used to detect hydrofluoric acid, which changes its color to yellow.

All the test-papers are to be cut into narrow strips, and preserved in
closely stopped vials. The especial employment of the test-papers we
shall allude to in another place.


B. ESPECIAL REAGENTS.


13. _Fused Boracic Acid_ (BO^{3}).--The commercial article is
sufficiently pure for blowpipe analysis. It is employed in some cases
to detect phosphoric acid, and also minute traces of copper in lead
compounds.


14. _Fluorspar_ (CaFl^{2}).--This substance should be pounded fine and
strongly heated. Fluorspar is often mixed with boracic acid, which
renders it unfit for analytical purposes. Such an admixture can be
detected if it be mixed with bisulphate of potassa, and exposed upon
platinum wire to the interior or blue flame. It is soon fused, the
boracic acid is reduced and evaporated, and by passing through the
external flame it is reoxidized, and colors the flame green. We use
fluorspar mixed with bisulphate of potassa as a test for lithia and
boracic acid in complicated compounds.


15. _Oxalate of Nickel_ (NiO, [=]O).--It is prepared by dissolving the
pure oxide of nickel in diluted hydrochloric acid. Evaporate to
dryness, dissolve in water, and precipitate with oxalate of ammonia.