The "melt"
will slowly dissolve, but the solution should be hastened by keeping the
liquid moderately acid with hydrochloric acid. When the "melt" has
dissolved, clean and remove the platinum-dish, and evaporate the
solution to a paste. Continue the evaporation to dryness on a water-bath
(not on the hot plate), and whilst drying stir with a glass rod, feeling
at the bottom of the dish for any unfused particles, which, if present,
can be detected by their grittiness. If there is much grit, it will be
necessary to repeat the assay; but with a small quantity it will only be
necessary to refuse the grit and silica after ignition.
During solution of the "melt" and evaporation (which may be carried on
together), a clear solution will not be obtained, a flocculent silica
will separate out, and towards the end of the evaporation the mass will
get gelatinous. The drying of the jelly must be finished on the
water-bath; first, because at this temperature the silica is rendered
insoluble in hydrochloric acid, whilst the solubility of the alumina,
iron, &c., is unaffected, which would not be the case at a much higher
temperature; and second, because the gelatinous residue requires very
cautious drying to prevent loss from spirting.
When dry, the substance is moistened, and heated with strong
hydrochloric acid, and the sides of the dish are washed down with water.
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