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MAY LECTURE OF THE INSTITUTE OF METALS, BY PROF. W. H. BRAGG.

On the occasion of the sixth annual May Lecture of the Institute of Metals, delivered on May 4, at a meeting of the Institute in London, Prof. W. H. Bragg, D.Sc., F.R.S. (Nobel Prizeman), gave an interesting account of the new method of applying the properties of X-rays to the study of crystal structure, including the structure of certain metals.

The method, it was shown by the Professor, results in the determination of the exact relative positions of the atoms of which the crystal is composed. It is not successful in every case as yet, because of the lack of practice and experience of the experimenters in the new field and because some of the interpretations are not fully understood. There is no lack of indications; but we are not yet fully aware of the meaning of all of them.

It is natural to attack first such crystals as have an obviously simple structure and consist of few elements associated in simple proportions. It is also of great advantage if the crystals belong to a group of isomorphous members, such as the rock-salt series. This series, in fact, has everything to recommend it to the experimenterits form is simple, that of the cube; its symmetry is high; it contains two elements only, in equal proportions, e.g., sodium or potassium associated with chlorine or iodine; and there are several members of the series, so that we can watch the effect of changing one element at a time. The constitution of this series was, therefore, one of the first to be examined and made plain.

The constitution of the diamond, which has also been determined, presented a rather more difficult task, because the arrangement of the atoms is not so simple as that of the rock-salt series; although its form is cubic, its symmetry is high, and it contains atoms of one kind only.

Of the metals which will naturally be of especial interest to the Institute of Metals, silver and copper, and by inference gold, have been shown to possess a very simple structure, in which the atoms are arranged as in the piling of shot. Bismuth and antimony have a distorted arrangement; but these two, as well as zinc, have not been completely determined. A beginning had been made with iron. The war has, however, stopped all work of the kind on this metal.

This new field of research, according to Prof. Bragg, depends on a principle already known. When a regular train of waves falls upon a surface separating two media, part is reflected and part goes on. If the part that goes on meets another separating surface, a second portion is reflected and some of this emerges from the second medium in the same direction as the beam reflected from the first surface. It will happen in general that the two reflected beams are out of phase and to that extent destroy one another. Whether they do so or not depends upon the relation between the wave-length, the angle of the inclination of the beam to the reflecting surfaces, and the distance between the surfaces. In this way are explained the colours of the soap film, of the thin layer of oil on the surface of a liquid, of the colours of steel when being tempered, and so on.

If the reflecting surfaces are many in number, not two, the effect is made more intense and at the same time more precise. It occurs in the beautiful colours of potash

crystals as shown by Lord Rayleigh. These crystals are formed of alternating layers, twinned across their surfaces of separation, and for some obscure reason the thickness of all the layers is the same though it varies from crystal to crystal. When white light containing all wave-lengths is incident upon such a crystal, at a certain angle, then only that wave-length is reflected for which the proper relation between the wave-length, angle, and spacing holds good. If the angle is altered the wave-length which is reflected is no longer the same. Hence the beautiful play of colours which the crystal shows.

It is an essential cause of the success of this wonderful effect that the wave-length and the spacing are not very different in amount.

We now pass on to the case of the X-rays. These consist of waves-so, indeed, these very experiments tell us— which are something like ten thousand times shorter than the wave-length of light. To obtain the parallel effect we must look for reflecting surfaces which are ten thousand times closer together than the twinning surfaces of the chlorate of potash crystals, and these are separated from one another by only the forty-thousandth of an inch or thereabout. These also nature has provided in the layers of atoms in the crystal.

It may seem curious that a layer of atoms should act as a reflecting surface: but after all it is not necessary that such a surface should be continuous. A row of iron railings, for example, can act as a reflector of sound waves A natural face of a crystal contains, no doubt, a layer of atoms arranged regularly; and behind the natural face are other layers all similar, and placed at regularly increasing distances behind it. Thus, all the conditions for this peculiar reflection experiment are present, and we actually find that when a pencil of X-rays of a definite wave-length are allowed to fall upon the face of the crystal, and the crystal is gradually turned round so as to alter the angle of incidence, the reflection of the beam as a whole is non-existent, except when the angle is right. Then it flashes out strongly. When this angle is observed, the relation of the wave-length to the spacing is known.

The instrument used is called the X-ray spectrometer. It has no lenses because X-rays cannot be refracted, and the rays are invisible, so that in place of the telescope appears a chamber containing gas, which is ionised by the X-rays. The resulting electrical effect is observed in an electroscope. It is important that the measurement of the result is quantitative, so that in this respect the new spectrometer has an advantage over the old.

In this way, if we use always the same X-ray, we can compare the spacings between the layers parallel to one after another of the natural faces parallel to the crystal; and in this way we arrive finally at the crystal structure. The instrument is not at all difficult to use, and the observed effects are large and precise, so that it is quite easy to get numerical results. The interpretation is not always quite so easy. One part of it comes readily, viz., the number of molecules to each unit of the pattern of the crystal-the unit being the smaller part, which, being reflected again and again without alteration of orientation or distance from its neighbours forms the complete crystal. For instance, the unit of pattern of potassium sulphate contains four molecules; the unit of pattern of antimony contains two atoms.

The far greater difficulty lies in the determination of the way in which the atoms are arranged in the unit. These data are sufficient, but the interpretation is hard. To understand how it is attempted, and in some cases achieved, is best explained by models.

South African Association for the Advancement of Science.--(Honorary President, His Majesty the King).— The Fourteenth Annual Session of the Association will be held at Maritzburg, from Monday, July 3, to Saturday, July 8, 1916, inclusive, under the Presidency of Prof. L. Crawford, M.A., D.Sc., F.R.S.E.