Contrary to the old idea, it is now almost universally considered that, instead of being of volcanic origin, graphite consists of the remains of long-dead organisms, and in this respect is closely related to coal. This hypothesis, however, fails to explain one point, namely the crystalline nature of graphite; for even anthracites, which form the oldest coals known to have had their origin in the decomposition of organic substances, do not reveal the faintest traces of crystalline structure. The upholders of the theory that graphite was formed by the action of plutonic forces adduce, in support, the fact that graphite can actually be produced, in certain chemical processes, at high temperature. Molten cast-iron in cooling causes the separation of carbon in the form of graphite; and the same substance is also formed, in large quantities, in gas retorts, through the decomposition of certain carbonaceous compounds when brought into contact with the glowing walls of the retorts. Recent investigations, however, have shown that the temperature necessary for the transformation of non-crystalline carbon into crystalline graphite is by no means so high as was formerly supposed; and it is now known that the change takes place at as low as red heat. Possibly the two theories could be reconciled by the assumption of a very old coal--such as is found, for instance, as anthracite in many parts of the world--being so strongly heated, by plutonic action, as to change into graphite. Native graphite crystallises in the form of hexagons, mostly tabular; but really well-developed crystals are of extremely rare occurrence, and by far the greatest quantities of this mineral are found in the condition of dense lumps, in which only the crystalline structure, and not any decided crystals, can be discerned. The hardness of the mineral fluctuates within fairly wide limits, ranging from 0·5 to 1·0. The sp. gr. averages 1·8018–1·844, but, in the case of impure lumps may increase to 1·9–2·2. The following analyses will give some idea of the considerable divergence existing between graphites from different deposits:-- SIBERIAN GRAPHITE 1 2 Carbon 94·28 40·55 Ash 5·72 56·56 Water -- 2·80 PORTUGUESE GRAPHITE Carbon 42·69 Water (chemically combined) 3·96 Ash 53·35 BOHEMIAN GRAPHITE