372 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



divalent metals Ca, Mg, Mn (manganous), Fe (ferrous), 

 Cu, Zn, Co, Ni in their simple and double sulphates (Ann. 

 Chem. Phys., 1821, 19, 416), and of the tervalent metals 

 Cr, Mn, Fe, Al in the spinels MgAl 2 O 4 ,Fe 3 O 4 , etc. The way 

 was therefore open to link up the atomic weights of nearly all 

 the metals with those deduced by Avogadro's hypothesis 

 for the non-metals chlorine and sulphur. 



Mitscherlich's observations on isomorphism, confirmed as 

 they were by Dulong and Petit's Law of Atomic Heats, led 

 Berzelius in 1827 (Jahresbericht, 1828, 7, 67-78) to write 

 the oxides of iron as FeO and Fe 2 O 3 instead of FeO 2 and 

 FeO 3 , and to publish a new table of atomic weights in which 

 the values for nearly all the metals were halved. He argued 

 from the isomorphism of the sulphates and chromates that 

 chromic acid must be CrO 3 , compare SO 3 ; chromic oxide 

 was therefore Cr 2 O 3 . The isomorphism of the salts of 

 chromium, manganese, ferric iron and aluminium gave the 

 formula Mn 2 O 8 , for manganic oxide, Fe 2 O 3 for ferric oxide, 

 and A1 2 O 3 for alumina. But if ferric oxide was Fe 2 O 3 , 

 ferrous oxide must be FeO, and this type of formulae must 

 also be given to all those oxides the salts of which were 

 isomorphous with the ferrous salts. 



Exceptions to the law of isomorphism. There are many 

 exceptions to the law of isomorphism. On one hand, 

 many substances of similar structure crystallise in totally 

 different forms. Thus Mitscherlich found that the usual 

 form of acid sodium phosphate, NaH 2 PO 4 , was quite different 

 from that of the acid arsenate, NaH 2 AsO 4 . This difference 

 is due to the POLYMORPHISM of the phosphate, which forms 

 two types of crystals, only one of which is isomorphous with 

 the arsenate. 



On the other hand, perfect isomorphism is often found in 

 substances of dissimilar composition. The most important 

 case is that of the ammonium salts in which the compound 

 radical NH 4 may be displaced by a single atom of sodium 



