222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



In spite of the obvious uncertainties that pervade all these dis- 

 cussions, there seems to be considerable evidence, from a considera- 

 tion of the crystal structures of such compounds as have been more or 

 less carefully studied, for the existence of two distinct types of solid 

 compounds: (1) Polar compounds, wherein the bonding between the 

 atoms, or at least between certain groups of atoms, is polar; and (2) 

 valency compound®, the atoms of which are bound to other atoms by 

 holding electrons in common. A consideration of organic compounds, 

 none of which have thus far been successfully studied using the 

 X rays, forces us to a third kind of compound; (3) the molecule- 

 forming compounds, built of groups of atoms (the chemical mole- 

 cules) held together presumably by relatively weak stray fields of 

 force. It will be noticed that in solids of the first two types no mole- 

 cules in the chemical sense of the word exist ; each crystal or piece is 

 a single chemical individual. 



