CONTEMPORARY ADVANCES IN PHYSICS 393 



rangement of the atoms; but where the advertisement is wanting, the 

 inner order may be none the less precise. Nearly every solid substance 

 owns it; metal, brick, stone and sand, wood, cotton, wool and bone 

 approach in varying degrees to crystalline perfection; so do films of 

 grease and films of liquid, and even in the middle of a liquid mass there 

 are traces of regular arrangement. The diffraction patterns disclose 

 all this, revealing the fine details of crystalline structure even where 

 the eye sees nothing but a shapeless mass. 



Even with the beautiful finely-formed crystals of the minerals in 

 museums, the diffraction-pattern teaches more than the crystal- 

 lographer could learn without it. I would not disparage the crystal- 

 lographers. Perhaps there are few physicists who realize how far 

 they went before the time of X-rays, and certainly any who thinks 

 that it was Laue's work which showed the world how to tell whether 

 a crystal is cubic has specialized in his science not wisely but too well. 

 Organic chemists also made many inferences about the arrangement 

 of atoms in large organic molecules, which the X-rays are now begin- 

 ning to verify. But the X-rays in these few years have carried us clear 

 beyond the farthest reach of inference to which chemist or crystal- 

 lographer could have aspired. 



Another service of diffraction is its disclosure of the ways in which 

 the tiny crystals making up an ordinary piece of metal are distributed, 

 their orientations in particular; these are liable to variations whenever 

 the metal is twisted or extended or rolled or hammered or annealed, 

 and it may some day be possible to explain from them the variations of 

 the mechanical properties of the mass. 



Yet another service of diffraction, and a very great one to the 

 physicist, is the information which it gives about the individual 

 group of atoms — sometimes indeed about the individual atom — which 

 is repeated over and over again to form the crystal. The beams of 

 the diffraction-pattern shooting off in their various directions may be 

 regarded as the beams proceeding in those same directions from any 

 individual group of atoms, tremendously amplified by the cooperation 

 of the other groups which go with it to make up the entire crystalline 

 network. The amplification-factor can often be estimated; and 

 dividing the observed intensities by it, one obtains an idea of the 

 diffraction-pattern which one group of atoms by itself would form, 

 and from this in turn may infer something about atoms. 



Diffraction-patterns are not formed exclusively with X-rays ; crystals 



may build them out of waves of another sort. Towards 1924 Louis 



de Broglie suggested that electricity and matter are partially wave-like 



in nature. The philosophy of physicists had changed since 1912, 



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