462 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



TABLE D 

 PERIODIC CLASSIFICATION OF THE ELEMENTS ! 



In this table the number of columns has been increased 

 to nine, in order to include the newly-discovered RARE 

 GASES, shown in Group O. 



He <3'99, Ne 20*2, Ar 39*88, Kr 82*92, Xe i3o'2, Nt 222-4. 



The discovery of these six gases not only adds an additional 

 column to the table, but justifies its subdivision into seven 

 chief periods. These periods are as follows : 



(1) The "hydrogen-period," which may perhaps contain 

 one or two other light gases. It may be extended, if 

 desired, to include helium, in which case neon would be 

 transferred to the end of Period II, and so on down the 

 series. 



(2) Two short periods of eight elements. Each period 

 starts with one of the rare gases, and both periods are similar 

 and complete. In Table E, the atomic weights are given 

 in round numbers and are seen to differ on the average by 

 1 6 units ; if exact atomic weights are taken the average 

 difference is 16*06, agreeing somewhat closely with the 

 atomic weight of oxygen. The average differences between 

 Period III and the initial portion of Period IV is i8j units. 



1 See footnote, p. 459. 



