Since both calcium
oxide and hydroxide are white substances, it was thought that a brownish,
greyish, or blackish residue must necessarily point to incipient
polymerisation of the gas. This view would be correct if calcium carbide
were prepared in a state of chemical purity, for it also is a white body.
Commercial carbide, however, is not pure; it usually contains some
foreign matter which tints the residue remaining after gasification. When
a manufacturer strives to give his carbide the highest gas-making power
possible he frequently increases the proportion of carbon in the charge
submitted to electric smelting, until a small excess is reached, which
remains in the free state amongst the finished carbide. After
decomposition the fine particles of carbon stain the moist lime a bluish
grey tint, the depth of shade manifestly depending upon the amount
present. If such a sludge is copiously diluted with water, particles of
carbon having the appearance and gritty or flaky nature of coke often
rise to the surface or fall to the bottom of the liquid; whence they can
easily be picked out and identified as pure or impure carbon by simple
tests. Similarly the lime or carbon put into the electric furnace may
contain small quantities of compounds which are naturally coloured; and
which, reappearing in the sludge either in their original or in a
different state of combination, confer upon the sludge their
characteristic tinge.
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