Lead dioxide, Pb0 2, also known as "puce oxide," occurs in nature as the mineral plattnerite, and may be most conveniently prepared by heating mixed solutions of lead acetate and bleaching powder until the original precipitate blackens.
Ammonium sulphide blackens it, and it is soluble in solution of ammonium acetate, which distinguishes it from barium sulphate.
The mastication causes a copious flow of saliva of a brick-red colour, which dyes the mouth, lips and gums. The habit blackens the teeth, but it is asserted by those addicted to it that it strengthens the gums, sweetens the breath and stimulates the digestive organs.
It blackens easily under the influence of smoke, and the town has consequently a somewhat gloomy appearance.
The bark of the tree contains a thick, resinous, acrid sap which blackens on exposure to the air.
