492 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 

 TABLE F. ATOMIC NUMBERS AND ATOMIC WEIGHTS cont. 



Atomic weights and atomic numbers. Total number 

 of missing elements. The discovery of a real system of 

 integral atomic numbers has supplied just that element of 

 mathematical exactness which was lacking in the relation- 

 ships put forward by Prout, by Dobereiner, by Dumas, and 

 others. The agreement between these atomic numbers and 

 the consecutive numbering of the elements is such that 

 every confidence may be felt in an enumeration of the 

 elements on this new basis. Thus between Ba(56) and 

 Ta(73) there are sixteen places, of which fourteen are filled 

 by rare-earth elements of known atomic weight ; Mendele'eff 

 was probably wrong when he assumed nineteen l vacancies in 

 this interval. Apart from one gap at 61 in the rare earth 

 series, and one gap at 72 between the rare-earth elements 

 and tantalum, the new atomic numbers admit only two 

 vacancies between aluminium and gold, namely, Mendele'eff s 

 Ekamanganese(43) = 99 (?) between Mo(42) and Ru(44) 

 Trimanganese (75) = i87(?) between W(74) and 05(76). 



1 The discovery of the gases of the helium family would have 

 increased Mendeleeff's estimate to twenty. 



