of Copper and of Iron. 13 7 



edges of the prifm on which they -re inclined. When thefe 

 new planes have acnaircd an incrtafe of fufficient extent 

 to make the primitive planes of the prifm totally difiippear, 

 and to replace ihem, the cryftal is chanced to a tricdral 

 pyramid with a truncated apex ; the hafe and truncated 

 apex of which are equilateral triangles. (Fig. 21.) VV'hen 

 it happens that the cryftal has, at the fame time, gone 

 through this niodilication and that which replaces the I'olid 

 angles of its other extremity, and thefe two modifications 

 have commenced at the very origin of the formation of the 

 cryUal, there is a particular period of its progrefs, in whichi 

 the cryftal is lengthened into a hcxaedral prifm, with acute 

 triangular ifofccles planes, having for their bafes two equi- 

 lateral triangular planes, perfectly equal. (Fig. 21) After 

 this period, if the aft of cryftallization continues, the cryllal 

 alTumes the appearance of an extremely acute rhomboid, the 

 acute folid angles of which are replaced, more or lefs com- 

 pletely, by an equilateral triangular plane, (fig. 23.) and 

 finifhcs at lalt by taking the form of a perfeiA rhomboid. 

 (Fig. 24.) 



All thefe varieties, though lefs common than thofe of the 

 firft modification, are vet frequently to be met with, except- 

 ing that of fig. 22. which is extremely I'are, and of which 

 I have feen only two or three cryilals : in general, however, 

 the crydals of thefe varieties arc very fniall, and their form 

 cannot be well feen without the allillancc of a magnifying 

 glafs. 



It frequently happens that two of the elongated triedral 

 prifms (fig. 10.) are clofely united to each other by one of 

 the fides of the prifm ; whence refults a kind of niacle 

 (fig. 25.) the form of which is a rhomboldal tctracdral prifin 

 of 60'^ and 120'"'; but there is always difcerniblc, on the ter- 

 minal furfaces of thefe prifms, a very fine tranfverfe line, AB, 

 on the fmall diagonal of the rhomboidal i)lane of thefe fur- 

 faces ; this line fliows the place of union of the two cryllals 

 of which the made confills. 



Sometimes the two comi)oncnt cryftals of this kind of 

 macle belong to the tricdral prifm, which has a fecondary 

 plane in the place of one of its edges; it then has the form 

 either of a hexactlral prifm that has four of its fides (two and 

 two in oppofition) broader than the others (fig. 26.), or of one 

 that has only two oppofile fides broader (fig. 27.), or of a 

 regular one, according to the width of the fecondary planes: 

 in all thefe forms, the line A B, indicating the j)lace of union 

 of the two crydals, is perceptible. 



It is not very common, as I have already obfervcd, to 

 5 meet 



