518 Prof. A. Ogg and Mr. Lloyd Hopwood : Critical Test 



from the zircon lattice by removing the Zr atoms and the 

 oxygen atoms associated with them, and substitute the Si 

 with Ti atoms. The oxygen atoms seem no longer to be 

 able to remain in the position they had in zircon, the 

 molecular axes all turn through an angle of 90° so as to 

 become parallel to the tetragonal axis, and so form the stable 

 lattice of anatase. 



(5) Also, in the case of anatase, the deformation of the 

 lattice from the cubic form is due to the oxygen atoms. 

 The molecular axes being all parallel, the lattice most 

 expands in the direction of the tetragonal axis to make room 

 for the oxygen atoms, and thus make the ratio c/a greater 

 than unity. 



(6) The absolute dimensions of the lattices of zircon and 

 xenotime are very nearly the same. The volume of the 

 lattices of the Zircon group shows a small but regular 

 increase with increasing atomic number. 



Also, the distance from a central atom to one of the 

 oxygen atoms associated with it increases with increase of 

 atomic number of elements belonging to the same family. 

 Generally the distance is smaller for a greater affinity to 

 oxygen. 



In conclusion, I wish to express my indebtedness to 

 Professor W. C. Brogger, for supplying me with the crystals 

 necessary for the research, and to Mr. H. Schjelderup for 

 valuable assistance in making the observations. 



LVI. A Critical Test of the Cry static-graphic Law of Valency 

 Volumes; a Note on the Crystalline Structure of the 

 Alkali Sulphates. By A. Ogg, M.A., Professor of 

 Physics, University College, Grahamstown, South Africa, 

 and F. Lloyd Hopwood, M.Sc.(Lond.), A.R.C.Sc, 

 Assistant in Physics, University of London, University 

 College *. 



rriHE complete investigation of the structure of crystalline 



substances involves two distinct inquiries. The first is 

 concerned with the various ways in which the structural units 

 may be arranged to form a homogeneous structure, and the 

 second with the nature of these units. 



While it is generally believed that the geometrical theory 

 of crystal structure is now complete, the laws governing 

 the relations between crystalline structure and chemical 

 * Communicated by Prof. W. H. Bragg, F.R.S. 



