THE ATOMS OF THE ELEMENTS. 



21 



correlated properties. It was just as though, in a com- 

 pany of soldiers raised in the same village, little groups of 

 brothers could be picked out, one here and one there, owning 

 to a strong family likeness. These little groups of corre- 

 lated atoms are known as the Triads of Dobereiner. Three 

 such groups are given below: 



ATOMIC WEIGHT. 



Calcium, 40' 1 Chlorine, 35*4 Sulphur, 321 



Strontium, 877 Bromine, 80'0 Selenium, 79*2 

 Barium, 134'4 Iodine, 126'8 Tellurium, 127*5 



The elements calcium, strontium and barium show chemi- 

 cal similarities to a remarkable degree. They look much 

 alike, act much alike, and in their very variations vary by 

 steps. The same may be said of chlorine, bromine and 

 iodine, or of sulphur, selenium and tellurium. 



Now the curious discovery alluded to above lies in the 

 fact that if you take the mean of the atomic weights of the 

 first and third elements in any of these groups, you will 

 obtain, approximately, but not exactly, the atomic weight 

 of the middle one. Thus: 



Atomic weight. 

 Calcium, 40.1 



Barium, 137.4 



Chlorine, 

 Iodine, 



35.4 

 126.8^ 



2)162.2 

 Mean 81.1 



Atomic weight 

 of Strontium, 



87.7 



Atomic weight 

 of Bromine, 



80.0 



Sulphur, 

 Tellurium, 



32.1 

 127.5 



Mean 



Atomic weight of Selenium, 79.2 



