The resistance of a graphite to oxidising agents is greater the higher the temperature to which it has previously been exposed. Graphites which are easily attacked by a mixture of fuming nitric acid and potassium chlorate are rendered more resistant by strong heat in the electric furnace.
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.
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.
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, 7. A macle or twin crystal, showing its formation from an octahedron with curved edges.
Some crystals of diamonds have their surfaces beautifully marked with equilateral triangles, interlaced and of varying sizes (Fig. 15). Under the microscope these markings appear as hollow depressions sharply cut out of the surrounding surface, and these depressions were supposed by Gustav Rose to indicate the probability that the diamonds had at some previous time been exposed to incipient combustion. Rose pointed out that similar triangular striations appeared on the surfaces of diamonds burnt before the blowpipe. This experiment I have repeated on a clear diamond, and I have satisfied myself that during combustion before the blowpipe, in the field of a microscope, the surface is etched with triangular markings different in character from those naturally on crystals (Fig. 16). The artificial striæ are very irregular, much smaller, and massed closer together, looking as if the diamond during combustion flaked away in triangular chips, while the markings natural to crystals appear as if produced by the crystallising force as they were being built up. Many crystals of chemical compounds appear striated from both these causes. Geometrical markings can be produced by eroding the surface of a crystal of alum with water, and they also occur naturally during crystallisation.