Determinations of this kind are made in
platinum dishes enclosed in lead or copper vessels in a well-ventilated
place. Fluor-spar is useful as a flux in dry assaying; it renders slags,
which would otherwise be pasty, quite fluid. Fluorides generally are
fusible, and impart fusibility to substances with which they form weak
compounds. Their fluxing action does not depend on the removal of
silicon as fluoride.
~Detection.~--Fluorides in small quantity are easily overlooked unless
specially sought for. In larger amounts they are recognised by the
property hydrofluoric acid has of etching glass. A watch-glass is
warmed, and a layer of wax is melted over the convex side. When cold,
some lines are engraved on the waxed surface with any sharp-pointed
instrument. The substance to be tested is powdered; and moistened, in a
platinum dish, with sulphuric acid. The watch-glass is filled with cold
water and supported over the dish. The dish is then carefully warmed,
but not sufficiently to melt the wax. After a minute or two, the glass
is taken off, and the wax removed. If the substance contained fluorine,
the characters will be found permanently etched on the glass. An equally
good, but more rapid, test is to mix the powdered substance with some
silica, and to heat the mixture in a test tube with sulphuric acid.
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