201 
Although the ideas of chemists on the classification and 
quanti valence of elements have greatly changed during 
recent years, there is no question that the alkaline metals, 
lithium, sodium, potassium, rubidium, and caesium, belong 
to the group which I have classified under H n. Chemists 
are also agreed that silver, notwithstanding the great diver- 
gence of some of its characteristics from those of the alkaline 
metals, also belongs to the same group. Now some of the 
physical and chemical properties of copper and mercury are 
more nearly allied to those of silver than to metals of other 
groups, and recent investigations have shown that silver 
may, like copper, be regarded as bivalent, since many of its 
compounds can be represented by formulae exactly analo- 
gous to those of cuprous compounds with which they are 
isomorphous * The position of Hg, A g, and Cu, as alternate 
members of the series H n, indicate their relationship with 
sodium, and are thereby brought into still closer connexion 
with Li, K, Eh, and Cs. That a relationship exists between 
sodium and silver, by the isormorphism of their anhydrous 
sulphates, and in other ways, has already been pointed out 
by Gdling. The greater specific gravity of sodium, while 
possessing a lower atomic weight than potassium ; its pas- 
sivity in the liquid state to the action of chlorine ;*f* and its 
inferior volatility and oxidability to K, confirm the relation- 
ship of Na to the heavy metals of the series. 
From what cause elements possessing physical properties 
so widely different, should be associated alternately in 
regular order in the same series, can only, in the present 
state of knowledge, be a subject of speculation; but, if the 
views which I have enunciated on the formation of the 
types H?i— H7n be correct, it may be conceived that after 
the transition of the cosmical vapours from the spiral to the 
annular form, the gaseous material of each pair of members 
* Quantivalence of Silver,— Wislieenus, Watts, Die. Cliem., 2nd 
Suppl., 1088. 
f Watts, Die. Chem., Suppl., 1030. 
