30 THE NEW KNOWLEDGE. 



they closely resemble one another and are distinctively of 

 the same type; indeed, we may go further, and say that their 

 relationship is so pronounced that we are able to call these 

 groups veritable natural families. 



As an example of such group relationship let us consider 

 the elements of Group II. These elements are in the order 

 of their atomic weight: Beryllium, magnesium, calcium, zinc, 

 strontium, cadmium, barium, mercury and radium. Now, in 

 studying these elements, the curious fact crops up that while 

 all the elements of the group resemble each other and are of 

 the same blood, so to speak, they may, nevertheless, be 

 divided into two sub-groups, the members of which are 

 peculiarly like each other. These sub-groups are: 



Sub-group A. Sub-group B. 



Calcium Beryllium 



Strontium Magnesium 



Barium Zinc 



Radium Cadmium 



Mercury 



The relations between the two sub-groups may fairly be 

 expressed by saying that calcium, barium, strontium and 

 radium are sister elements and bear only a cousinly relation- 

 ship to beryllium, magnesium, cadmium and mercury. 



The curious nature of this double relationship in a group 

 is accentuated by the fact that every other group is divided 

 in the same way into two sub-groups showing similar rela- 

 tionships. Attention, has been drawn already to a double 

 periodicity in the curves of the atomic volumes and melting- 

 points, and this double relationship in the group is the logical 

 consequence of it. How clearly related in their properties 

 these sister elements are, may be seen in the subjoined 

 table (Fig. 6). 



