The crystallographic information on the subject of graphite is scanty. It has generally been accepted as hexagonal with a well-marked basal cleavage; but besides the cleavage face few other faces have ever been observed on it. Kenngott, the earliest observer, seems to have obtained the best crystal, and measured planes, to which he gave the indices 10͞11 and 11͞21, making angles of 58° and 70° 18' respectively with the basal plane 0001. (These results are strikingly confirmed by X-ray measurements, which make the corresponding angles 58° 8' and 70° 13'.) Nordenskiöld considered graphite monoclinic, on account of the variability of its angles; but his conclusions were questioned by Sjögren, who, in a very full paper, adduced a number of reasons (twinning, percussion and etch figures, thermal conductivity) to show that it was hexagonal. Quite recently the researches of Gaubert have added a knowledge of the optical properties of graphite. In very thin flakes it is transparent, uniaxial and negatively birefringent, with a refractive index of about 2. This settles definitely that it has trigonal hexagonal symmetry. The X-ray analysis of graphite has lagged considerably behind that of the diamond. Bragg in 1914 made a measurement of the spacing of the cleavage planes, finding it to be 3·42 A. U., while Ewald, in the same year, took a Laue photograph of a crystal, perpendicular to the axis, confirming its hexagonal symmetry. The fuller interpretation of its structure was attempted by Hull and by Debye and Scherrer in 1917 by the powder method. Both were able to assign a structure to the element, but these structures have different lattices and belong to different crystal systems. An examination of the original papers shows that the observations of neither investigator are in very good agreement with the structure they propose, and the observations only show the roughest agreement with each other. This is especially marked in the matter of intensities. On the whole, Hull’s results are more plausible, because he separated the K
α
and K
β
lines of Mo by screening, whereas Debye and Scherrer often mistook α for β lines, as will be shown subsequently. Since then Backhurst has made some measurements on graphite, from the point of view of its expansion and the effect of temperature on the reflection intensities.
The diffusion of gases through metals