3oS Count de Bournon's Description of 
we cannot therefore hope to find, in the sand, any crystals of 
consequence, except such as have by accident escaped this first 
search ; some of these, however, I have had the good fortune to 
meet with. 
Among the beautiful series of crystals of this substance which 
I have been so happy as to procure, and to place in the several 
collections with the care of which I am entrusted by the friend- 
ship of their proprietors, there are four, in Mr. Grevilles 
collection, that I think it right here to take notice of. The forms 
of these crystals appear to me to be hitherto absolutely un- 
known ; for the Abbe Hauy, who may be justly considered as 
the most learned of those who devote themselves to the study 
of crystallography, does not even mention them, in the treatise 
on mineralogy he has just published. 
One of these forms, is a complete tetraedron, as in Fig. 43. 
It is produced by the enlargement of four of the faces of the 
octaedron, at the ex pence of the other four, which it has entirely 
caused to disappear. There are, in the same collection, many 
other crystals which are passing into this form, and are more or 
less advanced towards it. One of them, in which theie still 
remain some traces of the octaedron, which had entirely dis 
appeared in the preceding, deserves also to be mentioned. This 
variety, which is more common than the preceding, is repre- 
sented in Fig. 44. 
The second of the above forms, is a very acute rhomboid, the 
rhombic planes of which have iao° for the measure of their 
obtuse angles, and 60° for the measure of their acute ones. 
Fig. 45. This crystal is produced by the enlargement of six of 
the faces of the octaedron, at the expence of two opposite faces, 
