464 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



(3) Two long periods of eighteen elements. These two 

 periods are again precisely similar to one another, with the 

 single exception of a missing element, of atomic weight 

 about 100, which is required to fill a gap below manganese, 

 and between molybdenum and ruthenium. The average 

 difference between the atomic weights of the long Periods 

 IV and V is nearly 46, a difference which persists between 

 Periods V and VI, until the elements of the rare earths 

 are reached, when the difference between analogous elements 

 jumps suddenly to 89, but finally drops back to 54 when 

 the rare earths are passed. This is shown in Table E, 

 which carries the differences forward to the end of the list 

 of elements. 



(4) A long period of some thirty-two elements, inclu- 

 ding the elements of the rare earths. Mendeleeff assumed 

 that the two long periods, including series 4-5 and 6-7, 

 would be followed by three more long periods of identical 

 type, including series 8-9, 10-11, and 12-13 (?). He there- 

 fore left spaces (loc. cit. p. 151), for a group of transition- 

 elements of atomic weight about 150, between periods 

 8 and 9, and for another group of atomic weight about 

 250, at the end of period 12, to correspond with the group 

 Os, Ir, Pt, of atomic weight about 193 between series 10 

 and n. He was also obliged to assume that there was an 

 element missing in each of the following families : 



Cu Zn As 



Ag Cd In Sn Sb 



Au Hg Tl Pb Bi 



Lothar Meyer, on the other hand (Ann. Chem. Pharm., 

 1870, 7, Suppl., 356), suppressed these gaps and assumed 

 that the differences between analogous elements increased 

 abruptly from 16 to 46, and then again from 46 to 90 on 

 passing from Ba Sr to Ta Nb (see quotation, p. 459). 

 None of Mendeleeffs hypothetical transition-elements 



