Tbcory of ' Cry flail Jation. i^q 



founded with the edges a b, a g, af, &c. {Jig. 23.) of the nu- 

 cleus, which fcrve as lines of departure for the decrements. 

 Hence it follows, that to extracT this nucleus' it will be ne- 

 ceflary to make the planes of the lections pais along the dia- 

 gonals in quefiion, as has been already remarked. 



The Common Topaz {Fig. 25). 



Hulls et Topaze du Brczil. Daubenlon. Tab. Miner, edit. 

 1792, p. 7, iv. 1 and 2. 



The inclination of the trapezoid /';■ t m to the adjacent 

 plane r t ey 136° ; of the tune plane to k ry z 124 26'; 

 of the plane t m g e to m I ig 03 °. 



The primitive form of this topaz is that of a right qua- 

 drangular prifm by {Jig. 24), the bales of which are rhom- 

 bufes, having the angle h or r — 124 26'. Theory fliews 

 that in regard to the integral molecula the height ry is to 

 the fide r n almoft in the ratio of 3 to 2. 



The pyramidal fummit of the topaz refults from a di 

 ment by two ranges of fraall prifms on the edges x r, r //, 

 n b, b x, of the fuperior bafe of the primitive form. The 

 planes / m g c, I m g e {Jig. 25), on one fide, and b k z />, 

 b u dp on the other, arife from a decrement by three ranged 

 on each fide of the edges n v, x q {Jig. 24), which decre- 

 ment remain-; fufpended at a certain term, and leaves four 

 rectangles / ry e, k ry v, I b c i, u b c d {Jig. 35), parallel 

 to the planes of the primitive form. The effect of this decre- 

 ment is reprefented jig. 26, where the rhombus bnrx is the 

 fame z&Jig. 24 ; and all the fmall rhombufes by which it is 

 fubdivided, or which are exterior to it, reprefent the bafes of 

 as many molecule. The lines * d, x z, n i, n c, are directed 

 accordn!;;' to the law of decrement already pointed out, and 

 the hues < J, c /, y z,y e, anfwer to the planes of the prifrn, 

 :h are not fubjeit to this law. 



3. Decregje on the Angles. 

 'I be obfervation which gave birth to the prefent theory* 



bv 



