give us a sensible result were it not for the regularity of the crystalline arrangement. One atom cannot do enough by itself, nor a number of atoms acting independently ; but since the billions of atoms are in regular array, they can be as much more effective than a disordered mass as a dis- ciplined army is better than a mob.

And lastly, the X-rays have shown us crystals where we had never suspected their existence : they are to be found in rocks and minerals, metals and alloys, bones and teeth and muscle, cotton and silk and wool, rubber and paints, and, indeed, almost everywhere. And always the forms, sizes, characteristics and quantities of these crystals arc important determinants of the properties of the substances in which they are found.

Thus the new crystallography lies before us : a field of research where we move as yet with untrained and hesitating steps. But even so far as we have gone, we find unexpected beauties of ordered structure ; we begin to see in Nature's designs the minute details on which the properties of all solid substances depend.