202 Prof. W. J. Sollas. 



gives a spectrum less bright, but twice as long and particularly 

 developed in the blue and violet spectrum.* 



The passage of silver iodide from one form to the other is, as has 

 been stated, sudden and abrupt. This necessarily follows, from the 

 geometrical conditions of the case. On reference to fig. 5 it will be 

 seen by inspection that while a gradual descent of the atoms of 

 silver may take place so long as their centres are situated above the 

 centres of the atoms of iodine, against which they glide, yet directly 

 after they come to lie in the same plane, a sudden descent must take 

 place to a definite extent, which is given by the formula DOi DO 2 

 = n+2 2ri cos0, or by the equivalent -/(Sr^ 2 4^), where n is 

 the radius of the large spheres, r 2 of the small spheres, and Oi0 2 the 

 distance through which a sudden descent occurs. 



The instantaneous descent of the silver which thus takes place is 

 accompanied by a sudden change of volume in the compound itself ; 

 as determined by Rod well, the volume diminishes from 1 '01575 to 

 TO, as the temperature passes above 142, the maximum density of 

 the salt then being attained. In the collocation of spheres which 

 we have imagined, it is possible to compare the bulk before and 

 after the critical point is passed very simply. A tetrahedron is 

 constructed by joining the centres of three spheres of iodine below, 

 which are in contact with one of silver lying on the axis above, with 

 a fourth of iodine, also on the vertical axis and attached to the 

 single sphere of silver. The ratio of the volume of this tetrahedron 

 is directly proportionate to the whole volume of the structure, 

 whether in the cubic or hexagonal systems, and at the critical point 

 the volumes are directly proportionate to the heights of the respec- 

 tive tetrahedra. From this we find that the volume before contraction 

 is to that after as 114 : 100, amply sufficient, and, it might be 

 objected, superfluous ; for the contraction, as observed by Rodwell, 

 only amounted to from 1016 to 1000. This is a case, however, in 

 which theory proves more correct than observation, for Mallard and 

 Chatelier have shownf that Rodwell's results are erroneous, owing-, 

 as they remark, to his having deduced the cubical expansion from 

 the linear extension, as though silver iodide were an isotropic body. 

 These observers were able to bring about the transformation from the 

 hexagonal to the cubic structure by the application of pressure 

 (4,000 kilograms to the square centimetre), and they found that 

 the ratio of the volumes before and after change, was as 116 to 100, 

 which gives a coefficient ten times as great as that of Rodwell, and 

 very closely in agreement with that (114) which we have theoretically 



* Wernicke, ' Pogg. Ann.,' vol. 143, vide Rodwell, ' Proc. Boy. Soc.,' vol. 25, 

 p. 206. Wernicke accounts for this by supposing some of the iodine to be liberated 

 from combination. 



f 'Bull Soc. Min.,' vol. 7, 1884; ' Journ. Phys.,' vol. 4, p. 305, 1885. 



