Extract

The primitive crystal of quartz is considered to be an obtuse rhomboid, of which the angles are given by Haüy in his ‘Tableau comparatif,’ &c. as being 94° 24′ and 85° 36′: that of the sulphate of barytes is a quadrangular prism with rhombic terminations, the angles of which according to the same authority are 101° 32′ 13″ and 78° 27′ 47″. The results of some attempts to verify these admeasurements by subjecting the natural planes of the crystals of both these substances, as well as some regular fragments of the latter, to the reflecting goniometer, form the particular object of the present communication.

The first attempts to ascertain by this means the angles of the rhomboid of quartz, were made upon some minute primitive crystals from Bristol: seven of these gave incidences on the one angle varying from 94° 12′ to 94° 17′, and on the other from 85° 44′ to 85° 52′; not more than two or three agreed. But the reflections afforded by these crystals were not perfectly clear, probably owing to their enclosing a multitude of very minute diverging fibres, perhaps of thè oxide of iron. Numerous attempts were afterwards made upon some crystals obtained under the name of Bornholm diamonds; many of which exhibit, on the primitive crystal, the planes of the modifications producing the pyramids and the prism, not in a very advanced state; the measurements they afforded were much the same as, and by no means more regular than, those obtained from the