OP ARTS AND SCIENCES. 81 



the axial dimensions of the two orthorhombic forms and of the 

 hexagonal form, if referred to corresponding orthorhombic system 

 of axes : — 



For SbClg a = 1.139 b = 0.902 c=l 



„ SbBrg a = 1.150 b= 0.940 c = l 



•„ Sblg a = 1.155 ^^ = 0.923 c=l 



It thus appears that, constructively, iodide of antimony is closely 

 isomorphous with the two allied compounds, and the small apparent 

 differences in the axial dimensions of the three forms are no greater 

 than the uncertainties in these values, themselves arising from the im- 

 perfect measurements of some of the angles. 



But although the rhombic prism of 60*^ or 120*^ may imitate the 

 external aspect of an hexagonal form, as is frequently the case with 

 the micas and vermiculites, yet (as the optical relations of these very 

 minerals show) the two classes of forms may still remain perfectly dis- 

 tinct ; and it does' not, therefore, by any means follow that the rhombic 

 prism passes into the hexagonal system when the prismatic angle be- 

 comes 120°. We have, however, shown, in the paper just referred to, 

 that, by a species of interlamination, — interlaminar macling, we may 

 name it, — the orthorhombic crystals of foliated minerals frequently 

 imitate the structure, as well as the forms, of the hexagonal system ; 

 and although there is an obvious distinction between such structures 

 and homogeneous crystalline masses, like calcite or the substance we 

 are considering, yet analogy would suggest, that, even the true hexag- 

 onal structure may result from a more fundamental macling of the 

 same kind ; and we advanced the theory, that it might be the result of 

 what we called molecular macling. According to this theory, the 

 crystalline molecules of hexagonal forms are, in some cases if not in 

 all, groups of three simpler molecules, each of which (so far as its 

 chemical constitution is concerned) is a unit in itself, and possibly 

 under certain conditions may act as a unit ill a crystalline structure, 

 and probably always becomes isolated when the substance is converted 

 into vapor. Our theory also assumes that the members of these 

 groups are united among themselves in the same relative positions as 

 the diagonals of a regular hexagon, so that the optically uniaxial 

 character of hexagonal crystals, is an effect of such grouping, and the 

 hexagonal form an obvious result of the juxtaposition of the six-sided 

 groups. Further, we suppose that the simpler molecules are of such 

 a nature that, when united as individuals in positions parallel to each 



VOL. XIII. (n. S. V.) 6 



