THE VALENCY AND SPECIFIC HEAT OF THE METALS 573 



hydrogen. One method is verified by another, and all the processes for 

 the accurate determination of the equivalents require the most accurate 

 methods to avoid the absorption of moisture, of further 9xidation, 

 volatility, and other like circumstances which influence exact weigh- 

 ings. The description of the methods necessary for the attainment of 

 exact results belongs to the province of analytical chemistry. 



For univalent metals, like those of the alkalis, the weight of the 

 equivalent is equal to the weight of the atom. For bivalent metals 

 the atomic weight is equal to the weight of two equivalents, for 7i-valent 

 metals it is equal to the weight of n equivalents. Thus aluminium, 

 Al=27, is trivalent, that is, its equivalent = 9 ; magnesium, Mg=24, 

 is bivalent, and its equivalent =12. Therefore, if potassium or sodium, 

 or in general a univalent metal, M, gives compounds M 2 O, MHO, 

 MCI, MNO 3 , M 2 S0 4 , &c., and in general MX, then for bivalent 

 metals like magnesium or calcium the corresponding compounds 

 will be MgO, Mg(HO) 2 , MgCl 2 , Mg(]STO 3 ) 2 , MgSO 4 , fec., or in general 

 MX 2 . 



By what are we to be guided in ascribing to some metals uni- 

 valency and to others bi- ter- quadri-. . . n- valency 1 What obliges 

 us to make this difference 1 Why are not all metals given the same 

 valency for instance, why is not magnesium considered as univalent ? 

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

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

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

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

 sodium and potassium. These compositions were so expressed before, 

 why has this been changed now ? 



These questions could only be answered after the establishment of 

 the conceptions of multiples of the atomic weights as the minimum 

 quantities of the elements combining together to form compounds in 

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

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

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

 pounds, 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 =7 5, and its compounds cor- 

 respond, like the compounds of nitrogen, with the forms AsX 3 and 

 AsX 5 ; for example, AsH 3 , AsCl 3 , AsFl 5 , As.,O 5 , etc. It is evident 

 that we are here dealing with a metal (or, better, element) of two 

 valencies, and which is furthermore in no way 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 



