THE CHEMISTEY OF SOLIDS ^ 



By Prof. Cecil H. Desch, F. R. S. 



It is remarkable how little we know with any certainty about 

 the chemical properties of solids, although the idea of a solid is 

 so fundamental. At the present time we always begin the study of 

 chemistry with the gases on account of the simplicity of their mathe- 

 matical treatment, but it must be remembered that this simplicity 

 is the result of long study and of many discoveries. To the un- 

 scientific mind the solid is simpler, because more tangible. When 

 men have tried to understand gases, they have expressed themselves 

 in terms of solids. The atom, however attenuated it may have 

 become in recent years, was in the first instance essentially a solid 

 sphere, and the elasticity of gases has been explained in terms of 

 the collision of elastic solid particles in motion. 



Our conception of liquids has been based in the same way on the 

 idea of moving particles, themselves thought of in terms of the 

 solid state. Yet, of solids themselves, whilst our knowledge of their 

 physical and mechanical properties is very extensive, our chemi- 

 cal information is of the most meager kind. It was an old doctrine 

 that chemical reactions could onl}^ proceed in the gaseous or liquid 

 states, so that chemical action on a solid was always preceded by 

 the tearing off of atoms from the surface under the influence of 

 electrical forces. That view can no longer be maintained. Chemi- 

 cal reactions can occur within or at the surface of a solid, but the 

 experimental difficulties are sometimes such as to make the exact 

 investigation of the subject a difficult matter. 



In the modern conception of a solid, the atoms are characterized 

 by a regular arrangement in space, that arrangement being repeated 

 so as to build up a crystalline lattice. Crystals and aggregates 

 of crystals are thus the only true solids, gases being regarded as 

 under-cooled liquids of high viscosity. The X-ray method devel- 

 oped by Laue and by W. H. and W. L. Bragg has made it possible 

 to determine, not only the class of a crystal, but also the exact lat- 



1 From the presidential address delivered at Southampton on Aug. 31 hefore Section B 

 (Chemistry) of the British Association; reprinted by permission from Nature, No. 2921, 

 vol. 116. Oct. 24, 1925. 



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