12 THE FITNESS OF THE ENVIRONMENT 



similar positions. Thus the alkali metals, 

 lithium, sodium, potassium, rubidium, and 

 caesium, occupy the crests of the waves; the 

 halogens, fluorine, chlorine, bromine, and 

 iodine, fall about midway between crests 

 and troughs, and a little further study dis- 

 closes a host of other corresponding relation- 

 ships. 



Similar periodic variations may be shown 

 to occur in other physical properties of the 

 elements ; — the melting points, the boiling 

 points, the magnetic characteristics, etc. 

 Even more striking are the periodic varia- 

 tions in chemical properties, including the 

 general characteristics which first led to the 

 idea of rational classification, and more spe- 

 cific qualities like the combining powers for 

 hydrogen, oxygen, and other elements. 



The clearest proof of the value of the 

 periodic classification has been the predic- 

 tion of "new' elements, and accurate fore- 

 knowledge of their properties. Thus when 

 Mendeleeff first described the system, the 

 element germanium, discovered by Winkler 

 in 1886, was unknown; but from the proper- 

 ties of the elements surrounding a gap in the 

 system the Russian chemist was able to 

 predict its properties with almost incredible 

 exactness, as the following table shows. 



