S56 071 Crystallography. 



proEjress to make, before geometry can have the data neces- 

 sary for submitting to a precise and rigorou? theory the 

 combined forces of the diftVrcnt agents which concur in 

 crystalHzation, and ascend from thefacts already establish- 

 ed, to other facts more general, and more allied to the 

 Iriie causes aliich depend immediately on- the will of the 

 Supreme Being. It is a rich mine, the working of .which 

 is merely commenced, and which waits for more favourable 

 times and more scientific workmen to follow the vein to 

 the greatest depth. 



Of Crystals with a Moiety reversed, and of those ivhich 

 - appear to penetrate each other. 



We have hitherto considered crystallization as impress- 

 ing on its results the character of the greatest possible per- 

 fection, and producing nothing but isolated forms, exempt 

 from every thing that could aflect their purity and symmetry. 

 It remains for us to describe certain accidents, which, un- 

 der the appearance of exceptions or anomalies, still possess 

 a latent tendency towards the same laws to which the struc- 

 ture is subjected, when nothing deranges their progress or 

 disturbs their harmony. 



In ordinary crystals, the faces adjacent to each other al- 

 ways form saliant, and never re-entering, angles. But 

 crystalline forms also exist which present these last angles ; 

 and Rome de I'lsle was the first who observed that this 

 effect took place wheii one of the two moieties of a crystal 

 was in a reversed position with respect to the other*. A 

 very simple example will enable us to conceive this re- 

 versal. 



Let us suppose that B d (fig. 44) represents an oblique 

 prism with rhomboidal bases, situated in such a manner 

 that the panes AD d a, C D c/ c, are vertical, and B D arc 

 the acute angles of the base ; and the latter proceeds in a 

 risina; direction from A to C. Let us, besides, suppose that 

 the ))rism is cut into halves, by means of a plane which 

 should pass bv the diagonals drawn from B to D, and from 

 b to d, and that, the half situated on the left remaining 

 fixed, the other half is reversed without being separated 

 from the former. The crystal will be presented under 

 the aspect which v\e see in fig. 4.7, where the triangle Z''^'c', 

 which was one of the halves of the lower base (fig. 44j, is 

 now situated in the upper part (fig. 45), and forms asaliant 

 angle with the fixed triangle ABD, while the triangle 



•if Rijine ik Chic. — Cry«al. t. 1. Introd. n. 9S. 



BCD 



