90 PROCEEDINGS OF THE AMERICAN ACADEMY 



After heating to 120°, complete change. 

 „ „ „ 107°, no change. 

 110° 



m° 



55 55 55 -"^i-*- 55 55 



1 19° 



55 55 55 -'-■'■'' 55 55 



„ „ 55 1 1 4°, partial change. 

 „ „ „ 114° complete „ 



These results were completely confirmed by similar experiments, all 

 of which indicated that 114° is very closely tlie temperature at which 

 the change first begins ; and this result is in complete accordance with 

 the fact we have before stated, that the red iodide of antimony, when 

 sublimed below 114°, is completely converted into the yellow modifica- 

 tion. 



It is evident, from the above experiments, tliut the point of the 

 change we Iiave been discussing is fully as sharply marked as tlie melt- 

 ing point of a solid; and, by referring to the table of melting points 

 on page 84, it will be seen that 114° (the temperature at which the 

 change takes place) is the very point at which, theoretically, the 

 normal iodide of antimony ought to melt. Evidently, then, the yellow 

 orthorhombic iodide does undergo incipient fusion at this point ; and 

 the molecules, becoming thus fiee to move, regroup tliemselves, and the 

 more stable sti'ucture of the red hexagonal iodide results. 



Here, then, we have certainly a most remarkable confiimation of the 

 theory we have advanced in regard to tlie molecular structure of hexa- 

 gonal forms. The two isomers we have just described have enabled 

 us to show that absolutely the same external form is compatible with 

 the differences of structure which distinguish the orthorhombic from 

 the hexagonal system ; and this fact, only probable before, is now de- 

 monstrated. Secondly, the conditions under which one of these isomers 

 changes into the other indicates clearly that the difference between the 

 two substances is simply a difference of grouping of the same molecules, 

 and also that in the red modification the molecules are more intimately 

 united than in the yellow. When we attempt to go further, and ex- 

 plain what this more intimate grouping is, we of course soon enter 

 the region of theory ; but the analogy furnished by the superimposed 

 mica plates is certainly very strong. We may now be said to know 

 that the structure of an hexagonal crystal can be produced by a more 

 intimate grouping of the molecules of an orthorhombic crystal, when- 

 ever the dimensions are such that the same external form is compati- 

 ble with these two types of internal structure. We also know that the 



