On a difficulty in Isomorphism. 47 



Instances, 

 I Si Protosulphateoftin; of iron; of manganese. 



^ \ Persulphate of mercury — of copper. 



II. A A S Protosulphate of mercury — of copper. 



III. A §2 Persulphate of tin. 



. C Persulphate of iron — corresponding sul- 



IV. A A S3< phate of manganese — sulphate of alumina 



(, — of chrome. 

 These four descriptions of sulphates contain each one 

 atom of oxide. The first and second contain each one atom 

 of acid ; but, what demands most attention, the third con- 

 tains two atoms of acid, and the fourth, three atoms of acid. 

 But, admitting that each of these four descriptions of sul- 

 phates contains one atom of oxide, and that the first and 

 second contain each one atom of acid, analogy would lead us 

 to expect that the third and fourth sorts would also contain 

 one atom of acid in each. Sulphurous acid, on being farther 

 oxidized, so as to become sulphuric acid, does not, in con- 

 sequence of having acquired more oxygen, combine with 

 more potash, in order to form a neutral salt, and, indeed, 

 hypo-sulphuric acid, which is undoubtedly more oxidized 

 than the sulphurous, combines, for the same sulphur, 

 with only half as much potash. Chemists, guided, as 

 they may conceive, only by the result of analysis, may, 

 it is true, choose to regard, as a rule of combination, 

 that any oxide, the basis of a salt, requires, for neutralliza- 

 tion, as many atoms of acid, as itself contains of atoms 

 of oxygen. But we are too apt to regard, as the result 

 of analysis what is merely our own arbitrary expression 

 of that result. The objection to our admitting, as a law 

 of nature, that oxides combine with acids according to 

 such a rule, lies in this : that not only does such a rule 

 imply that oxides, in their combinations, observe a law 

 different from what bodies not oxides observe, but it im- 

 plies that one class of oxides observe a rule of combina- 

 tion, different from another class of oxides. This will at 

 once appear on considering the following Table, where 

 A and X stand for any two oxidizable metals, and where 

 the combination of the oxides of each metal is represented 

 as taking place, reciprocally in the same number of atoms 

 of each oxide as the other combining oxide contains of atoms 



