Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/263



Cyanite.

Fig. 7.

On submitting some brilliant crystals to the reflecting goniometer, it became evident that no reliance could be placed on their natural planes. The form of the primitive crystal is an oblique prism. The incidence of M on T, given by Haüy as 106° 6′, varied very much; 106° 6′, 106° 10′, 106° 20′.

This substance is considerably hard and brittle; but in the attempt to cleave it in the direction of its natural joints, the same means did not succeed that usually does with other substances possessing those characters. The pincers always bruised the laminæ in separating them, which was fatal to precision. The most effectual means to avoid this, I found to be that of placing the crystal on a table, and supporting its under part in such a manner that the laminæ to be separated should be perfectly at right angles with the table. A sharp penknife then being placed in the desired direction, a smart blow with a light hammer usually produced the effect. Several fragments procured in this manner agreed in the incidence of M on T, 106° 15′, and that of T on M on the adjacent plane over the edge A 73° 45′; the former being 9′ more than the measurement obtained by Haüy from the natural planes by means of the goniometer in common use.

A regular fracture in the direction of the terminal planes of the primitive crystal is not so easily obtained as those parallel with the lateral planes. I obtained one considerably brilliant, but not sufficiently so for the use of the reflecting goniometer.