OF ARTS AND SCIENCES. 99 



the mouoclinic differs chemically from the orthorhorabic iodide, only in 

 containing a small amount of impurity. The molecules are supposed 

 to be similarly constituted and similarly grouped together ; but, by the 

 adhesion of the impurity to the molecules, a certain difference of form 

 results. This difference, although usually regarded as fundamental 

 (for it is the difference between two crystalline systems), does not 

 appear, as thus viewed, either great or essential ; and the fact that 

 similar differences of form are also met with in the mineral kingdom, 

 among the micas, the vermiculites, and the chlorites, without the corre- 

 sponding differences of chemical composition and physical qualities 

 which an essentially different crystalline structure would imply, tends 

 to sustain our theory. The subject is obviously one of great impor- 

 tance, and we may hope that the further study of these artificial prod- 

 ucts may serve to elucidate what has been a very obscure chapter in 

 the science of mineralogy. We propose to continue the investigation, 

 and as soon as larger and better crystals of the several conditions of 

 antimonious iodide can be prepared, we shall repeat and complete the 

 optical measurements. Meanwhile, we are studying the allied iodides, 

 which promise further results. Every one wlio has had experience 

 with this kind of work knows how easily an observer may fall into 

 error by mistaking in these optical phenomena the delicate shades or 

 features on which important distinctions frequently depend, and this is 

 especially true when, as in the present case, the conditions are not the 

 most favorable. An examination of larger and more perfect crystals 

 will undoubtedly correct some of our data, and may modify some of 

 our conclusions. The results here given are the best that could be 

 obtained with the material at our command, and must be regarded as 

 provisional until better material can be secured. 



Tschermak maintains that the Muscovite micas are monoclinic crys- 

 tals, of which the acute bisectrix makes a very small angle with the 

 plane of cleavage ; and it is possible that the crystals of antimonious 

 chloride, bromide, and iodide, which we have studied, should partake 

 of a similar structure ; and that the thin, rhombic plates of antimonious 

 iodide obtained by sublimation should differ from the monoclinic 

 crystals of the same compound only in their habit. At least, with our 

 present imperfect measurements, we cannot disprove such a theory; 

 although the not necessarily incompatible theory advanced above 

 seems to us the more probable of the two, and the only one which is 

 consistent with the facts as they at present appear. Yet, if the other 

 view should prove to be the more correct, the general result of the 

 discussion in this paper would not be affected : only we must extend also 



