chapter Five 



THE CONSTITUTION OF LIQUIDS WITH 



ESPECIAL REFERENCE TO 



SURFACE TENSION PHENOMENA 



The epoch-making work of Laue and Bragg has given us a clear insight 

 into the structure of crystalline bodies. In the case of inorganic crystalline 

 substances, it has been found that the atoms are arranged according to 

 definite lattices so that certain groups of atoms are repeated in a regular 

 manner. In this arrangement, however, there is nothing to indicate where 

 one group ends and the next begins. Thus, in a crystal of common salt, 

 the chlorine and sodium atoms alternate in regular order, but are not 

 grouped together into pairs. Any division of the crystal into molecules is 

 thus purely arbitrary. We must therefore conclude that solid bodies are 

 not built up of molecules, but of atoms arranged in definite ways ; or 

 rather, we should regard a crystal as a single large molecule. 



From this viewpoint the forces holding solid bodies together (cohesion, 

 etc.) are chemical forces, and phenomena such as evaporation are strictly 

 chemical phenomena. So far, in the study of crystal structure, only in- 

 organic bodies have been investigated. It is certain that in solid organic 

 bodies the atoms are arranged in groups which are usually identical with 

 the molecules found when the substances exist as vapors. We may call 

 these group molecules. There are good reasons for believing that in crystals 

 of an organic substance these group molecules are held together by chemi- 

 cal forces (secondary valence) to form single large molecules. 



A similar theory is now proposed for the structure of liquids. Each 

 atom in a liquid is regarded as being chemically combined with all adja- 

 cent atoms. This chemical union may in general be of two kinds, charac- 

 terized by primary and secondary valences. The atoms held together by 

 primary valence usually constitute group molecules, in which the atoms 

 are more or less rigidly attached to each other in definite arrangements. 

 The secondary valence serves to hold the group molecules together. In 

 very many cases (such as most inorganic liquids) the group molecules are 

 very much simpler than the usual "chemical molecules" and may often be 

 entirely absent. Thus, in such substances as water, concentrated sulphuric 



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