124 THE SPIRIT OF THE SOIL 



than with why it happens. Experiment has shown 

 that the different atoms have varying affinities. 

 We may regard these affinities as hooks sticking out 

 from the smooth surface of the atom, one hook corre- 

 sponding to one affinity. Of the commoner elements 

 in agricultural chemistry, we can state that carbon 



r r 



has four affinities * C -* usually written more simply 



i -i 



C ; Hydrogen one, H ; Oxygen two, O ; 



I J 

 Nitrogen sometimes three and sometimes five, N 



I I 



or N ; Phosphorus five, P ; Sulphur six, 



ix 



or two, S or S ; Sodium one, Na ; and 



I \ 

 Potassium one, K . 



The statement can be made generally that there 

 cannot exist any compound in which any hooks are 

 left free, or, in chemical terms, all affinities must be 

 satisfied. Thus, while the substance water exists, 



O TT\ 



Tj_!T~ ~Z_jr, usually written S/O, the substance 



OH cannot exist because it would leave an affinity 

 unsatisfied, thus : 



In organic chemistry especially it is of vital 

 importance to know how the atoms are arranged in 

 the molecule, because the different groups give very 

 different results, and two substances may contain 

 exactly the same atoms with the same number of 



