HPHT Synthetic Diamond | High Pressure High Temperature Carbon Page 31



	ARTIFICIAL PRODUCTION OF DIAMOND	233

	and pressures has not only confirmed his results but has thrown, it is hoped, additional light on the causes operating to produce diamond in iron. The experiments under high pressure in steel moulds, where heating of the charge was effected by a central core through which current was passed, enabled Hannay's experiments with Dippel's oil to be tried under much higher pressures, and more thoroughly than is possible with steel tubes in a furnace. The Appendix * gives some indication of the many substances and chemical reactions tested. The results were chiefly negative. The few that were favourable were generally attributable, as has been said, to the presence of iron. It was noticed that the iron seldom contained diamond unless when so situated in the charge as to cause equal cooling on all sides, and it will be remembered that the experiments under atmospheric pressure showed this condition to be essential for the formation of diamond. In some of the experiments of this group considerable gaseous pressure existed, up to 6000 atmospheres, but it is doubtful if in these the right kind of gas was present or a sufficiency of heating or carburisation of the iron occurred. On the whole, therefore, it would appear that all, or nearly all, the chemical reactions as such, under pressures up to 6000 atmospheres, have given negative results. The experiments on very rapid cooling would seem to dispel the theory that carbon can be caught in a state of transition, and to lead us to the conclusion that quick cooling is not in itself a cause of the occurrence of diamond in rapidly cooled iron. Moissan observed that when the spherules of granulated iron were cracked, or contained geodes, no diamond was ever found in them, and he attributed this to want of mechanical pressure. The experiments we have made not only corroborate this fact, but they tend to show, we think conclusively, that the cracks in the spherules act by allowing a free passage for the occluded gases to escape, and the geodes by providing cavities in which the gases can find lodgment without much gaseous pressure occurring in the metal.f Further, the experiments have shown that iron when it sets does not expand with appreciable force, and that the only compressive forces that are brought to bear on the interior are those arising from the contraction of the outer layers. Our experiments further show that when a crucible of molten iron is subjected to pressure more than three times as great as can be produced by these contractile forces, the yield of diamond is not increased. On the other hand, when the conditions of the experiment operate to imprison the occluded gases, then the yield of diamond is about the same as if the * Not reproduced. t Conversely they may act to allow gases to enter the metal.