xvm THE CLASSIFICATION OF THE ELEMENTS 469 



12345678 

 Li. 2 BeO B 2 3 CO 2 N 2 O 5 



( includes 



Na 2 MgO A1 2 3 SiO 2 P 2 O 5 SO 3 



The "typical" hydrides and oxides at the head of Men- 

 dele'effs tables were selected so as to fit into a regular 

 series. Thus in Group I. the higher oxides of sodium and 

 potassium, Na 2 O 2 and K 2 O 4 , were rejected as " peroxides " 

 (which did not form salts) ; but the typical salt-forming 

 oxides, cupric oxide, CuO, and auric oxide, Au 2 O 3 , were 

 also set aside in favour of the less stable oxides Cu 2 O 

 and Au 2 O. The regular ebb and flow of valency with 

 increasing atomic weight is, however, one of the most 

 striking features of the periodic classification, and is set 

 out in a very effective way in a chart attached to the 

 Faraday Lecture which Mendeleeff delivered before the 

 Chemical Society in 1889 (Trans. Chem.. Soc , 1889, 55, 

 facing p. 656), where all the salt-forming oxides are 

 tabulated in a series of well-defined waves. These 

 regular changes of valency afford the sole justification for 

 crowding the elements into eight groups, to which a new 

 Group o is now added to include the inert gases of the 

 helium family, which appear to be devoid of all combining- 

 power. Moreover, no excuse, but those arising from identity 

 of valency in selected compounds, can be given for 

 including in the same column elements so diverse as sulphur 

 and chromium, chlorine and manganese, or potassium and 

 copper. 



Periodicity of atomic volumes (L. Meyer, 1869). The 

 most remarkable feature of Lothar Meyer's paper is a 

 diagram, which shows the ebb and flow of the ATOMIC 

 VOLUME 1 of the solid elements as the atomic weight 

 increases. 



1 The atomic volume = At. wt. x specific volume or At. wt.-r- density. 



