Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/265

 and transparent, and bearing at first sight as much the aspect of quartz as of corundum. From this, I succeeded in obtaining, among others, four very minute portions with brilliant and perfectly reflecting planes. These were procured by the assistance of the pincers; but it must be confessed that, as the directions of the natural joints were not at all visible, it was more by chance than regular design that they were obtained at all. Owing to the extreme hardness of the substance, I found it requisite, after placing the specimen in the pincers, to envelope it and them in a piece of cloth, to prevent the escape of the fragments; as the force which it is requisite to use would otherwise have caused them to fly in various directions. The same mode was for similar reasons pursued in regard to the oriental ruby and the sapphire: the latter may be cleaved with the utmost beauty and regularity in one direction; in the others it is difficult.

One of the fragments of corundum yields the measurements o£ 86° 4′ and 93° 56′; the other three, each 86° 4′. Two minute portions of the sapphire give each 86° 4′; another 93° 56′. One fragment of the oriental ruby yields 86° 4′. In the whole seven corresponding measurements of 86° 4′ and two of 98° 56′, which therefore I consider to be the true value of the angles of the primitive rhomboid.

Sulphate of Strontian.

Fig. 2.

The form of the primitive crystal of the sulphate of strontian is considered to be a right prism with rhombic bases. Its angles are, according to Haüy, 75°. 12′. and 104°. 48′. On submitting several crystals with perfectly rejecting planes to the goniometer, I found