This modification is represented by fig. 19. Pl. 16. and consists in a decrease on the four lateral solid angles of the primitive form, by which each is replaced by a plane, perpendicular to the axis passing through those angles.
Fig. 20. shews this modification in a more advanced state, and has been added not because it has been thus observed, but in order that the combination of the planes of this modification with those of the primitive form, may be the more readily traced in fig. 21. in which it occurs, though but rarely. In the fine collection of tins, in the possession of Mr. Sowerby, there is a specimen of
- The crystals of this substance, when on the matrix, have so greatly the appearance of being confusedly grouped, that little can be done towards describing them, on account of their splendour and numerous facets, without first detaching them from the matrix, which on account of their brittleness requires considerable care. The mode best adapted for preserving as well as for observing them, I first noticed in the scientific collection of the Count de Bournon, in which, insulated crystals are placed on wax. For this purpose I have used the common green taper cut into pieces of about an inch in length, and placed the crystal at one end. There are between 4 and 500 crystals of this substance so arranged in my collection, including every one described in the series belonging to this paper, and, being placed in that series according to the method adopted by the Count de Bournon (that is, according to their modifications) little or nothing need be said upon any of the individual crystals.
But in order to render the series more perfectly intelligible, I have taken especial care to place the drawing of every crystal throughout the series iu the same point of view, except in a few instances, for the sake of illustration. I am aware that an attention to this circumstance affords material facility to those who may desire to become acquainted with the crystallization of the substance, in tracing the modifications through their various combinations: and the same care has been observed, not to introduce in the series of any modification, the figure of a crystal exhibiting the planes of any other modification, that has not preceded it.
to adopt it as the integrand molecule. Indeed, it may fairly be doubted, whether, considering the present state of mineralogical knowledge, much benefit has accrued from the attempts that have been made to determine that of many other substances.