On the Intimate Structure of Crystals. 293 



deduced. Under the influence of rising temperature, the change 

 was smaller, from 111 to 100, but this value is said to be merely 

 approximate. That the change of volume should be smaller is not 

 unintelligible ; the direct effect of pressure is to bring about a 

 change of configuration, which is unstable, since the substance 

 reverts to its original state directly the pressure is relieved; the 

 direct effect of heat, on the other hand, is to produce an expansion 

 of the atomic volumes, and the change of configuration follows only 

 as a consequence of this. In the difference between the value 111 

 and 116 we should have a measure of this increase in atomic 

 volume, but for the probability that an expansion of atomic volume 

 may take place, as a consequence of the change of configuration 

 which results from the action of pressure. It is of interest to note 

 that the amount by which the pressure must be reduced to reverse 

 the operation and bring back the substance to the original state is 

 only one-half of that required to produce the change directly ; the 

 reversal also takes place much more rapidly than the direct trans- 

 formation. 



It is to the beautiful observations of Fizeau that we owe our 

 knowledge of the change in dimensions of silver iodide measured in 

 relation to its crystalline axes. Along the axis c, Fizeau found a 

 negative coefficient of expansion of 0*00000397; along two rect- 

 angular axes in a plane normal to c he found a positive coefficient of 

 0'00000065. These were for a mean of 40 over an interval of from 

 10 to 70; the coefficient of variation Aa/A0 was found to be for 

 c 4*27, for the other axes T38. Thus for the mean temperature 

 the coefficient of contraction is six times that of the coefficient of 

 expansion. A very significant relation, but not more so than the 

 fact that the contraction increases as the temperature rises, and, as 

 Bodwell's observations seem to prove, the increase becomes very 

 considerable as the temperature rises above 70. Every geometer 

 will perceive at once that these relations are in absolute har- 

 mony with the conception we have framed of the ordering of the 

 molecules in the crystal. They directly depend on the changing 

 ratio of the sine and cosine of the angle 9 as the small sphere (Ag) 

 of the figure (fig. 6) is squeezed out from the three larger spheres 

 (I) against which it is pressed (only one of these spheres (I) is 

 shown in the figure). As the iodine atoms approach each other, 

 with fall of temperature, the line aC will revolve round a as a 

 centre, and thus the vertical parameter will expand as the hori- 

 zontal contracts. From the coefficients given by Fizea.u, it follows 

 that at his mean temperature of 40 the angle 9 is 9 18' ; from the 

 crystallographic measurements of Zepharovich, made presumably at 

 the ordinary temperature of the air, and therefore at or about 15 

 this angle appears to have become 22 17', as calculated for me by 



