90 ORIGIN AND NATURE OF LIFE 



for atom with oxygen to form compounds 

 containing only two atoms, and this leads the 

 chemist to suspect that these must be divalent 

 elements. Now, on forming compounds of 

 these with chlorine the suspicion is confirmed, 

 for each unites with the chlorine under 

 ordinary conditions to form compounds in 

 which two atoms of chlorine are united 

 with one of the calcium or copper. In the 

 same way elements which act as triads, 

 tetrads, pentads, and even hexads can be 

 identified. 



In chemical interchanges and variations 

 from compound to compound, it may generally 

 be said that where chemical affinities permit 

 of it, one monad can replace another, or a 

 dyad atom may be replaced by two of a 

 monad, or by one atom each of two different 

 monads, and so on ; but circumstances often 

 arise which make this rule inapplicable. For 

 example, where a dyad is acting as a connecting 

 link between two groups of atoms it cannot 

 be replaced by two monads without the 

 compound splitting into two smaller com- 

 pounds to each of which one of the monads 

 attaches itself. 



Another complication arises because of 

 the tendency possessed by elements with 

 higher valencies to lower their apparent 



