88 ORIGIN AND NATURE OF LIFE 



completes gradually the list of elements all 

 equal to hydrogen and to one another in 

 valency, and as no elements in nature are 

 found of a lower order of valency, such, for 

 example, as would unite two of their atoms 

 to one of hydrogen or any others of the 

 above group, it becomes obvious that the 

 valency possessed by hydrogen, and its 

 equals, is the minimal valency. These ele- 

 ments are accordingly called monads, or 

 univalent elements. 



The next order of valency amongst the 

 elements is such that an element belonging 

 to this group possesses double the affinity 

 of the monad, and these elements are called 

 dyads or divalent elements. This is a large 

 group of which oxygen may be taken as 

 the type. As stated above, water shows 

 the divalent nature of oxygen, for one atom 

 of oxygen is united in it to two atoms of 

 hydrogen. The same runs through all the 

 oxides of the univalent metals mentioned 

 above for two of each of these unite with one 

 of oxygen, when they combine spontaneously 

 with oxygen to form oxides. Compounds are 

 also known in which, instead of two atoms of 

 each alkali metal, one atom of the univalent 

 metal and one of hydrogen unite with the 

 divalent oxygen to form compounds called 



