10 THE FITNESS OF THE ENVIRONMENT 



to the means of the atomic weights of chlorine 

 and iodine, of calcium and barium, and of 

 sulphur and tellurium respectively. More 

 general relationships between the atomic 

 weights and properties of the elements were 

 first pointed out by Newlands in 1864 and 

 were extended by Mendeleeff and Lothar 

 Meyer a little later. Out of these studies 

 has arisen the law that the properties of the 

 elements are periodic functions of their atomic 

 weights. 



The essential characteristics of this law are 

 best illustrated by a consideration of the 

 relative volumes occupied by atoms of the 

 various elementary substances, the so-called 

 atomic volumes, which may be expressed by 

 dividing atomic weights by specific gravities. 

 The facts are graphically represented upon 

 the accompanying diagram, where atomic 

 weights are plotted horizontally, atomic vol- 

 umes vertically. 



Beginning with lithium the volumes fall 

 to boron and carbon, then rise irregularly to 

 sodium. A second fall leads to aluminium, 

 a second rise to potassium, and then the rises 

 and falls of the curve are repeated until, 

 among the elements of higher atomic weight, 

 gaps break the continuity of the relationship. 

 On the whole curve similar elements occupy 



