On/ Crystallography. ^ 107 



Ctid decisive proofs which confiriTi this last opinion, and I 

 have tveti deiiion'trated ihat the dimensions and ang'es of 

 these iTUiIecuies were invariable in all primitive crysiaU of 

 one and the same substance. I shall not slop here agam to 

 obviate the dillicuiuts which opposed roy progress, be- 

 cause I have had the salislaclion of observing that the an- 

 swers si5g<res;ed themselves as if spontancouslv to those who 

 are ni posessiou of the theory, and because it appears to me 

 that this liieorv bas been taken up by (he only author who 

 studied it in the same points of view v-'ith myself*. 



Bui every form of the integrant rr.olecule varies in its di- 

 mensions, or in the measurement of its angles, accordineto 

 the species to which it belongst. The p.aralK-iopipedou is some- 

 times oblique-anekd, and sometimes right-angled : some- 

 times it presents the form of the rhomboid, and at other 

 times that of the cube, which is the most perfect amonsj the 

 forms of this kind. In some cases the triangular prism is 

 merely isoscele ; in other cases it is equilateral ; and in this 

 last case, the relation between its height and the side of lis 

 base varies in both species. The tetrahedron undergoes 

 analon;ous diversities. 



There exist, however, forms of integrant molecules, as 

 well as primitive forms which are common to several sub- 

 etances of diverse natures. For example, muriated soda and 

 sulphuretted iron both have the cube as their primitive form. 

 The regular octahedron is that of the ruby, and at the same 

 time that of native bismuth. In this case the elementarv 

 molecules, although difl'erent in all respects, are so arranged 

 that there results the same external contignralion nearly; a.« 

 in creoiirelry, we may compose a square in several ways by 

 assortments of figures which will differ from each other in 

 the various equares. l^esides, if observation has proved that 

 mineral snbsiances, distinct in their nature, sometimes ex- 

 hibit secondary crystals of the same form, (for instance, 

 regular hexahedral prisms with a diversity of structure which 

 would suppose one in the forms of the molecules themselve:s) 

 ouehi wt lobe surprised also to find in diflerent species inte- 

 grant molecules, the forms of which, similar externally, arc 

 owing to comltinatioiis of prir.ciples which cannot have anv 

 relation wilh each other? But it is worthy of atlention that 

 hitherto these forms common to several minerals were al- 

 ways those winch have a rtmaiKable charaeto- for simplicity 



• Thiorie dt U TVrrc, par I.awethcrie, '2d edit, tome i. p. ?,5 ct scq. 



f Observation only makoB known the nic.isiircmeuts (>f arT;'c'i, ami not tlie 

 reference of diniciisioDi; tijcory, however, Jurnishts us witli data for de« 

 \eflMni\ty: lilt, lutlci. 



ti and 



