crucible
had been plunged into water, while if the conditions are such as to
allow a free passage through the skin of the ingot, the yield is at
once diminished, even though the bulk pressure on the ingot is the same.
The
experiment, on compressing acetylene and oxygen, has shown that minute
crystals, probably diamond, are produced almost instantaneously in the
molten surface of metal exposed on one side to gases consisting of
carbon monoxide, carbon dioxide, and hydrogen at very high temperature
and at 15,000 atmospheres. Sir William Crookes' experiment described in
his lecture before the British Association at Kimberley in 1905 is
somewhat analogous; cordite with a little additional carbon was fired
in a chamber, the pressure reaching 8000 atmospheres, a few crystals of
diamond were found and isolated; this result Crookes attributed to the
melting of the carbon under the temperature of explosion and
crystallisation under the pressure on cooling.
Under
the conditions of the experiment there would be a considerable amount
of the surface of the chamber melted and swept into the products of the
charge by the turbulence of the explosion, and the spherules of iron
would thus be carburised and cooled while still under heavy pressure.
In
the acetylene-oxygen experiment there is a molten surface with reducing
gases on one side at high pressure, and on the other metal impervious
to gases. In Crookes' experiment the globules of metal are surrounded
by gases at high pressure. In both cases the metal has solidified with
the occluded gases imprisoned by the high external gaseous pressure,
for we have seen that the pressure of occluded gases in highly
carburised iron when quickly cooled cannot exceed about 1000
atmospheres.
The
experiments under vacua from 75 mm. up to X-ray vacua have shown
generally that as the vacuum is increased the yield of diamond in the
crucible is diminished, and that below 2 mm. none has been detected.
But when alloys previously boiled at atmospheric pressure are quickly
heated up under high vacuum violent ebullition takes place, from the
large volume of gases liberated, and some of the contents are ejected
into the vacuum chamber before they have had time and sufficient
temperature to part with their occluded gases, and diamond occurs in
the spherules so ejected.
The gases occluded in cast iron which are given off when heated in vacuo have
been investigated by H. C. Carpenter and others, and the relative
amounts of the constituents are found to vary widely according to the
previous heat treatment and the nature of the gases in contact with the
metal while molten and during cooling; they are carbon monoxide and
carbon dioxide, hydrogen and nitrogen.
H. C. Carpenter (Journal of Iron and Steel Institute, 1911) states that, when heating up a bar of cast iron in vacuo in a silica tube, 'After the
