684 PRINCIPLES OF CHEMISTRY 



oxide CuO that is, the compounds CuX corresponding wfth 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 make a distinction between atomic weights and 

 equivalents. 



In this manner the valency, that is, the number of equivalents 

 entering into the atom of the metals may in many cases be established 

 by means of comparatively few volatile metallic compounds, with 

 the aid of a search into their analogies (concerning which see Chapter 

 XV.). The law of specific heats discovered by Dulong and Petit has 

 frequently been applied to the same purpose 3 in the history of chemistry, 

 especially since the development given to this law by the researches of 

 Regnault, and since Cannizzaro (1860) showed the agreement between 

 the deductions of this law and the consequences arising from Avogadro- 

 Gerhardt's law. 



Dulong and Petit, having determined the specific heat of a number 

 of solid elementary substances, observed that as the atomic weights of 

 the elements increase, their specific heats decrease, and that the product 



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

 multiple of the equivalent should be .ascribed to the atom, are: (1) The law of Avogadrc- 

 Oerhardt. 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 

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

 half as much oxygen again 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 

 of others is generally had recourse to, and is quite necessary, because, phenomena of 

 dissociation, polymerisation, &c,, may complicate the individual determinations by each 

 method. 



It will be well to- observe that a number of other methods, especially from the province 

 of those physical properties which are clearly dependent on the magnitude of the atom 

 (or equivalent) or of the molecule, may lead to the same result. I may point out, for 

 instance, that even the specific gravity of solutions of the metallic chlorides may serve 

 for this purpose. Thus, if beryllium be taken as trivalent that is, if the composition of 

 its chloride be taken as BeCl 3 (or a polymeride of it), then the specific gravity of 

 solutions of beryllium chloride will not fit into the series of the other metallic chlorides. 

 But by ascribing to it an atomic weight Be =7, or taking Be as bivalent, and the composi- 

 tion of its chloride as BeCl a , we arrive at the general rule given in Chapter VII., Note 28. 

 Thus W. G. Burdakoff determined in my laboratory that the specific gravity at 15/4 

 of the solution BeCl s + 200H 2 O = 1-0138 that is, greater than the corresponding solution 

 KCl + 200H a O (-1-0121), and less than the solution MgCl 3 +200H a O (-1-0208), as would 

 follow from the magnitude of the molecular weight BeCl a -80, since KOl-74'5 and 

 MgCl,-95. 



