Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/271

 pyramid, varying from 78° 35′ to 78° 48′. One elongated crystal similar to fig. 10, gave P′ on the opposed plane over the summit 103° 40′. All these crystals reflected with uncommon brilliancy. One of eight crystals, sufficiently bright to afford clear reflections on each of the four elongated planes P P (fig. 10.) and more nearly approaching to agreement than any of the others, gave on the two obtuse angles 101° 12′ and 101° 15′, and on the two acute 78° 40′ and 78° 43′. Another among them gave one incidence of 78° 35′, and another of 78° 48′ on the two acute angles.

I am induced to be thus particular, not only on account of the vast difference between the measurements given by Haüy and those obtained by means of the reflecting goniometer, but also because it is scarcely possible to find another in the whole range of mineral substances, which seems to unite more decidedly the characters of perfect crystallization and brilliant surface. It seemed therefore pre-eminently adapted to the use of the reflecting goniometer. The result however proves that the sulphate of lead is one among many minerals, on even the perfect reflections of whose natural planes no reliance can be placed without numerous coinciding results; if indeed it would be possible to obtain them at all.

In the endeavour to cleave this substance parallel with its natural joints, I was at first greatly foiled by its extreme brittleness, which without great care causes its fracture to assume the conchoidal form. The directions for finding the joints given by Haüy, are quoted in his own words; if I comprehend them they are not accurate, but they are not expressed with his usual perspicuity.

As the elongated planes present the largest surfaces, it was an inducement to attempt a cleavage parallel with them in the first instance, on the presumption of their being planes of the primitive octahedron; but after destroying a large number of crystals, I was