Authors seem agreed that the black-lead ore is an altered carbonat, but they seem not to have been so well agreed as to the nature of the blue-lead ore. In the cabinet of Colonel Gibbs, there are specimens which appear satisfactorily to illustrate both these subjects. The black-lead is by the blowpipe alone reducible to metallic lead; there is one specimen in the cabinet referred to, which is blackened on what appears to have been the under side, and seemingly by the contact of sulphuretted hydrogen gas; that which was probably the upper part remains unaltered, and is beautiful white carbonat of lead; this appearance is the more striking, because the piece is large and full of interstices, by which the gas appears to have passed through. The blue ore is in large six-sided prisms of a dark blue or almost black colour; where the prisms are broken across, they present an unequal appearance; sometimes they are _invested_; and sometimes slightly, and at other times deeply, _penetrated_ by sulphuret of lead, having the usual brilliant foliated fracture. The part which looks like sulphuret of lead is easily reducible by the blowpipe, but not the whole crystal, as authors appear to imply; for if that part of the crystal which does not present the appearance of galena is heated by the blowpipe flame, it is not reduced, but congeals into the garnet dodecahedron, with its colour unaltered: these crystals are therefore phosphat of lead, and they appear to be either an original mixture of phosphat and sulphuret of lead, or the phosphat has somehow in part given up its phosphoric acid, and assumed in its stead sulphur, perhaps from the decomposition of sulphuretted hydrogen.