CONDUCTION THROUGH GASES 141 



ference results, and the resultant effect is negligible. 

 If instead of homogeneous sodium light we used 

 white light, the different coloured components 

 would produce bright lines at different angles, 

 and a coloured spectrum would be formed at C 

 at right angles to BC. 



Now, if X-rays be regular wave-trains at all, 

 their wave-lengths must be much shorter than 

 those of visible light. No glass plates would be 

 thin enough to give a reflection spectrum. But, 

 if the atoms in a crystal be situated in regular 

 layers, it is possible (i) that they might act as 

 superposed reflecting plates, and (2) that the 

 wave-lengths of X-rays might be of the same 

 order of size as the distance between the layers 

 of atoms. 



This was found to be the case. In particular, 

 the Braggs have thus proved the wave-lengths 

 of X-rays to be about io~^ or one hundred 

 millionth of a centimetre, and have discovered 

 many interesting facts about the structure of 

 crystals. 



For instance, a photograph of the X-ray 

 spectrum from a crystal of rock salt, shows that 

 layers of high reflection are interspersed with 

 layers of low reflection. Hence it is concluded 

 that layers of sodium atoms lie between layers of 

 chlorine atoms. It is the atam and not the 

 molecule which is important in crystal structure. 

 Indeed, the crystal must be regarded as one 

 enormous molecule of formula Na^Cl,^. 



Again, the X-ray spectrum from a diamond 

 shows that the carbon atoms each lie at the 

 centre of a tetrahedron, and are linked together 

 in six-membered rings, corresponding exactly to 



