On the Intimate Structure of Crystals. 291 



expansion the atoms of silver will descend deeper and deeper 

 between the atoms of iodine, and the distance between successive 

 sheets in a vertical direction will diminish. This corresponds to 

 contraction along the vertical axes with rise of temperature. On 

 this hypothesis the anomalous contraction of silver iodide ceases to 

 be an anomaly, but follows as a natural, though by no means neces- 

 sary, consequence of its crystalline structure. 



At the critical point, when the hexagonal is exchanged for 

 cubic symmetry, the atoms of iodine possess, as already shown, a 

 diameter of 3*439. Let them be represented in plan in the relative 

 positions they must occupy for open cubical packing. They are 

 shown in fig. 4 by a section taken at right angles to a trigonal 

 axis. 



Measured along the edge of the cube, the intervals between the 

 atoms from centre to centre must, on this system of packing, be to 

 the diameters precisely as v x 2 : 1. Find the diameter of a small 

 sphere that will just fit in between the three spheres as represented 

 in the section. It is exactly 2*17687. But we have already found 

 that the diameter of an atom of silver, both in the free state and in 

 combination, measures at ordinary temperatures 2*1715, an extra- 

 ordinary coincidence. The difference between this number and 

 that just obtained, 2*1769 2*1715 = 0*0054; again, the difference 

 between the diameter of the atoms of iodine in the alkaline iodides 

 at ordinary temperatures and the atoms of iodine in silver iodide at 

 142 is 3-4393*2 = 0*239, and these two numbers (0*0054 and 

 0*239) probably approximately represent the change in dimension 

 of the constituent atoms of silver iodide as the temperature of this 

 substance is raised through a range of 130. Calculating from the 

 coefficients given by Fizeau, the expansion of 0*0054, which we have 

 found for the silver, is just twice that which it would experience in 

 the free state. 



Up to the critical point, when the interval between the iodine 

 atoms has become large enough to allow the atoms of silver to pass 

 between them, the atoms of silver have been supported by the atoms 

 of iodine and have supported iodine in their turn all the atoms 

 have been subject to molecular pressure, but directly the critical 

 position is attained, the atoms of silver are driven by this internal 

 pressure into the middle of cubical clusters of atoms of iodine and 

 partly or wholly released from pressure, the atoms of iodine alone 

 then sustaining the crystalline fabric. It is not wonderful under 

 these circumstances that the spectrum of silver iodide should differ 

 markedly when observed below and above the critical point. Thus 

 Wernicke has observed that at a high temperature the spectrum (of 

 silver iodide), like that of solid and liquid iodine, contains no blue 

 nor violet light. In the normal state, below 138 C., silver iodide 



