of the Periodic System of the Elements. 



281 



to Na than to K, and helium appears to be a sufficiently in- 

 active element, we may infer that in the period ? — Li we 

 have only the portion of the curve EFGr ; then H would fall 

 about the point E, and helium at the point F. 



Fig. 1. 



So far we have left the valency of the elements out of sight. 

 But, as we know, there exist for each chemical value two 

 series of elements analogous amongst themselves ; according 

 to the usual manner of writing we have, for example, the 

 bivalent and trivalent elements arranged according to in- 

 creasing atomic weight : 



Be, Ca, Sr, Ba, — , — , 

 Mg, Zn, Cd, -, Hg, 

 and 



B, Sc, Y, La, Yb, -, 



AL 



Ga, In, — , Tl, — . 



