'THE VALENCY ANJD SPECIFIC HEAT OF THE METALS 583 



If this be done, taking Mg = 12 (and not 24 as now), not only is 

 a simplicity of expression of the composition of all the compounds of 

 magnesium attained, but we also gain the advantage that their com- 

 position will be the same as those of the corresponding compounds of 

 sodium and potassium. These combinations were so expressed formerly 

 -why has this since been changed ? 



These questions could only be answered after the establishment of 

 the idea of multiples of the atomic weights as the minimum quantities 

 of certain elements combining with others to form compounds in 

 a word, since the time of the establishment of Avogadro-Gerhardt's law 

 (Chapter VII.). By taking such an element as arsenic, which has 

 many volatile compounds, it is easy to determine the density of these 

 compounds, and therefore to establish their molecular weights, and 

 hence to find the indubitable atomic weight, exactly as for oxygen, 

 nitrogen, chlorine, carbon, &c. It appears that As = 75, and its com-- 

 pounds correspond, like the compounds of nitrogen, with the forms 

 AsX 3 , and AsX 5 ; for example, AsH 3 , AsCl 3 , AsFl 5 , As 2 O 5 , &c. It is 

 evident that we are here dealing' with a metal (or rather element) of 

 two valencies, which moreover is never univalent, but tri- or quinqui- 

 valent. This example alone is sufficient for the recognition of the 

 existence of polyvalent atoms among the metals. And as antimony 

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

 (just as potassium is analogous to rubidium and caesium) ; so, 

 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, wliich is itself volatile, gives several 

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

 118, vapour density ;= 61*5), 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 = 16), is a repre- 

 sentative 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* 



13 



