96 PROCEEDINGS OF THE AMERICAN ACADEMY 



was attended with some efflorescence, and the crystals generally be- 

 came more or less opaque ; but sometimes they retained their trans- 

 parency quite perfectly, and we noticed instances in which the change 

 spread through these crystals as through the orthorhombic plates before 

 described. It is evidently the same change in both cases ; and the best 

 explanation we can give of the phenomena is, that, while the fixed 

 carbonaceous occlusion, by loading the molecules, renders them less 

 mobile, and thus tends to prevent the change, the volatile iodine, on 

 the other hand, in breaking away from its attachment, destroys the 

 unstable equilibrium of the molecules, and thus induces the change 

 at a lower temperature than it would otherwise take place. 



The habit of the clinorhombic or monoclinic crystals of antimo- 

 nious iodide varies considerably under different conditions between 

 prismatic and tabular forms. As in the case of the hexagonal iodide, 

 the presence of an excess of iodine in the solution seems to determine 

 the production of elongated prisms, while, after the solution has been 

 freed from iodine, more tabular crystals are deposited. We measured 

 numerous angles on crystals of these different types ; and although 

 the forms were obviously the same, and the similar angles very nearly 

 equal, yet we observed differences in these angles amounting to fully 

 half a degree, even when the several measurements must have been 

 accurate to a minute. Such a variation in the angles was to be ex- 

 pected under the circumstances, and is wholly in harmony with the 

 theory we have formed of the structure of these crystals. The crystal 

 figured on Plate I. Fig. 6, and whose dimensions are given below, 

 was selected from the product of the most successful crystallization we 

 have yet made. On it were united all the forms we have observed on 

 any of the crystals of this modification of antimouious iodide. It had 

 a lemon yellow color, and was obviously of the tabular type. The 

 crystal was very perfect, and most of the angles admitted of accurate 

 measurement. The results are given below. They were verified by 

 numerous repetitions of the measurements in zones and by comparison 

 with similar angles measured on other crystals of the same prep- 

 aration. As usual, we deduced the elements of the crystal from the 

 measured values of three selected angles ; and it will be noticed that 

 the measurements of the other angles agree very closely with the 

 values calculated from these data assumed to be fundamental. Indeed, 

 we have seldom obtained better results with artificial crystals. 



