Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/260

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than that assigned to it by Haüy. However it must be allowed that, on taking into consideration the circumstances that all the brilliant crystals did not agree in yielding the same results, and of my being compelled for want of brilliant fractures, to depend on the natural planes, it is possible that the measurements on which I rely may not be absolutely correct.

Like most other hard and brittle substances, the hyacinth most readily yields to the pincers.


Fig. 2.

In regard to this substance also I rely on the measurements obtained by means of the reflecting goniometer on the natural planes. The form of the primitive crystal is a right rhomboidal prism (fig. 2) of which the admeasurement of M on M is given by Haüy as 129° 30′. Two of the only three crystals that were submitted to that instrument agree in affording, each two measurements of that angle 129° 20′, and each also two of M on the adjacent plane over the edge A 50° 40′. The other affords one of 129° 20′; only two of its planes give clear reflections; which on the faces of the other two crystals were remarkably clean and well defined. These crystals are from St. Gothard.


Fig. 4.

The form of the primitive crystal of anatase is an elongated octahedron of which the common base is square. Of nine isolated crystals in my possession only two are sufficiently brilliant for the