Diamonds, as was discovered by Lavoisier, yield on combustion nothing but carbon dioxide ; their identity with carbon was thus proved. When pure, they are colourless ; they are the hardest of all known sub stances, and possess a density of 3.514 at 18. When heated in absence of air in an electric arc, a diamond changes to a coke-like black substance. Diamonds of any appreciable size have not been formed artificially, but minute diamonds have been made by Moissan by dissolving carbon in molten iron heated to its boiling-point in an electric furnace, and then suddenly cooling the iron by plunging it into molten lead; the external surface of the iron solidifies, and encloses a molten interior. As iron possesses a greater volume in the solid than in the liquid state, the molten iron, containing carbon in solution, when it solidifies is under great pressure, for it is confined and hindered from expanding by the crust of solid iron ; under this pressure the carbon separates out in the liquid form, and in solidifying crystallises in octahedra with curved facets characteristic of natural diamonds. If, on the other hand, the iron is allowed to cool without any device to compress the interior, the carbon crystallises out in the form of graphite or plumbago, or, as it is sometimes termed, " black lead." This variety of carbon is also found native ; it forms hexagonal plates, is soft, and is slippery to the touch. Lastly, many compounds of carbon when heated to redness decompose, and leave the carbon in an amorphous or non crystalline form. Varieties of these are gascarbon, deposited in the necks of gas-retorts ; oilcoke, left as a residue after the distillation of certain oils ; sugar-charcoal, the residue on heating sugar in absence of air ; and wood-charcoal, the product of the distillation of wood. All of these are black, more or less hard substances. When heated to whiteness in an electric arc, they are transformed into graphite. They all contain a trace of hydrogen, from which they can be freed by heating to redness in a current of chlorine. At the temperature of the electric arc, carbon volatilises with out fusion and condenses as graphite ; it is only when it is heated under pressure, as described, that it can be made to melt.
