Theory of CryJlaUization. 37 



the rhomboid * of the fame fpar whofe large angle is about 

 loi^ , we fhall, at firft, be inclined to believe that each of 

 thefe two forms is entirely foreign to the other. But that 

 point of union which efcapes our notice, when we confine 

 ourfelves to the confideration of the exterior form, becomes 

 fenfible when we penetrate into the internal mechanifm of 

 the ftructure. Let me here then be permitted to trace 

 things from their origin, by relating how the obfervation 

 from which I fet out, and which has become the key of the 

 whole theory, firft prefented itfelf to my mind. 



I was holding in my hand a hexaedral prifm of calcareous 

 fpar, fimilar to the one I have mentioned, and which had 

 fallen from a group of which it formed a part. The frac- 

 ture prefented a very fmooth furface, fituated obliquely, like 

 the trapezium/) fut {Jig. i), and which made an angle of 

 1 35 , both with the refidue a b cfp h of the bafe, and with, 

 the remainder t u e/oi the plane i n e f. Obferving that 

 the cuneiform fegment pfutin feparated by this fracture 

 from the cryftal had for vertex one of the edges of the bafe, 

 that is to fay the edge i «, 1 wilhed to know if I could detach 

 a fecond fegment in the part to which the next edge c n be- 

 longed, by employing for that purpofe the blade of a knife 



* I give the name of rhomboid to a parallelopipedon ae (fig. 4) termi- 

 nated by fix rhombufes equal and fimilar. In every rhomboid two of 

 the lb'.id angles fuch as a, e, oppofite to each other, are formed by the junc- 

 tion of three equal plane angles. Each of the other fix. folid angles is 

 formed by a phne angle equal to each of the preceding, and by two other 

 angles of a different roeafure, but equal to each other. The points a,e, are 

 the fummits, and the line a e the axis. In any one whatever a b dfoi the 

 rhombufes, which compofe the furface, the angle a, contiguous to the fum- 

 mit, is called the fuperior angle; the angle d the inferior angle ; and the 

 angles b and / are the lateral angles. The fides ab, af, are the fuperior 

 edges ; and the fides bd, df the" inferior edges ; bf is the horizontal dia- 

 gonal ; and a d the oblique diagonal. The rhomboid is obtufe or acute ac- 

 cording as the angles of the fummits are obtufe or acute. The cube is the 

 boundary of the rhomboids. 



D 3 direfled 



