Chemical and Physical Properties of Elements. 29 



sponding positions in the ascending series : thus the alkali 

 metals are the first members of certain series, the alkaline- 

 earth metals are the second, and the halogens the sixth 

 members. 



The systematic grouping into families, in accordance with 

 this conception, is shown in the table (Plate II. fig. 1), and 

 will be seen to be similar to, bnt not identical with, Men- 

 delejeff "s classification. 



Family relationships between elements may be of two orders 

 of intimacy. The halogen group, the group of the alkalies 

 and of the alkaline earths, are among the best examples of 

 complete family resemblance ; bnt there are other groups of 

 elements usually arranged in families which have a certain 

 number of points of resemblance, and yet differ in many im- 

 portant respects. For instance, there is the group consisting 

 of hydrogen, lithium, sodium, copper, silver, and gold : the 

 group comprising beryllium, magnesium, zinc, cadmium, and 

 mercurv: the group carbon, silicon, tin, and lead: and the 

 group nitrogen, phosphorus, arsenic, antimony, and bismuth. 

 It is evident that in these cases there is less complete resem- 

 blance between the various members of the family, than exists 

 in the case of the halogens and the other aforesaid perfect 

 families. A little consideration will show that the resem- 

 blances between some of the members of these imperfect sub- 

 families, as they may be termed, are often merely structural 

 resemblances of the various compounds formed. For example, 

 the lower chlorides of the subfamily sodium, copper, silver, 

 and gold have little more in common than the fact that the 

 metal in each case is monad, and united to one ecpuivalent 

 of chlorine : while the chlorides of the family of elements 

 lithium, sodium, potassium, rubidium, caesium, have this fea- 

 ture in common, as well as many others svhich are not common 

 to all the members of the subfamily — as, for example, solu- 

 bility in water, and stability under great variety of conditions. 

 In short, we may say that the members of a natural family of 

 elements resemble one another in most physical and chemical 

 characteristics : while the members of a subfamily chiefly 

 resemble the members of the family in atomicity, which 

 accounts for the similar structure of their compounds. But 

 the atomicity of an element seems to depend almost entirely 

 upon position in the primary septenary series ; and, upon 

 inquiry, we find that in all cases families and their collateral 

 subfamilies are alike in this respect. It may be thought that 

 too much stress should not be laid upon atomicity, a concep- 

 tion coming of late to be thought less rigidly applicable by 

 chemists. But we here employ the notion of atomicity under 



