Theory of CtyJlalVifathm 385 



G, B, D, A ; from which it refults that any one of the 

 faces, fuch as CLGO, is common to two rhomboids, one 

 of which would have its fummit in C, and the other in G, 

 and which would themfelves have a common part in the in- 

 terior of the cryftal. 



It may be farther remarked, that a line G S (Jig. 69.), 

 drawn from any one G {fg. 68.) of the folid angles com- 

 pofed of three planes, as far as the centre of the dodecaedron, 

 is, at the fame time, the axis of the rhomboid, which would 

 have its fummit in G, and one of the edges of that which 

 would have its fummit in C {fg. 68 and 69.). The com- 

 pofmcr rhomboids then have this property, that their axis is 

 equal to the fide of the rhombus. With a little attention it 

 will be eafily feen, that in each tetraedron, fuch as C L G S 



(fg- 6 9-)j au tne ^ aces are e( l ua * aru * f imuar ifofceles tri- 

 angles. 



If we fhould continue the divifion of the dodecaedron by 

 fe&ions paffing between thofe which we have fuppofed to be 

 directed towards the centre, and which fliould be parallel to 

 them, we fhould obtain tetraedra always fmaller, and ar- 

 ranged in fuch a manner, that, taking them in grOupes of 

 fixj they would form rhomboids of a bulk proportioned to 

 their own. 



The tetraedra, which would be the term of the divifion, 

 were it poffible for us to reach it, ought to be confidered as 

 the real moleculae of the garnet. But we (hall fee, that, in 

 the paffage to the fecondary forms, the laminae of fuperpo- 

 fition, which envelop the nucleus, really decreafe by ranges 

 of fmall rhomboids, each of which is the aflemblage of fix 

 of thefe tetraedra. 



The fulphure of zinc or blend has the fame ftrufture as 

 the garnet. I have divided by very clean fe&ions fragments 

 of this fubftance, in fuch a manner as to obtain fueceffively 

 the dodecaedron, the rhomboid, and the tetraedron. 



V01.I. Cc 2. Tra- 



