80 PROCEEDINGS OF THE AMERICAN ACADEMY m 



Evidently, theu, so far as yet appears, the iodide of antimony is not 

 isomorphous with the corresponding bromide and chloride, although 

 there is no group of compounds which we should by analogy expect 

 to find more closely isomorphous than the chloride, bromide, and iodide 

 of the same element. It has been shown, however, by other crystal- 

 lographers as well as by ourselves, that the hexagonal forms are closely 

 related to the trimetric, and that when the angle of the rhombic prism 

 becomes equal to or even closely ajiproaches 60° or 120°, the last may 

 imitate, if they do not actually assume, both the external aspect and 

 internal structure of the first. We were, therefore, led to suspect that 

 we had before us another example of such a relation, and that the 

 iodide of antimony might thus be constructively isomorphous with 

 its allied compounds. The following calculation, moreover, strongly 

 sustained this view : — 



The large development of the pinacoid planes (010 and OTO) which 

 is very characteristic of the crystals of bromide of antimony, besides 

 the general form of these crystals (as shown by our figure), indi- 

 cate very clearly that these pinacoids are the analogues of the basal 

 planes of the hexagonal crystals of iodide of antimony ; and, if so, 

 then the isomorphism of the bromide and the chloride indicates that, 

 for the last compound, the analogues of these planes would be a 

 corresponding pair of pinacoids which does not appear on the actual 

 crystals. Again, we find the analogue of the angle, 120*^, on the 

 hexagonal section of the crystals of antimonious iodide, in the angle 

 98^, between the planes 101 and TOl on the crystals of antimonious 

 bromide ; and although the corresponding planes do not appear on the 

 crystals of antimonious chloride, yet the equivalent angle can be easily 

 calculated, and will be found to be 97"^ 26'. If now we compare the 

 tangents of the halves of these last two angles with § of the tangent 

 of 60", we shall obtain the following relations : — 



For SbClg tang. AS" 43' = 1.139 

 „ SbBrg tang. 49" = 1.150 

 „ Sbig f tang. 60" =1.155 



If, further, we take into consideration the third axis, this relation 

 will appear still more close and simple. Making, then, in the crystals 

 of antimonious chloride and bromide the half-axis c our unit, and 

 regarding for the time, as the vertical axis, the half-axis b, which cor- 

 responds to the vertical axis of the hexagonal form, giving also to this 

 last axis its known value, — we obtain the following comparison between 



