Mineralogy. 375 



2. P — x . i— ^ — Of). £~— • P + x . Fig. 15. Inclination of 



Jon/= 86° 4'; of h on edge x = 101° 6'; of g on the same = 151° 51'; 

 of t on the same 132° 31'. There occur many secondary faces ; the whole 

 has much the appearance of crystals of hemiprismatic Vitriol-salt. 



Cleavage, parallel to g, and to a face replacing the edge x of the prism, 

 imperfect. Fracture imperfect conchoidal. Surface, deeply streaked pa- 

 rallel to the edges of combination with y, particularly b and f, as indi- 

 cated in the figure ; the pyramids are smooth, t rough, though even. 

 Lustre intermediate between metallic and metallic adamantine. Colour 

 iron-black. Streak dark cherry-red. Opake, except in thin splinters, 

 where it transmits a deep blood-red colour. Very sectile. Hardness = 

 2.0... 2.5. Sp. Gr. — 6.-2...5A. 



Observations. — The chemical composition of this species, one of those 

 which were formerly comprised under the dark-red silver, has not been as 

 yet exactly ascertained. Before the blow-pipe, it gives results nearly 

 agreeing with those of rhombohedral Ruby-blende, but it contains only 

 about 35.00 ... 40.00 per cent, of silver, besides sulphur and antiraonv. 

 The only specimen of it, in the possession of Mr Von Weissenbach at 

 Freiberg, is supposed to have been found in the mine called Neue Hoff- 

 nung Gottes, at Braunsdorf, near Freiberg, in Saxony. It consists only of 

 crystals, and is not accompanied by any other mineral. 



A finely crystallised specimen from Hungary is in the possession of Mr 

 Brooke, which seems to have some properties analogous to the hemipris- 

 matic Ruby-blende. Yet its combinations appear to be tetartoprismatic, 

 and may therefore belong to another species. (Mohs, vol. ii. p. 606. 

 Transl. vol. iii. p. 42.) Professor Mohs remarks, in regard to the light 

 and dark-coloured varieties of Red Silver, that the difference between these 

 varieties, though originally founded on the different tints of colour and 

 streak of the two minerals, and on their lustre, which is dependent upon 

 them, is deeper rooted in the essence of these bodies than it would appear 

 at first sight. Though the forms do not seem to be very different, and the 

 peculiarities in the series of crystallizations be common to both, the spe- 

 cific gravity of the two substances is considerably different, being circum- 

 scribed, as far as our present information goes, within the limits of 

 5.8 ... 5A) for the dark-rtd, and of 5.4 . . . 5.6 for the light-red variety. A 

 dark-red cleavable variety from the Hartz gave 5.831, a light-red one, 

 also cleavable, from Annaberg, 5.524, and a crystallised one from the 

 Churprinz mine, near Freiberg, having the colour of the dark-red variety, 

 5.422. This subject deserves the particular attention of mineralogists, 

 though as yet it is impossible to settle any thing in regard to the determi- 

 nation of the species. 



30. Fergusonite, a New Mineral Species. 



Hemipyramidal, with parallel faces. P = 100° 28', 128° 27' Approx. 

 (a = V4-5-) 



