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ARTIFICIAL PRODUCTION OF DIAMOND 221
gressively
increased charges were used, and even with relatively small charges the
block cracked on the second round. With the 90 per cent, excess charge,
the block always split on the first shot, but this probably occurred
after impact, and not till the full instantaneous pressure had been
exerted, which was estimated to be greater than with the plain hole,
probably over 5000 tons.
Only graphite was placed at the bottom of the hole in these latter experiments, and the analysis yielded nothing.
Experiments on pressure in cast iron when cooled
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It
has been generally assumed that iron rich in carbon expands on setting,
and that this supposed property is a contributory cause in the
formation of diamond.
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Several
experiments were made by pouring iron saturated with carbon from the
electric furnace through a narrow git into a very massive steel mould,
closed at the bottom with a breech screw (Fig. 9). When cold, the
breech screw was easily removed, and there was no sign of any
appreciable pressure having come on the threads. Not being sure that,
because of capillarity, the corners of the mould had been quite filled,
a steel mandril was, immediately after pouring, forced down the
git-hole by a press giving a fluid pressure in the mould of 75
atmospheres. The observed pressure on the breech screw appeared not to
have exceeded this pressure. Highly carburised iron, therefore, does
not expand with any considerable force on setting.
The
reason why a lump of cast iron thrown into a ladle of molten metal
first sinks to the bottom and soon rises and floats on the surface is
probably that cast iron is about seven times stronger in compression
than in tension.
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