These laboratory diamonds burn in the air before the blowpipe to carbonic acid. In lustre, crystalline form, optical properties, density, and hardness they are identical with the natural stone. In several cases Moissan separated ten to fifteen microscopic diamonds from a single ingot. The larger of these are about 0·75 mm. long, the octahedra being 0·2 mm. The accompanying illustrations (Fig. 22) are copied from drawings in Moissan’s book _Le Four Electrique_. Along with carbon, molten iron dissolves other bodies which possess tinctorial powers. We know of blue, green, pink, yellow, and orange diamonds. One batch of iron might contain an impurity colouring the stones blue, another lot would tend towards the formation of pink stones, another of green, and so on. Cobalt, nickel, chromium, and manganese, all metals present in the blue ground, would produce these colours. A NEW FORMATION OF DIAMOND I have long speculated as to the possibility of obtaining artificially such pressures and temperatures as would fulfil the above conditions. In their researches on the gases from fired gunpowder and cordite, Sir Frederick Abel and Sir Andrew Noble obtained in closed steel cylinders pressures as great as 95 tons to the square inch, and temperatures as high as 4000° C. According to a paper recently communicated to the Royal Society, Sir Andrew Noble, exploding cordite in closed vessels, has obtained a pressure of 8000 atmospheres, or 50 tons per square inch, with a temperature reaching in all probability 5400° Ab. Here, then, we have conditions favourable for the liquefaction of carbon, and were the time of explosion sufficient to allow the reactions to take place, we should certainly expect to get the liquid carbon to solidify in the crystalline state.[7]