452 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



B. NUMERICAL RELATIONSHIPS AND THE PERIODIC 

 LAW 



Dobereiner's "triads" (1829). The existence of small 

 groups or " families " of elements became evident at a very 

 early date. The metals of the alkalis and of the alkaline 

 earths formed two of these natural groups. Amongst the non- 

 metals, the group of halogens and the elements of the oxygen 

 group were enumerated by Berzelius as typical constituents 

 of " haloid " and of " amphi " salts respectively. 



Dobereiner, in 1829, directed attention to the fact that a 

 determination by Berzelius of the atomic weight of bromine 

 agreed with a prediction of his own "that the atomic weight 

 of bromine would probably be the arithmetic mean of the 

 atomic weights of chlorine and iodine " ; thus 



351470+ 1 26-470 m 8o . 9?0 



where Berzelius found 78*383. So also in the case of sul- 

 phur, selenium and tellurium, ^ - = 80741, 



where the actual atomic weight of selenium was 79*263. 



In the case of the metals, similar relationships were 

 detected in the groups 



calcium, strontium, barium 

 lithium, sodium, potassium 



as well as in certain groups where the atomic weights differed 



but little, as in 



nickel, copper, zinc, 

 platinum, iridium, osmium, etc. 



(Pogg. Ann. der Physik., 1829, 15, 301-307.) These groups 

 of three elements, with atomic weights in arithmetical 

 progression, became known as Dobereiner's TRIADS. 



Dumas's "natural series" and "common differences " 

 (1859). The accuracy of the relationship discovered by 



