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



strongest in the reflexions from the (001) planes, except in 

 the case of ammonium sulphate, which gives a stronger 

 second order. These general considerations lead us to think 

 that there are planes of sulphur atoms with spacings equal 

 to half the side of the unit rhomb, with planes of metal 

 atoms lying between them. The presence of a very strong 

 first-order spectrum from the (111) planes of ammonium 

 sulphate would seem to indicate a face-centred lattice for the 

 sulphur atoms. 



It appears, then, that a lattice with representative points 

 at the corners and at the middle points of the faces (see 

 fig. 1) would explain the spectra and give four molecules per 

 unit rhomb. 



Fig. 1. 



The dimensions of the crystals are such that the tri- 

 angles OHA and BHGr are nearly equilateral. It has 

 therefore been suggested by von Federow that the space- 

 lattice of the rhombic sulphates is pseudo-hexagonal. Tutton 

 considers the evidence so strong for such a structure that he 

 has adopted it. 



The arrangement we have suggested would give the 

 representative points in the (001) planes at the corners 

 of hexagonal prisms. But the plane KLMN is derived from 



