CLASSIFICATION OF ELEMENTS. 147 



ferric acid, then specular iron would be a ferrate of the prot- 

 oxide of iron, as the other is a titaniate of the same base, and 

 their isomorphism would be intelligible, and present nothing 

 unusual. But if ferric acid does not exist, and it has hitherto 

 eluded research, then the isomorphism of the bodies in ques- 

 tion seems to imply that this relation does not require simila- 

 rity of constitution, but merely equality in the number of 

 atoms contained in the bodies which exhibit it. 



Eighth Class. Silver and gold. From their isomorphism in 

 the metallic state. Gold will thus be connected, through silver, 

 with sodium and the fourth class. 



Ninth Class. Platinum, palladium, iridium, osmium. From 

 the isomorphism of their double chlorides. The bichloride of 

 tin with chloride of potassium crystallizes in regular octohe- 

 drons, like the double bichloride of platinum and potassium, 

 and other double chlorides of this group ; which although not 

 alone sufficient to establish an isomorphous relation between 

 this class and the seventh, yet favours the notion of its exis- 

 tence (Dr. Clark.) 



Tenth class. Tungsten and molybdenum. From the isomor- 

 phism of the tungstates and molybdates, the salts of tungstic 

 and molybdic acids, W O 3 and Mo O 3 . Mr. Johnston has 

 observed that the chromate of lead is dimorphous, and corres- 

 ponds in the least usual of its forms with the molybdate of lead. 

 This establishes a relation between molybdic, chromic, sulphuric 

 and other analogous acids.* 



Eleventh Class. Carbon, boron, silicon. These elements 

 are placed together from a general resemblance which they 

 exhibit without any precise relation. They are not known to 

 be isomorphous among themselves, or with any other element. 

 They are non-metallic, and form weak acids with oxygen, the 

 carbonic, consisting of two of oxygen and one of carbon, and 

 the boracic and silicic acids, which are generally viewed as 

 composed of three of oxygen to one of boron and silicon. Si- 

 licic acid may, perhaps, replace alumina in some minerals, 

 but this is uncertain. 



Of the elements which have not been classed, no isomor- 

 phous relations are known. They are mercury, which in some 

 of its chemical properties is analogous to silver, and in others 



* Phil. Mag. third series, vol. 12, p. 387. 

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