476 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



of the table as the element having the greatest atomic weight 

 that is known. 



Correction of atomic weights, (b) Equivalents 

 corrected. Certain elements, which were found to be mis- 

 placed, when arranged strictly in the order of their atomic 

 weights, could be restored to their own families by assuming 

 smaller errors, due to the use of inexact equivalents. Thus 

 gold (196*2) was placed after osmium, iridium and platinum 

 (198-6, 1967, 1 96*7) ; later measurements have proved the 

 correctness of this sequence, thus : 



Os Ir Pt Au 



190-9 193-1 195-2 197-2 



On the other hand, three cases are known in which the 

 atomic weights refuse to conform to the order suggested by 

 the periodic classification. These are 



A K 



39-88 39-10 



Co Ni 



58-97 58-68 



Te I 



127*5 126-92 



The significance of these deviations is discussed below 

 (p. 494). 



Prediction of missing elements. Mendele'eff was so con- 

 vinced of the validity of the periodic law that, unlike 

 Newlands and Lothar Meyer, he altered atomic weights that 

 did not conform to his system and made detailed predictions 

 of the properties of unknown elements, which could be 

 verified or disproved by experiment. Missing elements, 

 were named by prefixing the Sanskrit numerals eka-^ dwi-, 

 tri- to the element in the odd or even series below which 

 vacancies were found. Detailed predictions were made of 

 the properties of 



ekaboron = 44, ekaaluminium l = 68, ekasilicon = 72. 



1 Mendeleeff gave a more detailed prediction of the properties of 

 ekaaluminium (Comptes rendus, 1875, 81, 970-971) after the discovery 

 of gallium had been announced by Lecoq de Boisbaudran. 



