78 PROCEEDINGS OF THE AMERICAN ACADEMY 



reflected light, the rhombohedral planes can be distinguished on the 

 edges. When formed during the slow evaporation of the solvent, they 

 are, as Schneider also noticed, larger and more tabular. We our- 

 selves have further observed, that, when the solution contains an 

 excess of iodine, the rhombohedral planes become much more domi- 

 nant, and the crystals greatly elongated in the direction of the vertical 

 axis. The basal plane is then often reduced to a small triangular face ; 

 and we have seen crystals in which it had almost, if not wholly, disap- 

 peared. Among such crystals, we have observed macles hemitroped on 

 the basal section ; and the rhombohedral planes are frequently strongly 

 striated parallel to the basal edges. 



Under circumstances similar to those just mentioned, especially when 

 the amount of free iodine in the solution is proportionally large, the 

 crystals frequently group together into stars with six rays. These rays 

 are formed by crystals elongated in the direction of one of the diago- 

 nals of the hexagonal section, and each by itself has the outward aspect 

 of the trimetric system. The rays often brancli, but in all cases at 

 the constant angles of 60° or 120° ; and the whole group preserves a 

 more or less regular hexagonal outline. Such groups may be regarded 

 as skeleton crystals, and their formation is probably determined by a 

 deficiency of the substance of the crystals in the mixed solution from 

 which they are formed. The polariscope shows that they have through- 

 out an hexagonal structure, and their formation indicates a tendency in 

 the crystals of this substance (often manifested in single crystals to a 

 less degree) to excessive development in a single direction, thus imi- 

 tating a trimetric habit. As we shall hereafter see, this habit is not 

 without its significance. 



When iodide of antimony is sublimed as described in the previous 

 paper (page 57), and also by Schneider {loc. ciL), it condenses in very 

 broad thin leaves or plates, which hang from the surfaces of attach- 

 ment by their edges. Even these, however, frequently exhibit on their 

 free edges, not only the hexagonal outline, but also the rhombohedral 

 planes ; and the polariscope shows that the surfaces of the leaves are 

 simply widely extended basal planes. 



Iodide of antimony is not hygroscopic, and for this reason the crys- 

 tals present conditions which are more favorable for accurate measure- 

 ments than the crystals either of the chloride or of the bromide of the 

 same element. Nevertheless, our results were not as constant as the 

 brilliancy of the crystals led us to expect ; and we met with variations 

 in the angles which we could not ascribe solely to imperfections of the 

 faces or to other causes of inexactness in the measurements. The 



