Results of Crystal Analysis. 95 



disturbing influence of oxygen should be 100, 62, 30, 8 

 of a normal spectrum of the orders 1, 2, 3, 4. 



As, in the case of silver, the value of 6 occurring in the 

 Debye temperature factor should be the same for both faces, 

 the rate of fall of the intensity will be different for face 

 (100) and (111) of silver, even if we correct for temperature 

 in the way given by Debye. 



So lono- as we know so little about the cause of the 

 intensity variation, it will be difficult to say anything definite 

 with regard to the explanation of the observed differences. 



It might be possible that the temperature factor is different 

 for different faces even in the case of a cubic crystal. On 

 account of the different elastic properties along different 

 point-planes, such an explanation might not be unlikely. 

 But if it should be impossible to account for the difference 

 of intensity variation by differences of the temperature 

 factor, we should probably have to suppose that the atoms 

 had different reflexion properties in different directions. 



Summary. 



1. The lattice of gold and lead has been determined, and 

 found identical with that of copper and silver. 



2. The structure of the Zircon group has been completely 

 determined. The Si as well as the Zr atoms are arranged in 

 tetragonal lattices of the diamond type. In the case when 

 Si and Zr are replaced by identical atoms Ti or Sn, we get a 

 simple prism-centred lattice for the metallic atoms. 



3. The tetragonal structure is not produced by symmetry 

 properties of the atomic centres, but by the tetragonal arrange- 

 ment of the oxygen atoms. 



4. The lattice has a sort of molecular structure with mole- 

 cules of the type M0 2 , where M is an atom of Si, Zr, Ti, or 

 Sn. The three atoms forming one molecule are situated on 

 a straight line and with M in a central position. This line 

 might be called the molecular axis. 



The positions of the oxygen atoms are determined, when we 

 know the directions of the molecular axes and the distance to 

 the central atom M (molecular distance). The fact that the 

 molecular distance is different for different central atoms M, 

 as also certain geometrical relations of the zircon lattice, goes 

 to support the view that the groups M0 2 form chemically 

 bound molecules. 



5. For all the minerals considered, the molecular axes are 

 equally arranged and are always perpendicular to the tetra- 

 gonal axis, which accounts for the fact that the ratio c\a is 

 smaller than unity and almost equal for all minerals. 



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