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228
EXPERIMENTS ON THE
to the amount of iron of any of our experiments, the largest crystals being 0-7 mm. in length.
This
experiment was repeated several times with the same result. The time of
cooling of the crucible, from switching off the current to the
temperature of setting, was 15 seconds, and probably sufficiently rapid
to allow a skin to be formed around the ingot before the centre was
solidified, for the configuration of the crucible and cover was such
as to ensure nearly equal and simultaneous cooling on all sides of the
ingot. At the time, vacuum was erroneously thought to be the chief
contributory cause and not the presence of carbon monoxide in large
proportion.
High vacuum experiments
The
molecular pump not having yet been evolved, a powerful pumping system
was arranged, consisting of three steam-jet exhausters in series,
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the
last ejector of the series discharging into a jet condenser with
separate air and water pumps, the former assisted by a steam jet. The
two steam-jet exhausters nearest to the exhausted chamber were fed with
highly superheated steam at 200 lb. pressure, and the suction pipe to
the chamber was 4 inches in diameter; the chamber, 2 feet 6 inches
diameter of spherical shape (Fig. 11). A vacuum of 1/6 mm.
absolute could be reached. The crucible was placed on a large block of
carbon, resting on the base of the chamber, and forming the bottom
pole. The cover was insulated from the chamber, and through an
oil-sealed gland passed a 2-inch brass
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