POPULAR SCIENCE 483 



correct. They indicate, however, how enormous these forces 

 of cohesion are in liquids. Most of the values quoted are 

 taken from a paper by Tinker (7). 



With such forces as these it will be readily appreciated that 

 the correction term, a/V 2 , in the van der Waals equation is 

 really of far more importance than the P term itself in the 

 case of the liquid state. 



We have now dealt with some of the more important pro- 

 perties of molecules qua molecules. The next point for con- 

 sideration is the structure of an individual molecule. We shall 

 deal with molecules of gases and liquids and leave the question 

 of solids until later, as the solid state appears to be charac- 

 terised by certain fundamental properties not shared by 

 liquids or gases. 



In general, molecules contain two or more atoms. The 

 exceptions to this statement are the rare gases ot the atmo- 

 sphere and the vapours of metals, which are monatomic. 

 Chemical reactions in the majority of cases are essentially atomic 

 reactions, i.e. they involve the transfer of an atom or group 

 of atoms from one molecule to another, or the building up 

 of quite new molecular structures by the assembling of atoms 

 obtained from other molecules. We shall refer to this very 

 briefly later. The question of the structure of the molecule 

 resolves itself, in the first place, into the question of the struc- 

 ture of the atom. This is considered in the next section. 



{To be continued) 



