157 



vibrations being therefore equal in this case. The waves reflected 

 under this difference of phase will thus totally destroy each other, 

 which was not the case when they were emitted by alternating layers 

 of zinc- and sulphur-atoms. 



With respect to the structure of diamond it may be remarked, 

 that the whole arrangement is such that each carbon-atom is sur- 

 rounded by four others, placed in the corners of a regular tetrahedron, 

 the centre of which is occupied by the first named carbon-atom. 

 This arrangement is in striking agreement with the suppositions 

 once made by Van 't Hoff and Le Bel with respect to the direc- 

 tion in space of the four valencies of the carbon-atom in general. 



However, in the simple cases of KCl and NaCl already, there are 

 a number of subtile discrepancies with the results of crystallogra- 

 phical research, for instance, in the symmetry of both salts, - 

 which are not accounted for in the structures derived by these 

 experiments. Special suppositions must, therefore, be made about 

 certain small displacements of the atoms from the normal positions 

 of equilibrium, to explain such differences. a ) 



Recently Debije 2 ), by means of his most ingenious and evidently 

 most universally applicable method of observation, studied the 

 structure of graphite and of other allotropic forms of carbon, which 

 all appear to have the same structure as graphite itself. He found, 

 that the carbon-atoms are placed here in the corners of regular 

 hexagons arranged in parallel strata, the fourth valency of the 

 carbon-atom being reduced to an extremely weak force. This fact 

 seems to prove that the carbon-atom can act as a centre either of 

 four or of three equal valencies, and the phenomena observed give 

 to some extent an explanation of the possible occurrence of deriva- 



x ) R. Gross, Jahrb. 1. Radioact. und Elektron., 15, 316, 319, (1919). 



2 ) P. Debije and P. Scherrer, Phys. Zeits., 17, 277, (1916); 18, 291, (1917); 

 P. Debije, ibid., 18, 483, (1917); P. Scherrer, ibid., 19, 23, (1918); A. J. Bijl, 

 Dissertatie, Utrecht, (1918); P. Scherrer, Nachr. d. Ges. d. Wiss. Gottingen, 

 (1918), p. 98; K. Schutt, Umschau, 22, 406, (1918); R. Gross, Jahrb. f. Radio- 

 akt. und Elektron., 15, 325. (1919). It is remarkable, that the method mentioned 

 allows one also to state the crystalline nature of the particles dispersed in some 

 colloidal solutions, such as those of gold and silver. 



During the war-time, the American physicist A. W. Hull (Phys. Review, N. S. 

 10, 661, (1917), came independently from the two investigators mentioned above, 

 to the invention of exactly the same method of observation. However, Debije's 

 publication dates from one-and-half a year before. Also the technical development 

 of Hull's method is almost identical with Debije's way of manipulation. 



