288 Prof. W. J. Sollas. 



itself in the cubic system, but in hemimorphic Hexagonal crystals. 

 For this reason we cannot, by the method we have previously em- 

 ployed, calculate the diameters of its component atoms, but a refer- 

 ence to the preceding table will show that the gross volume of the 

 iodine is not greatly in excess of that which it possesses in sodium 

 iodide, so that in all probability its atomic volume is the same, or 

 sufficiently similar to serve as basis for argument. The diameter of 

 the silver atom is then 2'1715, that of the iodine 3'1905. The ratio 

 of the diameter of the silver to that of the iodine falls consequently 

 below the limiting rates of 0'7286 : 1"0. The exact ratio both for 

 this and the constituents of the other silver compounds is given in 



the following list : 



Ratio of 

 diameters of atoms. 



Ag : Cl = 0-8928 : 1 

 Ag : Br = 0-7761 : 1 

 Ag : I = 0-6894 : 1 



From this it is evident that while the chloride and bromide are well 

 within the 0"728 limit, the iodide lies outside it ; and it follows from 

 our hypothesis that while the two former salts may be, as they are, 

 cubic, the iodide should not, but might be hexagonal hemimorphic, 

 as it is. 



When silver iodide is heated it contracts along the vertical axis (c) 

 and expands in directions normal to this, till at a temperature of 

 146 C. it passes per saltum into the cubic system, and then with 

 further rise of temperature expands uniformly. The transition 

 from the cubic to the hexagonal system on cooling is accomplished 

 almost with explosive violence, fragments of the solid iodide are 

 projected into the air, and deep clefts extend into its substance. 

 How on the basis of our reasoning is this curious transformation to 

 be explained ? Only on the supposition that the atomic volumes 

 expand as the temperature of the substance rises ; the mass, as a 

 whole, may contract, as it certainly does, but this is to be accounted 

 for by a change in the relative position of the atoms, the atomic 

 volumes themselves must be conceived as all the time expanding. 

 But it may be asked, is there any reason why the iodine and silver 

 should expand at the same rate, and if not, which is the more likely 

 to possess a higher coefficient of expansion, the iodine or the silver 

 atom ? The probabilities are all in favour of the iodine. The co- 

 efficient of expansion of silver in the free state is 0*000037 from 

 to the melting point, and 0'00002 from to 100 C. That iodine 

 has an immensely greater coefficient may be judged from the change 

 in its specific gravity with rise of temperature, thus at 40'3 

 its sp. gr. is 4-917 ; at 60, it is 4'886 ; at 79'6, it is 4'857 ; at 107, 

 just before it liquefies, it is 4'825 ; after liquefaction, it is 4'004 ; 



