383 theory of CryfialDfatiori. 



In fhort, feveial minerals are divifible into right tnan-« 

 gnlar prifms. Such as the apatite, the primitive form of 

 which is a regular right hexaedral prifm, divilible parallel to 

 its bafes and its planes, from which neceffarily refult ria;ht 

 prifms with three planes, as may be feen by infpe£ting 

 Jig. 68, which reprefents one of the bafes of the hexaedral 

 prifm divided into fmall equilateral triangles, which are the 

 bafes of fo many moleculae, and which, being taken two and 

 two, form quadrilateral prifms with rhombufes for their bafes. 



By adopting then the tetraedron in the doubtful cafe of 

 which I have fpoken, we fhould reduce, in general, all the 

 fbfms of integral moleculse to three, remarkable by their fim- 

 plicity; viz. the parallelopipcdon, the fimpleft of all the fo- 

 lids which have faces parallel two and two; the triangular 

 prifm, the fimpleft of all prifms, and the tetraedron, which is 

 the fimpleft of pyramids. This fimplicity may furnifh a 

 reafon for the preference given to the tetraedron in fparry 

 fluor, and the other fubftances of which I have fpoken. I 

 fhall, however, forbear deciding on this iubject, as the want 

 of accurate and precife obfervations leaves to theory nothing 

 but conjectures and probabilities. 



But the efTential object is, that the different forms to which 

 the mixt ftruftures in queftion conduct, are allorted in fuch 

 a manner, that their affemblage is equivalent to a fum of 

 fmall parallelopipedons, as we have feen to be the cafe in 

 Tegard to fparry fluor ; and that the laminse of fuperpofition, 

 applied on the nucleus, decreafe by fubtra£tions of one or 

 more ranges of thefe parallelopipedons ; fo that the bafis of 

 the theory exifts independently of the choice which might 

 be made of any of the forms obtained by the mechanical di- 

 vifion. 



By the help of this refult, the decrements to which cryftals 

 are fubjecl:, whatever be their primitive forms, are found 

 brought back to thofe which take place in fubftances where 

 this form, as well as that of the moleculae, are indivifible 



paral- 



