432 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The round brackets which Gerhardt used in formulating 

 his copulated compounds have been retained to the present 

 day as an integral part of the symbolism of structural 

 chemistry. The curly brackets of the theory of types, on the 

 other hand, have long since disappeared. 



G. VALENCY. 



Unequal combining-power of the elements indicated in 

 Gerhardt 's simple inorganic types, The theory of types 

 as developed by Gerhardt carried with it the idea that : 



(i) In compounds of the hydrogen type, a hydrogen- 

 atom could be linked to one radical, but only to one radical, 

 giving compounds such as 



(2) In compounds of the hydrochloric acid type, a 

 chlorine-atom could be linked to one radical, but only to one 

 radical, giving compounds such as 



C 2 



H 5 | C 2 H 3 Ch 



C1J C1J 



C1J C1J C1J ClJ 



Methyl chloride. Ethyl chloride. Acetyl chloride. Benzoyl chloride. 



(3). In compounds of the water type, however, two 

 radicals could be linked simultaneously to an oxygen-atom 

 as in 



1 As a consequence of Berzelius's dualistic conceptions of molecular 

 structure these acid-anhydrides had been regarded for many years as 

 the real acids, of which ordinary acetic acid, benzoic acid, etc., were 

 hydrates. They were, however, purely hypothetical compounds, until 

 Gerhardt in 1853 (Ann. Chem. Pharm., 1853, 87, 57-84, 149-179) 



