OF AETS AND SCIENCES. 95 



The perfect isomerism of the monoclinic crystals with the other 

 forms of antimonious iodide is shown by the readiness with which they 

 change into the hexagonal iodide even more strikingly than by the above 

 analyses. The general color of these crystals, like that of the ortho- 

 rhomic crystals, is yellow ; but the shade of color varies considerably 

 under different circumstances, depending obviously on the impurity 

 present. The purest crystals we have obtained had the same green 

 tinge as the orthorhombic plates represented by Plate II., although less 

 pronounced ; and from this the tint varied through lemon yellow to quite 

 dark brown, the brownish tinge evidently resulting from the free iodine, 

 which, as we have said, frequently adheres to the crystals. Now, when 

 heated, the monoclinic crystals (like the orthorhombic plates) acquire 

 a red color and hexagonal structure at a temperature below that at which 

 they melt. But the point of the change is by no means so definite as 

 in the former case ; and we observed circumstances connected with it ob- 

 viously depending on the impurities present, which have an important 

 bearing on the theory we have advanced above. 



In the first place, we found that the yellow monoclinic crystals would 

 bear a temperature of from 120" to 125'', without undergoing change. 

 The experiments were conducted like those with the orthorhombic 

 crystals already described (page 90) ; and comparative experiments 

 were made in which the two modifications were heated side by side. 

 The orthorhombic crystals were uniformly converted into the red 

 modification at 114°, while the monoclinic were only slowly altered 

 even at 125^*. Of one experiment, we have the note. Some yellow 

 crystals remain after heating for half an hour at 125'^ ; and of another, 

 The change is gradual : the color deepens ; the crystals become opaque, 

 and soon after melt. 



On the other hand, we observed that brownish crystals, which had 

 been deposited from a solution containing a large excess of iodine, 

 often reddened at the boiling point of water. In one experiment, 

 in order to obtain a direct comparison, we heated for one hour side by 

 side in a steam bath, portions of the following three different pre- 

 parations : — 



1. Greenish yellow orthorhombic plates obtained by sublimation. 



2. Lemon yellow monoclinic crystals similar to the one figured on 

 Plate I. Fig. 6. 



3. Brownish monoclinic crystals evidently occluding free iodine. 



Except the loss of evaporation, both 1 and 2 underwent no change, 

 but 3 was wholly converted into the red modification. The change 



