Fluorine is
shown by heating the assay with sulphate of potassa, in a closed tube
with a .strip of logwood-paper in the open end. The paper becomes
straw-yellow, and the glass is corroded. Another test is to heat the
pulverized mineral with concentrated sulphuric acid in a shallow dish
of platinum (or lead), over which a plate of glass covered with a coat
of wax, through which lines have been drawn with a piece of
sharp-pointed wood, is placed. If fluorine is present, the glass is
etched where exposed.
Boracic Acid.—The
mineral alone, or moistened with sulphuric acid, when melting, colors
the flame momentarily green. If the assay be heated with sulphuric
acid, and alcohol added and set on fire, the flame is colored green
from the vapors of the boracic acid.
Carbon, pulverized
and heated with saltpetre, detonates, leaving carbonate of potassa.
Carbonic acid is not easily discovered with the blowpipe, but the
minerals containing it effervesce in hydrochloric acid, and the
colorless gas that escapes renders litmus-paper red. In solution it
forms a precipitate with lime-water, which is again dissolved with
effervescence in acids.
Silica, before
the blowpipe, alone is unchanged ; is very slowly acted on by borax,
very little by salt of phosphorus, but with soda melts entirely with a
brisk effervescence into a clear glass. The silicates are decomposed by
salt of phosphorus, the silica being left in the bead as a powder or a
skeleton. Most of them melt with soda to a transparent glass. .Some
silicates are dissolved in hydrochloric acid, and this the more readily
the more powerful the basis, the less proportion of silica, and the
greater the amount of water they contain. Sometimes the acid only
extracts the basis, leaving the silica as a powder or jelly ; or the
silica too is dissolved, and only gelatinizes on evaporation. The
insoluble silicates may be first melted with
