Interference phenomena of X-rays in the case of irregularly arranged particles.

P. Debye and Scherrer.

The theoretical and experimental determination of the arrangement of atoms in a molecule, when there is a random mixture of an infinite number of such molecules, is described in Ber. d. Kgl. Ges. d. Wiss. zu Göttingen of 17 December 1915.

An attempt to determine theoretically and experimentally the regular arrangement of electrons in the atom: Ber. d. Kgl. Ges. d. Wiss. zu Göttingen of 27 February 1915.

A new method for studying crystalline and “amorphous” powders.
I. Description of the method. Phys. Zeit. 17, 277, 1916; Ber. d. Kgl. Ges. d. Wiss. zu Göttingen of 3 December 1915.

II. Interference phenomena in the case of liquids (benzene, etc.). Kgl. Ges. d. Wiss. zu Göttingen of 17 December 1915.

III. On the structure of graphite and amorphous carbons. Phys. Zeit. 18, 291, 1917.

All the works listed represent the theoretical and experimental development of a new method for studying the structure not only of large, well-formed crystals, but also of smaller particles, both solid amorphous ones and liquids.

The earlier methods of studying the structure of crystals by means of X-rays, given by Laue and by Bragg, required a well-formed crystal and knowledge of its crystalline form. The method described by the authors requires neither knowledge of the form nor exact orientation of the crystal and can be applied to the study of microscopic and submicroscopic particles. The interference patterns are obtained from the crystalline or amorphous powder under investigation, pressed into a column (diam. 2 mm, length 10 mm), placed along the axis of a sensitive film rolled into a cylinder. A beam of monochromatic X-radiation (from $Cu$ or $Pt$, containing, as is known, only two strong lines) is directed at the middle of the column, perpendicular to its axis. Each beam of monochromatic radiation gives, depending on the structure of the crystal, a series of interferences in the form of conical surfaces having as their common axis the primary X-ray beam incident upon the column. These conical surfaces, intersecting the film surrounding the column, leave on it traces in the form of a series of lines (see Figs. 1 and 2). In this way there were investigated first the familiar form of crystals, $LiF$ (see Fig. 1), in the rays of $Cu$ and $Pt$, then

Fig. 1.                Fig. 2.

“amorphous” silicon, which proved to be crystalline, with a structure analogous to that of diamond; finally, the structure of graphite and of coal was studied. The indications in the literature concerning the structure of graphite had hitherto been contradictory; Debye and Scherrer prove beyond doubt (see Fig. 2) that graphite crystallizes trigonally. The length of the side of the element of its structural lattice, containing 12 atoms, is equal to $4.69 \cdot 10^{-8}$ cm. The experiments show, moreover, that coal does not represent a special variety of carbon, but possesses the same structure as graphite, differing from it only in that its valences are arranged tetrahedrally. The crystals of coal are so small that such small crystals of graphite cannot be obtained by any grinding. Thus there are only two varieties of carbon: diamond and graphite.

Very important are the indications of one of the authors (P. Debye) that his method permits the study not only of molecules, but that, in this way, it will be possible experimentally to determine the position of regularly arranged electrons in the atom. The theoretical calculations of P. Debye confirm this.

N. Tsofro.