84 MOLECULAR STRUCTURE 



one faculty of combining, only one valency, as shown 

 graphically by a stroke : 



H 



On the assumption that this faculty of combination is in 

 the formation of the molecule used, and used up, mutually, 

 it follows that all elements which combine with hydrogen 

 according to the symbol 



XH (C1H), that is X H (Cl H), 



like chlorine, bromine, iodine, &c., have, like hydrogen, only 

 one valency, or are univalent. They can, therefore, like 

 hydrogen, be used to determine the valency. The elements 

 that combine according to the schemes 



-H 



are accordingly bi-, tri-, and quadrivalent. 



Simple as the fundamental conception of valency appears, 

 its further development is complicated because there are 

 elements which do not always appear with the same 

 valency, e. g. iron is bi- or trivalent according as it occurs 

 in the so-called ferrous or ferric compounds. Valency 

 would accordingly supply a very unreliable basis for 

 determination of constitution were it not that in the 

 department in which such determinations are important, 

 namely, in organic chemistry, the elements mainly con- 

 cerned do possess constant valencies. Hydrogen appears to 

 be invariably univalent, oxygen almost always bivalent, 

 carbon quadrivalent, so the somewhat empirical method 

 practically followed on these grounds is of the highest 

 importance, and usually leads directly to the object aimed 

 at. Thus a compound with the molecular formula CH 5 N 

 can only, remembering the quadri-, tri-, and uni valence of 

 carbon, nitrogen, and hydrogen, have the constitution 



H/ 



