(c.) Logwood Paper.—­Take bruised logwood, boil it in water, filter, and proceed as above.  Logwood paper is a very delicate test for free alkalies, which impart a violet tint to it.  It is sometimes 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.  The precipitate must be washed with caution upon a filter, and then dried.  It is employed in blowpipe analysis to detect salts of potassa in the presence of sodium and lithium.

16. Oxide of Copper (CuO).—­Pure metallic copper is dissolved in nitric acid.  The solution is evaporated in a porcelain dish to dryness, and gradually heated over a spirit-lamp, until the blue color of the salt has disappeared and the mass presents a uniform black color.  The oxide of copper so prepared must be powdered, and preserved in a vial.  It serves to detect, in complicated compounds, minute traces of chlorine.

17. Antimoniate of Potassa (KO, SbO^{6}).—­Mix four parts of the bruised metal of antimony, with nine parts of saltpetre.  Throw this mixture, in small portions, into a red-hot Hessian crucible, and keep it at a glowing heat for awhile after all the mixture is added.  Boil the cooled mass with water, and dry the residue.  Take two parts of this, and mix it with one part of dry carbonate of potassa, and expose this to a red heat for about half an hour.  Then wash the mass in cold water, and boil the residue in water; filter, evaporate the filtrate to dryness, and then, with a strong heat, render it free of water.  Powder it while it is warm, and preserve it in closed vials.  It is used for the detection of small quantities of charcoal in compound substances, as it shares its oxygen with the carbonaceous matter, the antimony becomes separated, and carbonate of potassa is produced, which restores red litmus paper to blue, and effervesces with acids.