84 ORIGIN AND NATURE OF LIFE 



values, for satisfying or saturating one an- 

 other. These two properties of affinity and 

 valency lead to formation of the chemical 

 compound, or single chemical molecule, which 

 carries us the next step forward in the process 

 of evolution leading towards life. 



The varying degrees of affinity may be 

 illustrated everywhere in chemistry with the 

 greatest ease, our own lives depend upon 

 such varying affinities at every moment, 

 and chemical affinity is seen daily in varying 

 degree in all the familiar objects around us. 

 But perhaps the best example to place before 

 the non-chemist is that seen in the rusting or 

 oxidation of metals. It is well known that 

 gold remains bright in the air, while silver 

 slowly tarnishes, lead tarnishes quite rapidly, 

 copper turns green, and iron coats itself with 

 a red rust. These metals are all elements, 

 and their rusting or tarnishing is due to their 

 surface layer uniting with the oxygen of the 

 air to form chemical compounds known as 

 oxides. The familiar occurrences mentioned 

 above illustrate varying degrees of chemical 

 affinity. Now, by methods which need not 

 be described here, the chemist can measure 

 the amount of energy set free or absorbed 

 when elements unite, and so obtains measures 

 for the chemical affinity. As a result 



