TENDENCY TO SELF-COMBINATION 95 



3. Tendency to Self -combination 1 . 



Another characteristic feature of affinity is the greater 

 or less tendency of similar atoms to combine together. In 

 univalent atoms this action is obviously limited to the for- 

 mation of diatomic molecules, which appears not to occur 

 amongst the metals (Part II, p. 60), but to be general 

 amongst the metalloids, although the halogen molecules 

 consisting of two atoms break up at high temperatures 

 (Part II, p. 26). 



The tendency can show itself over a wider area in the 

 case of the multivalent elements, since the molecules of the 

 elements themselves may consist of more than two atoms, 

 and compounds too may contain several similar atoms 

 attached together. Here again it is essentially among the 

 non-metals that this tendency is found, and that in very 

 varying degree. 



The tendency to self-combination is much the most 

 marked in the case of carbon; not only are compounds 

 in homologous series up to thirty and more carbon atoms 

 arranged in order known, but this accumulation does not 

 cause the least indication of reduced stability. And free 

 carbon is, according to pp. 54 and 71, most probably com- 

 posed of polyatomic molecules. 



Next comes silicon, although far behind carbon, since 

 e. g. the newly discovered silicomesoxalic acid 2 



HOJSi.SiO.SiO 2 H 



with only three silicon atoms arranged in series, is con- 

 verted by soda with evolution of hydrogen, into silica, in 

 which self-combination of silicon cannot be assumed. 



The property in question is found in sulphur to a certain 

 extent, e. g. potassium pentasulphide 



S S K 



KSSSSSK or >S< 



S^ \s K 



is capable of existence ; but the corresponding acid gives 



1 Van 't Hoff, Ansichten uber organische Chemie, ii. 240. 



2 Berl. Ber. 32. 1114. 



