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.
