196 THE FITNESS OF THE ENVIRONMENT 



VALENCE 



The principal theoretical foundation of 

 organic chemistry is the idea of valence. 

 Let us consider the chemical formulas of a 

 number of the simple compounds of hydrogen, 

 e.g. HC1, H 2 0, NH 3 , CH 4 , HI, HBr. It is 

 evident that in such formulas a single atom of 

 hydrogen is never represented as in union 

 with more than one atom of another element. 

 There are, however, cases where one atom of 

 hydrogen is in union with a single other atom, 

 e.g. HC1, HBr, HI ; or two atoms of hydro- 

 gen may unite with a single other atom, e.g. 

 H 2 ; or three atoms of hydrogen with one 

 other, e.g. NH 3 ; or four hydrogens with one 

 other, e.g. CH 4 . If the assumption be made 

 that discrete bonds or forces take part in the 

 union of atoms, hydrogen must possess but a 

 single such bond or valence. Otherwise com- 

 pounds of the type X — H — X, X — H<Q",or 



of some other type in which one atom of 

 hydrogen is in union with more than one atom 

 of lower valence, must exist, and this is con- 

 trary to fact. 



We may, therefore, employing a dash to 

 represent valence, write the constitutional 



