126 THE PATH OF SCIENCE 



this time distinguish large structures of compHcated con- 

 struction. He had seen rows of atoms linked together, but 

 never yet rings; nor had anyone else. This is how the idea 

 came to him: "Long rows, variously, more closely, united; 

 all in movement, wriggling and turning like snakes. And 

 see, what was that? One of the snakes seized its own tail 

 and the image whirled scornfully before my eyes. As though 

 from a flash of lightning I awoke." * 



But the picture Kekule had seen of the snake that had 

 seized its own tail gave him the clue to the most puzzling 

 of molecular structures, the structure of the benzene mole- 

 cule. For it Kekule suggested a closed ring of six carbon 

 atoms, to each of which a hydrogen atom is attached: 



H 



A 

 HC CH 



HC. €H 

 C 

 H 



This formula interpreted the behavior of benzene and its 

 derivatives in a satisfactory manner. For instance, it ex- 

 plained the fact that when t^vo hydrogen atoms in benzene 

 are substituted by other atoms or radicals, three different 

 di-suhstituted compounds can be obtained. Kekule pointed 

 out that these could depend on the position of the two sub- 

 stituted atoms in the ring. When they were next to each 

 other, they could be called ortho; opposite to each other, 

 para; and in the position where they were separated by one 

 hydrogen, he used the term meta compounds: 



* John R. Baker, Scientific Life, p. 13, London, Allen and Unwin, 

 1942. 



