1859.] Lithium, and their Congeners. 85 



while the gradation of lithium, sodium, and potassium, diverged in one 

 direction, the gradation of magnesium, zinc, and cadmium diverged in 

 another. 



Or, we might say, that, as regards their properties, potassium, barium, 

 and cadmium, are highly specialized forms, while lithium, calcium, and 

 magnesium are degraded or general forms, which, from the comparative 

 absence of special characters, approximate to one another. The sums 

 and means of the atomic volumes of the three groups of elements are 

 shown below. 



3)76-5 3)25-4 



25-5 8-4 



The atomic volume of the calcic family is observed to be exactly 

 intermediate between those of the lithian and magnesian families. 

 Similar relations exist between the atomic weights of the three 

 groups. 



When, irrespective of these curious numerical relations between the 

 different elements and groupings of elements, we find the groups 

 characterized by distinctive but correlated properties, and the members 

 of the groups associated by community of characters, and separated by 

 gradational differences only, we perceive that the possession, by each 

 element, of its own special properties, is not an accidental endowment, 

 but is a necessary result of the development of one general compre- 

 hensive plan. 



[W. O.] 



