Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/257

 blow with the hammer, even if the edge of the knife be precisely along the natural joints, is apt to have the effect of splitting it in various directions.

Quartz.

Fig. 5.

Since the time of presenting to the notice of the Geological Society, the memoir on the measurements of the angles of the primitive crystals of quartz and the sulphate of barytes, several other crystals of quartz with perfectly reflecting planes have corroborated the opinion therein stated, that the angles of the primitive rhomboid, which is slightly obtuse, are 94° 15′ & 85° 45′; which have also been further strengthened, and I may say confirmed to be their true value by co-incidences obtained by means of the reflecting goniometer from some fragments, exhibiting brilliant planes parallel with those of the rhomboid.

Crystals of quartz do not often present clear indications of their natural joints. By consulting Haüy, Traité Pl. XL. fig. 3. it will be observed that the primitive rhomboid is so situated in a dodecahedral crystal, that six of the twelve planes of the latter figure are alternately parallel with the primitive planes; the other six being the result of a modification explained by fig. 2 of the same plate. If therefore we would cleave a prismatic crystal of quartz, we are by the above circumstance assured, that by striking the prism diagonally and parallel with any plane of the upper or lower pyramid, it will be parallel with one or other of the planes of the primitive rhomboid, and, of course, in the direction of its natural joints. It will be well to attend to this observation, if