636 SCIENCE PROGRESS 



may be regarded as one complete molecule, but it is evident 

 that the ordinary concept of a molecule has vanished. We 

 are forced to regard the solid state as quite distinct from that 

 of the liquid or gaseous state even of the same substance. 

 From this point of view it is scarcely to be expected that any- 

 thing like continuity can exist between the solid and liquid 

 states, such as Andrews first discovered for the liquid and 

 gaseous states. 



To appreciate the reasoning upon which the above con- 

 clusion is based it is necessary to assume some slight acquaint- 

 ance with nomenclature of crystal structure. The Braggs 

 found in the case of rock-salt, to take a concrete case, that the 

 relative spacing of the spectra obtained depended upon the 

 particular face chosen for examination. That is, the diffract- 

 ing particles were spaced regularly, but the distance from 

 particle to particle varied according to the direction con- 

 sidered. This is, of course, quite to be expected, and does not 

 tell us anything with regard to the nature of the diffracting 

 centres themselves. The Braggs, however, observed that 

 whilst the spectra obtained from the (ioo) and (no) faces 

 diminished in intensity quite regularly as the order of the 

 spectrum increased (the normal behaviour), the spectra ob- 

 tained from the ( 1 1 1 ) face showed alternate strong and weak 

 lines. This alternation constitutes one of the most important 

 of the results obtained in these investigations. To grasp its 

 significance, it is necessary to make use of an experimental 

 fact established independently, viz. that the efficiency of an 

 atom of a substance to function as a scatterer of X-rays is 

 proportional to the atomic weight, the heavier the atom the 

 more intense the reflection. Applying this fact to the peculiar 

 spectra obtained from a particular face of rock-salt, the Braggs 

 concluded that the alternations in intensity indicate successive 

 layers of chlorine and sodium atoms. That is, the real physi- 

 cal units of which crystals are built up are atoms and not mole- 

 cules. Each atom has, as it were, a separate and individual 

 existence. Since any single sodium atom is united to six 

 chlorine atoms we see at once that our ordinary idea of 

 valency is breaking down badly in the solid state. 



One of the most interesting cases investigated bjr the X-ray 

 method is the structure of the diamond. It has been found 

 that the carbon atoms are arranged at the centres of tetrahedra, 



