252 HISTORY OF CHEMISTRY. [LECTURE XIII. 



On the assumption which is here made, the number of 

 valencies of other elements that can unite with n carbon atoms 

 which are united to one another may be expressed by the 

 equation : 



n (4 - 2) + 2 = 2n + 2. 



It is true that this species of mutual union of carbon atoms 

 does not pass with Kekule as the only one; he draws attention 

 to the fact that in benzene and its homologues a closer union, 

 " the next simplest," may be assumed. 



In order to indicate the standpoint which Kekule adopted 

 at that time, I may add that, so far as the value of for- 

 mulae is concerned, he is a follower of Gerhardt, and does 

 not conceive these as representing the arrangement of the 

 atoms, but merely as reaction formulae. Accordingly he re- 

 tains Gerhardt's mode of writing them, and assumes, as the 

 latter had done, that several rational formulae are possible for 

 one substance. Kekule is well aware that, starting from the 

 hypothesis of quadrivalent carbon, formulae may appear in a 

 new guise, but he avoids entering more particularly into the 

 matter. This is comprehensible when we recollect that Kekule 

 only attaches such a limited significance to formulae ; since he 

 believes that the physical properties of substances can alone 

 lead to the establishment of hypotheses regarding the arrange- 

 ment of the atoms. These views possess all the more interest 

 from the fact that it had already been stated in a very in- 

 fluential quarter that two formulae cannot be assigned to one 

 substance, and that one substance cannot be referred to dif- 

 ferent types. In collating the results of his excellent investi- 

 gation on the specific volumes of liquids, Kopp 8 showed that 

 these specific volumes can be calculated from the composition 

 of the substances if a certain specific volume is assigned to 

 each element. This volume is not the same in all cases, how- 

 ever, but is dependent upon the part which the element plays 

 in the compound. Thus, for example, according to Kopp, 



8 Annalen. 92, I ; 95, 121 ; 96. I, 153, 303; 97, 374; and especially 

 ioo, 19. 



