ARRANGEMENT OF ATOMS IN A CRYSTAL. 133 



ATTEMPTS TO ASCERTAIN THE STRUCTURE OF KNOWN 



CRYSTALS. 



The next problem is to determine which of these types 

 are physically possible and can be used to explain the 

 structure of known substances. 



It is customary to suppose that when a substance 

 crystallises, the gaseous molecules, or the liquid molecules, 

 or the free ions, coalesce to form complex groups to which 

 the name of crystal molecules is given, and the regular 

 arrangement of these clusters is supposed to constitute the 

 structure of the crystal. Further, it is usually believed 

 that the arrangement of the crystal molecules is determined 

 by their inherent symmetry, and the symmetry of each 

 crystal molecule is unhesitatingly ascribed to the arrange- 

 ment of the atoms within it. 



It is obvious that with such a string of assumptions as 

 are here necessary the reasoning may be to the last degree 

 fallacious. Even supposing that we are justified in regard- 

 ing the chemical molecule as having a configuration due to 

 the arrangement of rigid parts, it is difficult to see why the 

 symmetry of the crystal molecule must be the same as that 

 of the crystal ; and if the chemical molecules of which it 

 consists be differently orientated, it is still more difficult 

 from the symmetry of the crystal to draw any conclusion 

 regarding that of the chemical molecule. 



Returning for a moment to the illustration of the cubical 

 rooms employed above, it might be expected that if the 

 nature of such a structure were under investigation the first 

 step would be to ascertain the form of the rooms, and 

 afterwards to determine the distribution of the articles 

 which they contain. In the same way, although we do not 

 know whether the crystal molecule (if it exist) is to be 

 regarded as represented by the room itself or by any one 

 of the symmetrically distributed articles, it might at any 

 rate be expected that attempts would first be made to 

 ascertain how the crystal particles are arranged, rather 

 than to investigate their symmetry or that of the chemical 

 molecule. 



