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

This page has been proofread, but needs to be validated.

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.^[1]

↑ From the circumstance of the Count de Bournon having attributed to the angles of what he conceives to be the primitive prism, measurements nearly approximating to theme of P on P, and of P on the opposed plane over the elongated edge of a crystal similar to fig. 10, which is common to this substance, it may be assumed that he considers such a crystal to be a rhomboidal prism with diedral terminations. It will appear that I agree with the Count de Bournon in assuming the right rhomboidal prism with rhombic terminations as the primitive form; but differ from him in the manner in which that form lies (if it may so be said) in a crystal represented by fig. 10. He conceives the planes P P, to be primitive: I am on the contrary induced to suppose the planes P′ P′ to be primitive planes; we consequently differ in the angles of the prism.

[1] From the circumstance of the Count de Bournon having attributed to the angles of what he conceives to be the primitive prism, measurements nearly approximating to theme of P on P, and of P on the opposed plane over the elongated edge of a crystal similar to fig. 10, which is common to this substance, it may be assumed that he considers such a crystal to be a rhomboidal prism with diedral terminations. It will appear that I agree with the Count de Bournon in assuming the right rhomboidal prism with rhombic terminations as the primitive form; but differ from him in the manner in which that form lies (if it may so be said) in a crystal represented by fig. 10. He conceives the planes P P, to be primitive: I am on the contrary induced to suppose the planes P′ P′ to be primitive planes; we consequently differ in the angles of the prism.

[1]