I have already signified that there are various degrees of refractoriness to chemical reagents among the different forms of graphite. Some dissolve in strong nitric acid; other forms of graphite require a mixture of highly concentrated nitric acid and potassium chlorate to attack them, and even with this intensely powerful agent some graphites resist longer than others. M. Moissan has shown that the power of resistance to nitric acid and potassium chlorate is in proportion to the temperature at which the graphite was formed, and with tolerable certainty we can estimate this temperature by the resistance of the specimen of graphite to this reagent. CRYSTALLISATION The diamond belongs to the isometric system of crystallography; the prevailing form is octahedral. It frequently occurs with curved faces and edges. Twin crystals (macles) are not uncommon. Diamond crystals are generally perfect on all sides. They seldom show irregular sides or faces by which they were attached to a support, as do artificial crystals of chemical salts; another proof that the diamond must have crystallised from a dense liquid. The accompanying illustration (Fig. 14) shows some of the various crystalline forms of native diamonds. [Illustration: FIG. 14. CRYSTALLINE FORMS OF NATIVE DIAMONDS. To face p. 86.] No. 1. Diamond in the form of a hexakis-octahedron (the forty-eight scalenohedron), or a solid figure contained by forty-eight scalene triangles. According to Professor Maskelyne, this occurs as a self-existent form only in the diamond. No. 2. Diamond in the form of a hexakis-octahedron and octahedron. From Sudafrika. No. 3. Diamond in the form of octahedron with intersections. No. 4. Diamond from Brazil. No. 5. Diamond from Kimberley. No. 6. Diamond from Brazil. No, 7. A macle or twin crystal, showing its formation from an octahedron with curved edges. * * * * *