446 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



atoms can be replaced by radicals, was introduced by Kekule 

 in 1857. 



These simple inorganic types showed only one stage in the 

 construction of the molecule, namely, the linking together of 

 radicals either directly (in pairs), or through an atom of oxygen, 

 nitrogen or carbon, whereby two, three or four radicals could 

 be held together. A further subdivision of the molecule was 

 effected in the multiple types of Williamson (1852-1855), 

 and Odling (1855), the mixed types of Odling (1855) and 

 Kekule (1857), and finally the conjugated radicals of Gerhardt 



G. VALENCY. 



Gerhardt's simple inorganic types indicated that the different 

 elements possessed unequal combining-power ; thus hydrogen and 

 chlorine could only be linked to one radical, oxygen to two, 

 and nitrogen to three. Kekule therefore suggested, in 1857, 

 that the elements should be described, according to their 

 substitution-values, as monatomtc, diatomic, triatomic. This 

 idea could also be supplied to radicals, the sulphuryl and 

 carbonyl radicals being diatomic, as in sulphuryl chloride, 

 SO 2 C1 2 , sulphuric acid, SO 2 (OH) 2 , and carbamide, CO(NH 2 ) 2 , 

 whilst triatomic radicals were present in phosphoric acid, 

 PO(OH) 3 , glycerine, C 3 H 5 {OH) 3 , and trichlorhydrin, C 3 H 5 C1 3 . 

 The diatomic character of the carbonyl radical in potassium 

 carbonate, CO(OK) 2 , had already been indicated by Williamson 

 in 1851. Frankland, in 1852, had directed attention to the fact 

 that nitrogen, phosphorus, arsenic and antimony always com- 

 bined with three or five equivalents of other elements, and had 

 suggested that each element had a definite combining-power, 

 Odling, in 1855, had gone further and had represented the 

 combining powers or substitution values of the elements by 

 symbols such as H', Cl', O", S", N'", P'". Frankland and 

 Odling thus set forth clearly the idea now known as valency, 

 but they were hampered by an obsolete system of atomic 

 weights, and in the case of many elements could only indicate 

 that the double-atom A1 2 was tervalent, C 2 was quadrivalent, 

 etc. The quadrivalency of the single carbon atom was first 

 postulated by Kekule in 1858 ; he had already, in 1857, intro- 

 duced the marsh gas type, containing four hydrogen atoms, 



