Prismatic Sulphuret of Lead or Galæna

The mineral commonly known in England by the name Blue Lead Ore, appears to have been considered by Haüy to derive its form from that of Phosphate of Lead by a substitution of Sulphur for Phosphoric acid and Oxygen, if such a process may be understood by the passage of the one ore into the other: the idea of its being a mixture of Phosphate and Sulphuret of Lead may be correct, but does not explain the nature of its formation. Some specimens are friternally Phosphate of Lead, and in those which are wholly Sulphuret its granular texture and the uniformity of its substance shew clearly that it is not the immediate result of crystallization. It is found along with dark brown Phosphate of Lead, in large crystals, which agree with it in form, and of which I have added a figure at the top of the plate. It has often a brown surface, when it much resembles the unchanged Phosphate in external appeatance, but is duller: at other times the surface is metallic, and is then generally variegated with a blue tarnish like tempered steel. Huelgöet in Britany, and Zschoppau in Saxony, are the only places where it has been procured.