THE GROWTH OF CHEMICAL IDEAS 125 



Couper. Kekule wrote a most dramatic description of his 

 discovery. He was on a visit to London. He wrote: 



I sank into a reverie. The atoms flitted about before 

 my eyes. I had ahvays seen them in movement, these little 

 beings, but I had never succeeded in interpreting the man- 

 ner of their movement. That day I saw how two small 

 ones often joined into a little pair; how a larger took hold 

 of two smaller, and a still larger clasped three or even four 

 of the small ones, and how all span round in a whirling 

 round-dance. I saw how the larger ones formed a row and 

 only at the end of the chain smaller ones trailed along. 

 The cry of the conductor, "Clapham Road," woke me up 

 from my reverie, but I occupied part of the night in put- 

 ting at least sketches of these dream-products on paper. 

 Thus originated the structure-theory. 



While the molecules of a very large group of compounds, 

 the aliphatic compounds, could be built up as chains of car- 

 bon atoms, it was not possible to formulate in a similar 

 manner the aromatic compounds, which are characterized 

 by a relatively high proportion of carbon and never contain 

 less that six carbon atoms in the molecule. The simplest 

 member of this group is the hydrocarbon benzene. Benzene, 

 first isolated by Faraday, is shown by analysis to have the 

 composition Ce He- Since carbon atoms have a valency of 

 four, a compound with the composition Ce He should be 

 highly unsaturated, reactive, and unstable. The compound 

 C2 H2, acetylene, is, indeed, very unsaturated, reactive, and 

 unstable, as is evident when its structural formula HC^CH 

 is considered, for the two carbon atoms are attached to each 

 other by three bonds and can therefore add two atoms each 

 without dissociating. But benzene is not unstable or re- 

 active; it is stable and rather inert. 



In 1865, Kekule, then professor of chemistry at Ghent, 

 was engaged one evening in writing his textbook. "But it 

 did not go well; my spirit was with other things. I turned 

 the chair to the fireplace and sank into a half-sleep. Again 

 the atoms flitted before my eyes." His imaginative eye, 

 sharpened by repeated visions of a similar kind, could by 



