Theory of Cryfafflfation. 29.' 



edge, or the fame angle, is fubjecled to feveral laws of de- 

 crement, that fucceed each other. In a word, there are cafes 

 where the fecondary cryftal has faces parallel to thofe of the 

 primitive form, and which combine with the faces produced 

 by the decrements to modify the figuie of the cryftal. 



I cz\\ ftmple fecondary farms, thofe arifing from an unique 

 law of decrement, the effect of which entirely conceals the 

 nucleus; and compound fecondary jorms, thofe which arife 

 from feveral fimultaneous laws of decrement, or from one 

 fingle law which has not attained to its extent, fo that there 

 remain faces parallel to thofe of the nucleus, which concur, 

 with the faces produced by the decrement, to diverfify the 

 afpeft of the cryftal. 1 (hall foon make new applications of 

 theory to the compound fecondary forms, of which fyntadtic 

 iron ore has already prefented us an example. 



If amidft this diverfity of laws, fometimes infulated, fome- 

 times united by combinations more or lefs complex, the 

 number of the ranges fubtracted were itfe'.f extremely vari- 

 able; for example, were thefe decrements by twelve, twenty, 

 thirty or foity ranges, or more, as might abfolutely be pofll- 

 ble, the multitude of the forms which might exift in each 

 kind of mineral would be immenfe, and exceed what could 

 be imagined. But the power which effects the Afterac- 

 tions fej ms to have a very limited atftion. Thefe fubtrac- 

 tions for the moft part take place by one or two ranges of 

 moleculse. I have found none which exceeded four ranges, 

 except in a variety of calcareous fpar, forming part of the 

 collection of C. Gillet Laumont, the ftruclure of which I 

 have lately determined, and which depends on a decrement 

 by fix ranges ; fo that, it there exift laws which exceed the 

 de rements by four ranges, there is reafon to believe that 

 they rarely take place in nature. Yet, notwithftanding thefe 

 narrow limits, by which the laws of cryftallifation are cir- 

 cumfcribed, I have found, by confining myfelf to two of the 

 fimpleft laws, that is to fay, thofe which produce fubtraclions 

 by one or two ranges, that calcareous fpar is fufceptible 

 U 2 of 



