CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide
and hydrogen (which may perhaps come from cavities in the calcium carbide
itself), as being utterly insignificant from the practical point of view,
the carbide impurities of the gas fall into four main categories: those
containing phosphorus, those containing sulphur, those containing
silicon, and those containing gaseous ammonia. The phosphorus in the gas
comes from calcium phosphide in the calcium carbide, which is attacked by
water, and yields phosphoretted hydrogen (or phosphine, as it will be
termed hereafter). The calcium phosphide, in its turn, is produced in the
electric furnace by the action of the coke upon the phosphorus in
phosphatic lime--all commercially procurable lime and some varieties of
coke (or charcoal) containing phosphates to a larger or smaller extent.
The sulphur in the gas comes from aluminium sulphide in the carbide,
which is produced in the electric furnace by the interaction of
impurities containing aluminium and sulphur (clay-like bodies, &c.)
present in the lime and coke; this aluminium sulphide is attacked by
water and yields sulphuretted hydrogen.
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