Formation of Compound or Twin Crystals. 285 



with the general theory, and may be considered as evidence mere- 

 ly, that other powers operate on matter besides Crystallogenic at- 

 traction. 



A general explanation of the formation of secondary planes arises 

 from the nature of axial attraction, and has been adduced by Voltz. 

 The force of attraction in the line of these axes is necessarily in- 

 versely proportioned to the length of the axes ; that is representing 

 the axes by a, h, c, the force of attraction in the direction of each, 



111 a a 



will vary as - : t : - or 1 : t : -• If then any power should cause 



a variation in the proportion of the strength of attraction to the length 

 of the axes, we might expect secondary planes to occur, as the addi- 

 tion of particles would not take place in the same manner as if no 

 such variation had taken place. Such then must be considered the 

 origin of the modifications of the Primary forms. The variation in 

 the attraction necessary for each new plane may be stated simply on 

 mathematical principles. But I do not now propose to enter on the 

 consideration of this part of the subject, my object at present being 

 merely a general exposition of the theory with the view of explain- 

 ing the formation of Twin Crystals. 



Before proceeding, however, to the consideration of Compound 

 Crystals, a few remarks may here be made on the theory relative to 

 the forms of molecules, which seems to be so generally in vogue 

 among Chemists, and more particularly on the views of M. Gaudin, 

 who has lately attempted to reduce this Chemical theory, as it may 

 be called, to some shape and limits. 



This author has undertaken to point out the particular arrange- 

 ment of the atoms which takes place in the construction of the mo- 

 lecules of crystals. He has even proceeded so far in his investiga- 

 tions as to have succeeded in obtaining a compound molecule from 

 the elementary atoms of Feldspar, differing but 6' in the inclination 

 of its faces from the actual inclination in crystals of this mineral. 



In the first place, it may be remarked with regard to the theory 

 that the assumptions required by it are numerous and improbable. 

 To obtain the above result, the form and relative size of the atoms 

 ought to be known, as the primary forms of minerals have constant 

 proportional dimensions as well as angles of inclination between their 

 faces. To determine the former of these particulars, recourse might 

 be had, and it would seem justly, to the crystalline forms of the 

 elements, that is, such of them as are or may be rendered sohds. 



