xvn MOLECULAR ARCHITECTURE 437 



where Gerhardt's newer atomic weights were employed. In 

 particular, it must be noticed that if A1 2 is tervalent, Al 

 must be at least quadrivalent, to provide for the linking 

 together of the two aluminium atoms ; so also, if C 2 is 

 quadrivalent, the single carbon atoms must be at least 

 quinquevalent, etc. 



Kekule on the " marsh gas type " (1857) and the 

 quadrivalency of carbon (1858). The problem of molecular 

 structure was almost solved when Gerhardt's types (simple, 

 multiple and mixed) had developed into the conception of 

 valency. It was then seen clearly that oxygen and sulphur 

 might link together two radicals, whilst nitrogen and 

 phosphorus might unite three radicals. But the inner 

 structure of the radicals themselves remained obscure, until 

 Kekule detected the secret of their construction in the 

 quadrivalency of carbon ; the radicals, in fact, were held 

 together by their carbon atoms, which possessed the 

 remarkable power of uniting four other atoms or radicals. 



In 1857 he had referred some ten different compounds of 

 carbon to the MARSH GAS TYPE (Ann. Chem. Pharm^ 1857, 

 101, 204), thus deriving them from a hydrocarbon containing 

 four atoms of hydrogen 1 displaceable by other radicals. 

 In the following year, having accepted the atomic weights of 

 Gerhardt and of Cannizzaro, he was in a position to discuss 

 the combining-power of the single carbon atom in a marsh 

 gas of the formula, CH 4 , as follows : 



" If one considers the simplest compounds of carbon 

 (marsh gas, methyl chloride, carbon chloride, chloroform, 

 carbonic acid, phosgene gas, carbon sulphide, prussic acid, 

 etc.) it appears that the quantity of carbon, which chemists 

 have recognised as the smallest possible, as the atom, always 

 binds four atoms of a monatomic, or two atoms of a diatomic 

 element ; that in general the sum of the chemical units of the 

 elements combined with an atom is equal to four. This 



1 Kekule still wrote the formula of marsh gas as C 2 H 4 , taking the 

 atomic weight of carbon as 6. 



