twenty-fifth
heat it was noticed that in the water-cooled areas of the quartz tube a
lustrous black ring had formed. On being strongly heated, some of this,
evidently carbon, burnt off, leaving a white film, presumably silica.
This seems to show that a volatile silico-organic compound, containing
carbon, hydrogen, and silicon, was evolved from the iron on heating".
It
would appear from our experiments that probably a ferro-silicon
carbonyl is given off from the iron, for, as has been said, we observed
a corrosive action on carborundum by the gas evolved from iron borings
at red heat under a high vacuum, and the same action was produced by
gaseous ferro-carbonyl, and also by carbon monoxide, previously passed
over molten iron sulphide at atmospheric pressure.
Let
us consider what happens in an ingot or spherule when rapidly cooled
simultaneously on all sides. It is first surrounded by a thin coat of
solidified metal which, below 600° C, is impervious to gases. As the
coat thickens layer within layer, more and more gas is ejected by the
solidifying metal, and its semi-solidified centre, still pervious to
gas, receives the charge. As this process progresses the pressure may
rise higher and higher, though there may be a limit to the pressure
against which the metal is able to eject gas when setting. All we,
however, know is, that the mechanical strength of the ingot or spherule
places a limit of about 7000 atmospheres on the gaseous pressure, and,
as we have already mentioned in the case of some iron alloys, most of
the spherules are split or shredded, with an appearance consistent with
this view.
Crookes'
microscopical examination of diamonds with polarised light supports
this view. In his lecture at Kimberley, in 1905, he states: " I have
examined many hundred diamond crystals under polarized light, and with
few exceptions all show the presence of internal tension.
"
On rotating the polarizer, the black cross most frequently seen
revolves round a particular point in the inside of the crystal; on
examining this point with a high power we sometimes see a slight flaw,
more rarely a minute cavity. The cavity is filled with gas at enormous
pressure, and the strain is set up in the stone by the effort of the
gas to escape."
It
seems therefore probable, or indeed almost certain, from the
accumulated evidence, that the chief function of quick cooling in the
production of diamond in an ingot or spherule is to bottle up and
concentrate into local spots the gases occluded in the metal which,
under slow cooling, would partially escape and the remainder become
evenly distributed throughout the mass.
As
to the condition in which the gases exist within the iron at
temperatures above 500° C. little is known, though at 200° C. and at
180 atmospheres Mond has shown that iron penta-carbonyl is formed. The
intimate
