Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/270



The crystals of the sulphate of lead, as well those from Anglesea as those from Cornwall, have so great a tendency to become prismatic by the lengthening of what Haüy considers to be an octahedron, and the Count de Bournon the primitive prism, that from among several hundred crystals in my possession, I have been able to select only one, having the appearance of an octahedron with pointed apices, and that this elongation always takes place in the same direction is manifest both from the uniform position of the secondary planes in relation to those which are elongated, and by the measurements afforded by numerous crystals; for although in the latter respect there is not a perfect coincidence, the results are sufficiently near to assure us of the fact.

The common base formed by the meeting of the two pyramids of the octahedron described by Haüy as the primitive crystal, (fig. 9.) though rectangular, is not square. The meeting of two adjacent planes on one pyramid, with their continuous planes on the other, will therefore be at different angles, as is the case also in respect of the primitive octahedron of the carbonate of lead. The angle formed by the meeting of P′ with P′ is according to Hairy 78° 28′, and that of P with P 109° 18′. Six or seven crystals submitted to the reflecting goniometer varied in P' on P from 76° 4′ to 76° 20′. Nine crystals affording 16 measurements of P with P varied from 101° 12′ to 101• 28′; and the same crystals gave 17 measurements of P on the opposed plane of the same