On Crystallography. S40 



metif. The importance of the subject cannot be doabted, 

 and it appears to me to have been very undeservedly neg- 

 lected ot laie. 



I air, sir, your obetiient servant, 

 Nov. 17, 1809. Enceps. 



XLIX. On Crystallography. By M.Uaux. Translated 

 from the last Paris Edition of Ids Traite de Mineralogie. 



[Contin-jed from p. 302.] 



Of secondary Forms, the Molecules of which differ from 

 the Parallelopipedon. 



It is a character common to all the primitive forms to be 

 tlivisible parallel to their dit^erent faces. In the parallelo- 

 pipedon, this division, when not joined lo any other which 

 can take place in difierent directions, evidently leads to a 

 form of molecule similar to the nucleus. In the regular 

 hexahedral prism, it tjives as a molecule an equilateral tri- 

 angular prism, as we have already seen in another place- 

 In the octahedron, it seems to tend towards molecules of 

 two different forms, some tetrahedrons and others octahe- 

 drons. This mixed kind of structure takes place also with 

 tespect to the tetrahedron. But every probable reason 

 concurs to exclude the octahedron, and to adopt the tetrahe- 

 dron in preference, as being, in those cases, the true inte- 

 gral molecule. Under the head Filiated Lime more par- 

 ticulars will be found on this subject. 



If we divide in the same way the dodecahedron with 

 rhombic planes, the molecules will be, without equi- 

 vocation, tetrahedrons with triangular isoscele faces. Under 

 the article Garnet, every thing relative to this knid of struc- 

 ture will be found. 



With respect to the dodecahedron witli isosccle triangular 

 planes, we cannot extract the molecules that coniixise it 

 without dividing it in directions diflercnt from those which 

 would be parallel to the faces. The tranchant planes in this 

 case ought to pass by the axis, and by the ridges contiguous 

 to the sunmiits, whence irregular tetrahedrons result An 

 nioleculca. This point of theory will be treated of in the 

 article Quartz. 



The other primitive forms are also sometimes subdivided 

 in duections which are not parallel to their faces. We 

 have already had an example of this, relative to the rhom- 

 boid of tiie tournialiiie, whose subdivision, following planes 



which 



