| | | | | | | | | | |Chrome brown |Dissolves |Yellow |Blackened.|Blackened. | | |to greenish |solution, | | | | |yellow |black | | | | |solution. |residue. | | | | | | | | | |Cobalt brown |Dissolves |Unaltered |Blackened.|Unaltered. | | |to reddish | or | | | | |yellow |blackened. | | | | |solution. | | | | | | | | | | |Hatchett brown |Unaltered. |Becomes |Blackened.|Blackened. | | | |greenish | | | | | | blue. | | | | | | | | | |Humins, Bistre |Unaltered, |Give yellow|Unaltered.|Burn. | | |yellow |liquid. | | | | |liquid. | | | | +----------------+------------+-----------+----------+-----------+ _On Heating on Charcoal_:—Lead brown gives a lead bead in the reducing flame. Manganese brown and pyrolusite brown, when fused with saltpetre on platinum foil at a high temperature, give a bluish-green mass. Prussian brown and iron brown give a pale green borax bead in the reducing flame, which turns yellowish brown in the oxidising flame. Chrome brown heated, moistened with hydrochloric acid, and again heated, colours the flame green. It also gives a green borax bead. Cobalt brown produces a blue borax bead. Humin substances burn when heated on charcoal. REACTIONS OF THE BLACK PIGMENTS. Almost all black pigments consist of carbon, upon which reagents have no action. They should be at once heated on charcoal. If they burn away completely in the oxidising flame they consist of lamp black or carbon obtained by some process of incomplete combustion; if a white, infusible residue is left, the pigment is bone (ivory) black; if the residue is black the substance under examination must be ”neutral tint,” chrome black, or chrome-copper black. The two former give a pale green borax bead, whilst chrome-copper black gives a deep green bead, and when heated, moistened with hydrochloric acid, and again heated, it colours the flame green. CHAPTER LXXI. EXAMINATION OF LAKES. In examining lakes it is necessary to ascertain the nature of the organic colouring matter, the base with which this is united, and also the nature of the substances mechanically mixed with the lake to lighten its shade.