Theory ofCfyJlallifatkn. itf 



Valcareous Metaftatic Spar {Fig. 21*). 



Spath. calcatre a douze triangles. Daubcnton. Tall. Miner, 

 feint. 1792, p. 15. iv. 5. De Ylfie, Cryjiall. t. i. p. 530, 

 var. 1. 



Geomet. Cbaracl. Inclination of the triangle ofd to ofx, 

 IO4 28' 40", and to I d, 144 20' 26". Angles of the tri- 

 angle ofd;f = 101 32' 13"; <* = 54° 27' 30' ; = 24° 



G ' 17''. 



Geomet . Properties. The obtufe angle o/V of each tri- 

 angle is equal to that of the rhombus of the primitive form. 



The inclination- of the faces ofdj ofx is equal to that of 

 the rhombufes b ofd, ga fx (jig. 4), of the nucleus f. 



That part of the axfe of the dodecaedron which exceeds 

 on each fide the axis of the nucleus, is equal to that axis ; 

 or, what amounts to the fame thing, the whole axis of the. 

 dodecaedrori is triple that of the nucleus. 



The twelve fcalenc triangles that compofe the furface of 

 this variety have their lefler fide?, which are united two to 

 two on the fix edges b d, df.fx, gx, g c, I e (fig. 21), ex- 

 actlv fituated like trrofe marked \\ ith the fame letters Li 

 fig. 4, which gives an idea of the pofition of the nucleus in 

 i to the fecondary cryfral. 



It is thence feen that the laminae of fuperpofition decreafe 

 in a direction parallel to the inferior edges b d, df fx, &c. 

 or the edges of which we have been fpeaking. The theo- 

 retic calculus demonftrates that this decreafirig takes place 

 in two ranges of integral moleculaa ; and, as the laminae al- 

 wavs retain their rhomboidal figure, the fum of all their ex- 

 ternal angles, analogous to I, d, f x, g, c, produces fix 

 itudinal ridges, which form, alternately, the large and 

 mean fides d, of (Jig. 21), of the fcalcne triangles. It 



1, variety is commonly called dent d; cocben. The Enylifh call it 

 dog's tooth, dent de cbien. 



t The word metafialic fignifics tranfpofition, or the metaftcj/is of the an~ 

 to the fecondary cryftal. 



5 may 



