of Crystallized Bodies. 285 



particularly not of that prism, parallel to the faces of which 

 the composition takes place, to decide whether the obtuse edge 

 of it is greater or less than 120°. But so much is indubitable, 

 that the crystals neither are modifications of the cube, as is 

 supposed by Count Bournon, nor can be comprehended in the 

 rhombohedral system, as is taken for granted by others. 



I shall not enlarge here on the regular compositions met 

 with in Sternbergite, a new species, an account of which I 

 have laid before the Royal Society of Edinburgh. They near- 

 ly resemble Fig. 3. in their projection. No very remarkable 

 observations can be attached to them, and, besides, it would 

 require more particulars than I should properly indulge my- 

 self in, in the present place. 



Among the saline substances, occurring in compound crys- 

 tals after this law, the sulphate of potash deserves our notice. 

 The angle of the original prism being 120° 29', and regular 

 composition of three individuals very frequent, the forms were 

 long taken for rhombohedral ones, till Messrs Mohs, Brooke, 

 and Levy gave a more accurate description of them. Mr 

 Brooke, in particular, describes a composition resembling 

 Fig. 6 in its transverse section, which he obtained from a solu- 

 tion of the salt in distilled water. This is not, however, the 

 manner in which the usual apparently isosceles six-sided pyra- 

 mids are formed. This mode of junction is namely that of 

 Figs. 15 and 16, the planes passing through the edges of the 

 prisms, and producing a six-sided transverse section of the 

 compound group with four angles of 120° 29', and two of 

 119° 2', a kind of composition demonstrated likewise by the 

 optical discovery of it by Dr Brewster. As in the prismatic 

 melane-glance, all the faces bounding the compound crystal, 

 are such as are parallel to the principal axis of the fundamen- 

 tal form. 



Saltpetre presents the same phenomenon, as is evident from 

 its action on light, as observed by Dr Brewster. The crystal- 

 lographic properties have not yet been sufficiently ascertained. 

 Dr Forchhammer, however, informed me, that he actually 

 found a difference to exist in the angles of the six-sided prism, 

 hitherto supposed to be a regular one. 



The figure of snow is so nearly allied in its shape to some 



