436 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Odling in 1855 (Journ. Chem. Soc., 1855, 7, 3) had pre- 

 sented the same idea in a graphical form : 



" For the clear elucidation of the succeeding formulae, I 

 adopt in them, a simple plan of marking these different 

 substitution values, viz., by one or more dashes to the right 

 or left of the symbol, something after the fashion frequently 

 made use of in algebraical formulae : thus, 



H', an atom of hydrogen, .... 

 Bi"', an atom of bismuth, ....... 



having a value represented by three atoms of hydrogen, 

 etc." 



Water was thus represented as TJ/ rO", and alum as 



K'CT 



K'AL 



if the atomic weight of aluminium be doubled, to conform 

 with modern usage. 



Frankland and Odling were, however, greatly hampered 

 by their adherence to an old system of equivalents, from 

 which even Kekule himself did not wholly escape until 

 1867. The result was that Odling wrote tin and iron as 

 Sn' (stannous) Sn" (stannic) instead of Sn" and Sn"" 

 Fe' (ferrous) Fe 2 "' (ferric) instead of Fe" and Fe'" 

 He thus regarded two atoms of aluminium and two atoms 

 of ferric iron as tervalent, as indicated by the symbols A1 2 "', 

 Fe 2 '". In the same way, Kolbe and Frankland, in a paper 

 in which they foreshadowed the quadrivalency of carbon 

 (Ann. Chem. Pharm., 1857, 101, 257-265) were only able to 

 discuss the combining-power of a double atom C 2 of atomic 

 weight 2x6 (see Japp's " Kekule Memorial Lecture, "Trans. 

 Chem. Soc.) 1898, 73, 97-138; p. 130). Their conceptions 

 of valency or combining-power could not therefore be put 

 forward in the same convincing way as in Kekule's papers, 



