X-RAYS AND CRYSTALS 383 



to intense spots. Similar tables may be drawn up for the other 

 values of h 3 . 



If the point system selected be the correct one, it is of interest 

 to speculate what significance this has with regard to the 

 crystal structure. This question cannot be answered definitely 

 until results are forthcoming obtained with crystals of other 

 systems but it is interesting to note that there is already strong 

 evidence for associating this point system with a crystal such as 

 zinc blende. In the first place, zinc and sulphur have the same 

 valency and according to the theory of valency volumes of 

 Barlow and Pope the atoms should be arranged as if they were 

 spheres of equal volume in a system of closest packing con- 

 sistent with cubic symmetry. In order that this may be so, 

 the centres of the atoms must be arranged in this point system, 

 centres at the cube corners and at the middle of the cube faces. 

 Thus, if all the molecules of zinc and sulphur behaved in an 

 identical manner towards the light waves, they would give the 

 interference maxima actually found to exist. Again, the same 

 point system is repeated, though on a different scale, when only 

 those atoms are considered which are identical in every respect 

 as regards chemical nature, their neighbours in the crystal and 

 so forth. In the arrangement of the atoms assigned by Barlow 

 and Pope to zinc blende and similar crystals of compounds of 

 two atoms having the same valency, atoms of one kind are 

 grouped together four at a time in little tetrahedra, these 

 tetrahedra being again arranged in this point system ; there- 

 fore identical atoms, one from each tetrahedron, have the de- 

 sired arrangement. The element of a grating is that which, 

 in the ideal case, repeats itself indefinitely without variation 

 and the crystallographs give evidence of the arrangement of 

 elements in the crystal. In a crystal such as zinc blende, it is 

 possible to class together a certain number of atoms of zinc 

 and sulphur in such way that the assemblage contains a speci- 

 men of zinc and sulphur atoms of all modifications and so that 

 the whole crystal may be built up by packing together these 

 assemblages. It is these which probably form the elements of 

 the crystal grating ; they form by repetition the crystal pattern. 

 The simplicity of the interference pattern seems to show that 

 it does not concern itself with the arrangement of atoms within 

 these assemblages. Whether this is so or whether the atoms 

 themselves are the grating elements might be settled by experi- 

 25 



