614 SCIENCE PROGRESS 



chemical compounds, such as the simple halogen salts of potas- 

 sium. Assuming that the atoms of a crystal are arranged as 

 points in a space lattice, W. L. Bragg has shown that a structure 

 the same in all cases can explain the transmission patterns 

 obtained with this series of salts, the differences in the patterns 

 being due to the fact that the efficiency of an atom as a diffracting 

 centre increases rapidly with its atomic weight. If the atoms 

 are of nearly equal atomic weights, as in K CI, they are nearly 

 equivalent as centres; if one is at least twice as heavy as the 

 other, as in K Br or K I, the lattice formed by the heavy atoms 

 alone gives the pattern. The experiments also point to the 

 single atoms acting as diffracting centres, the lighter atoms not 

 being associated in any special way with, or grouped closely 

 round, the heavier atoms, but occupying intermediate positions 

 between the neighbouring heavy atoms, so that they can equally 

 well be considered as belonging to different ones. For instance, 

 an atom of sodium is equally close to six chlorine atoms in a 

 crystal of rocksalt. Thus in such a crystal a molecule cannot 

 be considered as having any individual existence; rather the 

 whole crystal constitutes one huge molecule. There can be no 

 doubt that the method is likely to prove very valuable in 

 examining further the structure of crystals. 



It would be expected that the heat motions of the atoms 

 would influence the diffraction pattern formed by the X-rays. 

 That the heat vibrations do not disturb the patterns at room 

 temperature might be immediately explained by Lindemann's 

 conclusion that at such temperature the distance of the centres 

 of the molecules — or atoms — only varies a few per cent, owing 

 to heat agitation. In an extended mathematical paper Debye 

 has come to the conclusion that the heat motions will not affect 

 the positions of the spots of the patterns, or their sharpness, 

 but only their intensity, increasing agitation causing the spots 

 to become fainter and fainter. The independence of the positions 

 and sharpness of the temperature have been experimentally 

 confirmed by de Broglie, the weakening of the spots at high 

 temperature by Laue and van der Lingen, so that Debye's 

 theory has been, at least qualitatively, confirmed. 



A good deal of rather more random work has been already 

 done on the diffraction of the X-rays by substances other than 

 crystals. Keene has shown that ordinary rolled metal sheets 

 give X-ray patterns owing to the metal possessing a crystalline 



