According to Caro the silicon may be present both as
hydrogen silicide and as silicon "compounds." A high temperature in the
generator will favour the production of the latter; an apparatus in which
the gas is washed well in lime-water will remove the bulk of the former.
Fraenkel has found that magnesium silicide is not decomposed by water or
an alkaline solution, but that dilute hydrochloric acid acts upon it and
spontaneously inflammable hydrogen silicide results. If it may be assumed
that the other silicides in commercial calcium carbide also behave in
this manner it is plain that hydrogen silicide cannot occur in crude
acetylene unless the gas is supposed to be hurried out of the generator
before the alkaline water therein has had time to decompose any traces of
the hydrogen silicide which is produced in the favouring conditions of
high temperature sometimes prevailing. Mauricheau-Beaupre has failed to
find silica in the products of combustion of acetylene from carbide of
varying degrees of purity. He found, however, that a mixture of strong
nitric and hydrochloric acids (_aqua regia_), if contaminated with
traces of phosphoric acid, dissolved silica from the glass of laboratory
vessels.
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