>cvm THE CLASSIFICATION OF THE ELEMENTS 459 



Pharm., 1870, 7, Suppl., 354-364), in which Mendele'efFs 

 suggestions were developed in a remarkable way by Lothar 

 Meyer. Meyer defined the law of periodicity as follows : 



"The properties of the elements are largely periodic 

 unctions of the atomic weight. Identical or similar pro- 

 oerties recur, if the atomic weight is increased by a definite 

 amount, which is at first 16, then about 46, and finally 88 

 :o 92 units " (loc. cit. p. 358). 



Thus there is a difference of 16 units between the alkali- 

 metals lithium and sodium, and again between sodium and 

 potassium. These elements resemble one another closely, 

 but differ widely from all the elements of intermediate 

 itomic weight. " Only the valency rises and falls regularly 

 .ind equally in the two intervals," thus 



Valency i 2 3 4 3 2 i 



j,j . Li Be B C N O F 

 Elements 



^ ^ A1 gi p s Q 



'loc cit. p. 358). 



Meyer's paper includes a complete classification of the 

 elements, which shows all the essential features of the 

 :amiliar table which Mendeleeff produced two years later, 

 ind in some points is in even closer agreement with the 

 nodern table on p. 462. 



The paper also contains a curve showing the variations of 

 ;ttomic volume as the atomic weight increases (see below, 

 :}. 472); this curve remains, even to-day, one of the best 

 illustrations that can be given of the principle of periodicity. 



Mendeleeff on "The Periodic Law" (1871). In his 

 complete paper on " The Periodic Law of the Chemical 

 Elements," Ann. Chem. Pharm., 1872, 8, Suppl., 

 [33-229), Mendeleeff, following Lothar Meyer, adopted a 

 compromise (compare Table D, 1 p. 462), between Newlands's 



1 This table differs from Mendeleeff 's table as follows : (i) An addi- 

 ional column O has been added ; (2) Series 2 and 3 are placed in the 



