Appendix. 53 



Other relations than those of place may be represented geomet- 

 rically, with advantage. 



The aromatic hydrocarbons, together, are known as the 

 benzoles the chief liquid distillates from coal tar coal tar 

 naphtha. Among the distillates from coal tar are two other 

 hydrocarbons, which are solid, and are of unusual importance in 

 industry, namely : naphthalene and anthracene. These are not 

 homologous with benzene, as members of the same series 

 (CnHan^), but belong each in another series related progressively 

 to the benzene series, with mathematical harmony, as follows : 



I 

 C 



Benzene C 6 H 6 or C 



C n H 2n _ 6 - - Single | 



Hexagon _Q 



C 



II 



C 



Naphthalene d H 8 



or C n H 2n _ 2x6 



Double Hexagon 



c 



\ 



\ 



C- 



\ 



C- 



