OF THE ANHYDROUS SULPHATE OF LIME. 191 



The broad faces, as well as thofe which terminate this fec-Luftre. 

 tangular prifm, have generally a very brilliant luftre ; the lon->- 

 gitudinal narrow ones faces are duller, and very frequently ftri- 

 ated in the direction of their length. The broad fides are be- 

 fides characterifed by a pearly reflection, fimilar to that which 

 is peculiar to the zeolite ftilbite, and this brilliancy remains 

 even after thofe faces have been divided : they alfo habitually 

 exhibit the interferon ((entrecroiffement) at right angles of 

 the joints belonging to the other faces. 



This cryftal prefents, along its longitudinal edges, a decreafe Secondaryforms. 

 which replaces each of thefe fides by a plane unequally in- °«a hedral 

 clined upon the adjacent faces : it forms with the broad fides 

 of the prifm an angle of 130°, and of 14-0° with the narrow 

 fides, (Fig. 2.) The prifm is therefore o&ahedral, and has 

 four edges of 130°, and four others of 140°. 



Thefe new planes frequently join each other, upon the nar- Hexahedral 

 row (ides of the prifm, and they then convert the octahedral P rlfm » 

 into a hexahedral form, having four edges of 130°, and the 

 two others of 100° (Fig. 3.) The pearly fides frequently 

 then continue to be the broadeft ; in other infiances, the fix 

 fides of the prifm are equal, or nearly equal with each other. 

 I have not yet feen any fecondary faces placed upon the ter- 

 minal edges. 



The cryftals upon which this defcription is founded are of 

 confiderable magnitude : fome are more than an inch in length. 

 They are of a fine flefii colour, and formed part of one of the 

 fpecimens brought from the Tyrol, in which the anhydrous Foreign admix- 

 fulphate of lime is mixed with muriate of foda or common '»« of muriate 

 fait ; but as this fait is intirely foreign to it, and only inter- 

 pofed between its parts, frequently even in a manner that is 

 very perceptible to the eye, the form of thefe cryftals ought 

 to be confidered as being really that of the pure anhydrous 

 fulphate of lime. Befides, 1 have feen in Mr. Greville's ca- 

 binet at London, a fpecimen in which this fubftance prefents 

 exactly the fame afpeet, and the fame colour, and which at 

 the fame time is totally defiitute of fait. In this, the anhy- 

 drous fulphate of lime is confufedly intermingled with a&inote 

 of a pale green colour, with fome parts of cupreous pyrites, 

 and of the black and very magnetic oxide of iron. The loca- 

 lity of this interefting fpecimen has not been preferved; but 

 there is every reafon to prefume that it came from Sweden. 



In 



