574 PRINCIPLES OF CHEMISTRY 



and bismuth are closely analogous to arsenic in all their compounds, 

 as potassium is analogous to rubidium and caesium ; therefore, 

 although very few volatile compounds of bismuth are known, it was 

 necessary to ascribe to them formulae corresponding with those ascribed 

 to arsenic. 



As we shall see in describing them, there are also many analogous 

 metals among the bivalent elements, some of which also give volatile 

 compounds. For example, zinc, which is itself volatile, gives several 

 volatile compounds (for instance, zinc ethyl, ZnC 4 H 10 , which boils at 

 118, vapour density =6 1-3), and in the molecules of all these com- 

 pounds there is never less than 65 parts of zinc, which is equivalent to 

 H 2 , because 65 parts of zinc displace 2 parts by weight of hydrogen ; so 

 that zinc is just such an example of the bivalent metals as oxygen, 

 whose equivalent =8 (because H 2 is replaced by O= 16), is a representa- 

 tive of the bivalent elements, or as arsenic is of the tri- and quinqui- 

 valent elements. And, as we shall afterwards see, magnesium is in 

 many respects closely analogous to zinc, which fact obliges us to regard 

 magnesium as a bivalent metal. 



Such metals as mercury and copper, which are able to give not one 

 but two bases, are of particular importance for distinguishing univalent 

 and bivalent metals. Thus copper gives the suboxide Cu 2 O and the 

 oxide CuO that is, the compounds CuX corresponding with the sub- 

 oxide are analogous (in the quantitative relations, by their composition) 

 to NaX or AgX, and the compounds of the oxide CuX 2 to MgX 2 , 

 ZnX 2 , and in general to the bivalent metals. It is clear that in such 

 examples we must distinguish metals of varying atomicity. 



In this manner the valency of many metals may be established 

 by means of certain and comparatively few volatile metallic com- 

 pounds, and by the aid of a search into their analogies (concerning 

 which see Chap. XV.). The law of specific heats discovered by Dulong 

 and Petit has frequently been applied to the same purpose 3 in the 



3 The chief means by which we determine the valency of the elements, or what 

 multiple of the equivalents should be ascribed to the atom, are : (1) The law of Avogadro- 

 Gerhardt. This method is the most general and trustworthy, and has already been 

 applied to a great number of elements. (2) The different grades of oxidation and their 

 isomorphism or analogy in general ; for example, Fe = 56 because the suboxide (ferrous 

 oxide) is isomorphous with magnesium oxide, &c., and the oxide (ferric oxide) contains 

 1 times as much oxygen as the suboxide. Berzelius, Marignac, and others took advan- 

 tage of this method for determining the composition of the compounds of many elements. 

 (8) The specific heat, according to Dulong and Petit's law. Regnault, and more especially 

 Cannizzaro, used this method to distinguish univalent from bivalent metals. (4) The 

 periodic law (see Chapter XV.) has served as a means for the determination of the 

 atomic weights of cerium, uranium, yttrium, &c., and more especially of gallium, 

 scandium, and germanium. The correction of the results of one method by those 



