434 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



"(i) Monobasic or monatomic, I, e.g. H, Cl, Br, K; 

 " (2) Dibasic or diatomic, II, e.g. O, S ; 

 "(3) Tribasic or triatomic, III, e.g. N, P, As." 

 (Ann. Chem. Pharm. 1857, 104, 132-133). 



A study of the multiple types and mixed types showed that 

 similar considerations could be applied to the radicals, 

 thus: 



" A monatomic radical can never hold together two mole- 

 cules of the types. 



" A diatomic radical can unite two molecules of the types 

 e.g. 



SO" 2 ,C1 2 SO/ H 



H JO CO")N 2 



Sulphuryl chloride. Sulphuric acid. Carbamide (urea). 



" A triatomic radical similarly unites three molecules of 

 the types, e.g. 



, C 3 H 6 '",C1 3 . 



Phosphoric acid. Glycerine. Trichlorhydrin. 



This "substitution-value" or combining power of ele- 

 ments and radicals is now described as VALENCY, elements 

 and radicals being described as UNIVALENT, BIVALENT, TER- 

 VALENT, and QUADRIVALENT (Lothar Meyer, Moderne 

 Theorien, 1864, 76), according as they combine with one, 

 two, three or four other univalent elements or radicals. 



Williamson (1851), Frankland (1852), and Odling 

 (1855) on combining power or valency. The concep- 

 tions thus clearly set out by Kekule were by no means 

 wholly new. Williamson in 1851 had represented the 

 conversion of "hydrate of potash" (potassium hydroxide) 

 into potassium carbonate by a scheme 



