On the Intimate Structure of Crystals. 289 



and at 151, a point just above the temperature at which the hex- 

 agonal form of silver iodide is exchanged for the cubic, it is 3'866. 



We may fairly therefore attribute the greater expansion to the 

 iodine, the silver probably undergoing but slight increase in bulk. 

 In what way does this help us ? If the disparity between the 

 volumes of the atoms were originally the cause which determined 

 the silver iodide to assume hexagonal instead of cubic symmetry, 

 how, by increasing this disparity, shall we render it cubic? Nothing 

 can be simpler. By the time the substance has reached 146 C. the 

 atomic volume of the iodine has become so great relatively to the 

 silver that the latter is lost in the interstices, and it is the iodine 

 alone which is directly operative in supporting the crystalline 

 edifice. Here we have a basis for calculation. If the iodine thus 

 forms the framework of the crystal, it must be, as we shall show 

 later, because it is packed on that open system which we have 

 already described and illustrated in the first part of this communi- 

 cation. From this the diameter of the iodine spheres can be calcu- 

 lated. 



The volume of silver iodide, as deduced from Rodwell's data, is 

 40 % 67, just above 142, the point which Eodwell gives as the critical 

 temperature for the change of system. Supposing, as we have 

 already stipulated, that the iodine is built up in most open cubic 

 packing, we find from this volume the number 3*439 as the length 

 of the diameter of the iodine atom. 



We shall make important use of this number directly, but we 

 must first endeavour to find a probable structure for the silver iodide 

 when crystallised in the hexagonal system. 



It will conduce to clearness to avoid discussion at this stage, and 

 to describe at once the structure which I am led to think is the only 

 possible one consistent with the crystalline and physical properties 

 of the compound. Let us regard the silver and iodine spheres as 

 directly united together to form molecules of Agl ; conceive these 

 placed on a horizontal plane with their major axes vertical, and dis- 

 posed in triangular order as in the diagram (fig. 1). The balls must 

 not be in contact laterally, but separated by definite regular intervals ; 

 over this first sheet place a second similarly formed, and so related to 

 the first that the silver atoms rest in the alternate intervals left between 

 the lower set of iodine balls taken in threes (fig. 2). The assemblage 

 so produced will be homogeneous, hexagonal, and hemimorphic. It 

 represents silver iodide as it exists at ordinary temperatures. Now, 

 bearing in mind the fact that every silver atom rests in the interval 

 between three iodine atoms, let the latter enlarge to a slight extent, 

 and simultaneously retreat from each other along the horizontal 

 plane to a small but uniform amount. This corresponds to the 

 expansion along the lateral axes on heating. But as a result of this 



