100 PROCEEDINGS OF THE AMERICAN ACADEMY 



to such monoclinic crystals as we have described the principles here 

 illustrated in regard to the relations of hexagonal forms. 



The chlorites, the vermiculites, and the micas, whose crystallographic 

 relations first suggested to us the theory of molecular macling, which 

 the new facts developed in this paper have so fully confirmed, are all 

 foliated minerals, of whose crystals the optical axis or acute bisectrix 

 is either normal, or inclined at only a small angle to the [)lane of easy 

 cleavage. With the crystals of antimonious iodide, both hexagonal 

 and monoclinic, there is also an easy cleavage, parallel to the basal 

 plane ; and there is also a similar, if not an identical, relation of the 

 optical axes. There is, however, no other evidence of a foliated struct- 

 ure, nor any sign of interlamination, such as we observed in those min- 

 erals. The crystals appear to be perfectly homogeneous, and (saving 

 their great brittleness) cleave more like crystals of topaz than those of 

 mica. The difference between the effect of interlamination and that 

 which, as we suppose, results from molecular macling, must not be over- 

 looked, although the optical phenomena in tlie two cases are so similar. 

 What we called, in our paper on the vermiculites, interlaminar macling 

 does not involve any essential change in the substance of the mineral ; 

 but molecular macling produces a new, although isomeric, substance. 

 The red and the yellow antimonious iodides are as different substances 

 as calcite and arragonite ; and, as we conceive, the difference in the two 

 cases is of the same kind. 



The facts developed in this paper all point to a more intimate rela- 

 tion between the different crystalline systems than has generally been 

 supposed to exist, and are in complete harmony with tlie opinion we 

 have frequently expressed, — that differences of crystalline system are 

 not necessarily more fundamental than corresponding differences of 

 dimension in the same system. 



Antimonious Oxi-Iodides (SbOI and Sb^OJj)- 



We have already, page 92, described the very remarkable chemical 

 reaction which takes place when a solution of antimonious iodide in 

 carbonic disulphide is exposed to the action of light and air. The re- 

 action is chiefly that expressed by the formula, — 



Sbig + O = SbOI + I-I ; 



but this is, to a very limited extent, accompanied by the more complex 

 reaction, — 



4 Sblg + 0, = SbAIa + 5 I-I. 



