Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/269

 hammer. Instead of a table, however, I frequently place the crystals of such substances as may be best divided by a blow, on a slab of steel, polished on one side for the sake of a perfectly level surface, which is advantageous because the resistance given by steel is greater than that of wood. A lighter blow is therefore effectual, and hence there is less danger of cracking the crystal in directions opposed to its natural joints.

Sulphate of Lead.

Fig. 9, 10, 11, 12, 13.

The form of the primitive crystal of this substance is considered by Haüy to be a rectangular octahedron (fig. 9.) the angle formed by the meeting of one plane of the upper pyramid, with the adjacent plane of the lower, being in one direction (P on P) 109° 18′ and in the other direction (P'on P') 78° 28′; and he says, “Cet octaédre se soudivise sur les arétes contigues”.

The Count de Bournon, for reasons given in his “Catalogue, &c.” (p. 357) considers the primitive crystal to be a right rhomboidal tetrahedral prism, with rhombic bases, of about 78° 30′ and 101° 30′ (fig. 11.) which are about the measurements of P on P, and P over the elongated edge of a crystal represented by fig. 10. He adds that the sulphate of lead is among those substances in which there is no trace of natural joints, no possibility of obtaining a cleavage.