OF ARTS AND SCIENCES. 83 



magnitudes the necessary dimensions were realized, although it is 

 probable that there are other conditions which must also concur to 

 produce this result. 



Evidently, we have before us just such an isomorphous series as our 

 theory anticipates, — a series of closely allied substances, in which the 

 orthorhombic passes into the true hexagonal structure ; and this fur- 

 nishes us with an excellent opportunity for testhig the theory we have 

 advanced. If the crystalline molecules of the hexagonal iodide of 

 antimony are really groups of three of the chemical molecules of this 

 substance, then we might hope to find another condition of this sub- 

 stance in which the molecules were united, as in the crystals of the 

 allied substances presenting rhombic forms with the angles of 60® or 

 120° ; and, if such could be discovered, it would be reasonable to 

 expect differences in the physical properties of the two isomers corre- 

 sponding to the differences of structure. We "were therefore led to 

 search for a rhombic modification of the iodide of antimony, and with 

 what success will soon appear. Before pursuing this subject, how- 

 ever, it will be best to describe some of the other physical properties 

 of the hexagonal iodide. 



The color of the hexagonal iodide of antimony is a'brilliant vermil- 

 ion red, which, however, in some preparations, is more or less tinged 

 with yellow, in consequence of oxidation, and the formation of an oxi- 

 iodide on the surfaces of the crystals. The solution of the iodide of 

 antimony in sulphide of carbon has a greenish yellow color, resembling 

 that of uranium glass, and strikingly contrasting with the brilliant red 

 color of the crystals which have been formed from it, — a fact whose 

 significance will hereafter appear. 



We made five determinations of the specific gravity of the hexagonal 

 crystals of iodide of antimony ; using for the purpose different prepara- 

 tions, and taking the specific gravity under petroleum which had been 

 rectified over sodium, — the only liquid we could find that did not act on 

 the substance. Even the rectified petroleum, however, acted slowly on 

 this, as it did on the allied substances ; but, during the time occupied 

 in the determination, the effect was very slight, and no considerable 

 error could have been thus caused. The following are the results of 

 these determinations, all made at a temperature of about 24° C: — 



