[ ^55 3 



LXVnr. On CnjstaUograpbj. % il^. Hauv. TrandaUd 

 from the last Paris Edition of his Traite dc Mincialogic. 



[Continued from p. 338.] 

 OF THE REPRESENTATIVE SIGNS OF CRYSTALS. 



JL HE relations which Serve to connect the difierent on- 

 ginary crystals of one and the same substance with one 

 common primitive form, are foundfd, as we have seen, on 

 laws of structure whose effect is to determine the number 

 and arrangement of the planes which' compose the suriace 

 of each crystal. By a necessary consequence the naturalist, 

 who is familiar with the progress of these laws, frequently 

 finds it merely requisite to have before his eves the primi- 

 tive form, and the explanation of the decrements which irs 

 angles or its ridges undergo, in order to represent the poly- 

 hevlron resulting from it, and to see in what manner, in 

 idea, we may efi'ect the metamorphosis of the nucleus from 

 which this polyhedron is derived. 



These considerations gave rise to an idea of translating 

 into a very concise languaee, similar lothat used in Algebra, 

 the various laws which determine the secondary crystals, 

 and thus to compose species of formulae representative of 

 these same crvstals. It is sufficient, in order to attain this, 

 to desisiniite by letters the angles and ridges of the primitive 

 form, and to accompany these letters by cyphers, which in- 

 dicate the laws of decrements undergone bv such angles 

 and such ridges, and the result of which is a certain 

 secondary form. I have endeavoured to confine the arrange- 

 ment of the letters within a regular order, corresponding 

 with that of the alphabet, so that this arrangement natu- 

 rally presents itself to every one. 



By attention to this point and some others concerning 

 the manner of placing the cy|)hers, it requires, in my opi- 

 nion, but a very few seconds to acquire the key of the me- 

 thod, and the principles which ought to serve as a rule for 

 applying them will always remain impressed on the me- 

 mory. 



\Vhen we have traced and brought into a very narrow 

 compass the different formulas, which will be like the theo- 

 retical images of the crystals relative to one and the same 

 substance, it will be equally easy to compare tlicui, either 

 with each other, or with the primitive form, which will 

 also have its expression, to follow the transitions of the 

 simpler to the more compound forms, to distinguish what 

 lljey possess in common, and whatever is pccii!:iu 'oeach ; — 

 Ffi in 



