POPULAR SCIENCE 635 



on a plate. This arrangement, however, would be far too 

 coarse to produce dispersion of the X-ray beam. In fact, we 

 require small obstructions of the dimensions of atoms or mole- 

 cules themselves to function as a grating for X-rays. The 

 idea that the ultimate particles composing crystals could func- 

 tion as the required grating for the rays appears first to have 

 been suggested by Laue. The ingenious device of using a 

 piece of crystal as a reflection grating is due to W. L. Bragg. 

 With this modification very much simpler results were obtained 

 than those of Laue himself. We have to think of a crystal as 

 built up of layers of particles, and at each layer the beam 01 

 X-rays suffers a partial reflection. If the beam itself is homo- 

 geneous, we can observe its single line spectrum, repeated at 

 intervals, that is, at certain angles with respect to the plane 

 of the grating, i.e. the layers of the crystal. This reflected 

 beam of X-rays cannot be detected by any optical method, 

 but, as the Braggs have shown, we can utilise the property 

 of ionising a gas, which is possessed by X-rays, to determine 

 the position of the reflected spectral line. By a well-known 

 expression in physical optics we can calculate the distance 

 apart of two successive diffracting planes or layers of particles 

 in the crystal, provided we know the wave length of the light 

 employed, and the angle which the diffracted beam makes 

 with the diffraction plane. These data were obtained by the 

 Braggs for a number of crystals, and for various positions of 

 one and the same crystal. As a result of these measurements 

 we now know the values of the spacing of the particles in a 

 number of crystals, that is the distance of the particles apart 

 as we consider them from different directions in the crystal. 

 To give a single illustration of the order of magnitude con- 

 cerned it may be stated that in the case of rock-salt (sodium 

 chloride) in one particular plane, the spacing is 2 x io" 8 cm. 

 We now come to a question of the greatest importance. Are 

 the particles, which have been referred to, the molecules, say 

 NaCl, or the separate atoms ? The Braggs have discovered 

 that the atom and not the molecule is the fundamentally im- 

 portant individual in crystals. In other words, the concept 

 of the molecule has disappeared for these solids. It is im- 

 possible to say whether a given sodium atom belongs to a 

 given chlorine or not, since any single sodium is symmetrically 

 placed with respect to six chlorine atoms. The whole crystal 

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