Summary: We here put together the results obtained by the above examination of the physical and geometrical properties of graphite.
1. The striæ or ridges occurring in graphite crystals from different localities arise from twinning, which takes place in every ridge according to several different laws. If the mineral is supposed to crystallise hexagonally, two twinning laws suffice to explain the structure of the crystal-complex; whereas on the supposition of a less symmetrical system of crystallisation, this twinning becomes far more complicated.
2. The same sort of twin crystals as the natural ones may be artiticially produced by bending crystal scales; thus it is evident that those surfaces which are twin-planes are also gliding-faces.
3. The percussion-figures of graphite may be referred to the gliding- faces, and are of another kind than the percussion-figures of micas and many other minerals.
4. The solid angles of graphite as well as the plane angles on the cleavage, show so little constancy that the geometrical structure of the mineral cannot be employed to deternine the system of crystallisation.
5. The combustion- and etching-figures are regular hexagons.
6. The melting-curve of the cleavage is a circle, as far as can be ascertained by means of the methods that have been used.
7. The nature of the twinning, together with the etching-figures and the melting-curce, make it probable that graphite crystallises in the hexagonal system. This is not contradicted by its geometrical properties, which are too inconstant to form the ground of any decision on this point.