Nature of

The second main sphere of industrial X-rays depends upon the fact that, when suitable technique is employed, the constituent atoms and molecules which form a crystal reflect X-rays in a perfectly definite and regular manner, producing what is known as an X-ray spectrogram.

Different materials, when suitably excited, produce different and characteristic X-rays, consequently X-rays may be used as a method of chemical analysis. The method affords a very reliable qualitative test, and considerable work has been done in develop ing it as a more practical system of quantitative analysis. In certain cases X-ray chemical analysis has advantages over the conventional method, for example in certain mixed salts it is im possible by ordinary chemical methods to say how the tiny crystals of the mixture are made up. Each element, however, yields its own characteristic X-ray wave-lengths under suitable stimulus, and therefore accurate information as to the constitution of a chemical mixture may be obtained by an examination of what is called an emission spectrogram. Impurities in materials may also be detected by the same technique.

It was due to the fact that each element emits characteristic X-rays that the missing element of atomic No. 72 was discovered in 1923 by Coster and Hevesy of Copenhagen' and called by them Hafnium, out of compliment to the place of its discovery (Hafnia was the old name for Copenhagen). Scientific con siderations led these investigators to examine the X-ray spectra, yielded by certain minerals, and in the course of their investiga tions they discovered spectrum lines of the frequency known to be characteristic of the missing element. (See HAFNIUM.) Practical applications of X-ray spectroscopy should be men tioned; but it is to be borne in mind that this aspect of the work was of much later development than radiography and involves a much more specialized knowledge, therefore it is practised chiefly in physical laboratories. Although many of the results have the greatest value in industry, yet the method, by reason of its specialized character, has not the same general use as radiog raphy.

All substances of a crystalline nature are suitable specimens for X-ray spectroscopic investigation, for example the investiga tion of the minute structure of cellulose by X-rays has had the greatest value in the textile and explosive industries.

In metallurgy this particular use of X-rays has achieved uni versal recognition as a method of investigating the structure of metals in a way that is quite impossible by any other agency. By reason of the limiting value of the wave-lengths of visible light, there is a degree of smallness beyond which no microscope will ever be of value ; X-ray spectroscopy, on the other hand, enables crystal structure to be studied with great accuracy. The effect of heat treatment of steels is a subject in which X-rays have con siderable contributory value. The change in crystal structure in metals consequent upon mechanical treatment, such as rolling, may be shown by the differences produced in the resulting X-ray 'Nature, vol. iii. p. 79 (Jan. 1923).

spectrogram. The study of strain in metals is greatly facilitated by the use of X-ray spectroscopy, inasmuch as any condition resulting in the alteration of atomic structure may show itself in properly prepared X-ray spectrograms. The method has also been of assistance to the metallurgist in his study of metallic alloy systems.

Interesting work has been done on the examination of pivots by X-rays. By reason of their structure many materials have much greater strength on one or two points on their surface than on others. The diamond, for example, may function perfectly as a pivot if the right spot is chosen to bear the strain. This par ticular spot depends upon the direction in which the constituent carbon particles are arranged. The regularity of arrangement that characterizes the diamond results in the formation of cer tain cleavage planes (or planes of maximum weakness), and a study of the arrangement of the carbon atoms in any particular diamond by X-rays will enable the experimenter to select that particular spot, with regard to these planes, most able to with stand wear. Explosives may be subjected to this special method of analysis in order to study the effect of atomic arrangement on their explosive properties, and their general sensitivity and stability; and also to detect impurities which may manifest them selves in the course of preparation or storage.