92 



Dr. L. Veoard on 



The results of the calculation are collected in Table VIII.. 

 giving the parameters € and the distances I (figs. 4, 5) for 

 the three crystals. 



Table VIII. 



Crystal. 



ZrSiO s 



(TiO a ) 2 

 (SnO a ), 



« ... 



150° 

 60° 

 lll°-5 

 112°-5 



5/24 

 1/12 

 0155 

 0-157 



1-08 

 1-99 

 2-08 



10- 



Photographs o£ a model o£ the zircon lattice are shown in 

 Plate III. 



§ 10. The Molecular Structure of the Lattice. 



The Rontgen-ray analysis has shown that crystals are 

 built up of atomic lattices ; and in a number of cases pre- 

 viously treated by W. H. and W. L. Bragg the arrangement 

 of the atomic lattices has left no room for such a thing as a 

 molecule. Suppose, for instance, that in a crystal of rock- 

 salt we fix our attention to a certain Na atom, we cannot 

 from the geometrical arrangement tell which CI atom is 

 associated with it. This fact ; however, does not without 

 further proof necessarily involve any fundamental change 

 in our conception of the chemical binding as taking place 

 between pairs of atoms (Na-Cl). 



The atoms might possibly be connected up in pairs in such 

 a way that all requirements of symmetry were fulfilled. 



In the case of rock-salt and similar substances there should 

 be four pairs of simple cubic lattices, and the lines connecting 

 each pair should be arranged with cubic symmetry ; but as 

 we have three equal directions in the crystal and four pairs, 

 such an arrangement does not seem possible. 



If, however, we regard our lattice for the Zircon group, we 

 notice that each of the Zr or Si atoms is associated with two 

 oxygen atoms ; thus the groups Si0 2 and Zr0 2 form a kind 

 of " molecular elements " of the lattice. This is not merely 

 a way of regarding the geometrical arrangement of the 

 atoms ; but we have reason to believe that the groups Si0 2 

 and Zr0 2 form chemically saturated compounds. First of 

 all, the fact that the oxygen atoms are closer to the Si than 

 to the Zr atoms goes to support this view, and, further, 



