382 HISTORICAL INTRODUCTION TO CHEMISTRY CH. xvi 



In the cubic system isomorphism is perfect, i.e. the angles for 

 the different substances are exactly equal. Examples are 



(1) the alums M I M III (SO 4 ) 2 ,i2H 2 O. 

 where M 1 == K, Na, NH 4 , &c. 



M' ni =A1, Fe(ferric), Cr, &c. 



(2) Spinel MgAl 2 O 4 Also MgFe 2 O 4 

 Gahnite ZnAl 2 O 4 MnFe 2 O 4 

 Magnetite FeFe 2 O 4 FeAl 2 O 4 

 Chromite FeCr 2 O 4 &c. 



In less symmetrical crystals the isomorphism is imperfect, but 

 mixed crystals may be formed if the angles do not differ more 

 than 5. The rhombohedral angles of a series of isomorphous 

 carbonates are given on page 369. 



Isomorphism often occurs in compounds of dissimilar structure, 

 e.g. CaCO 3 and NaNO 3 , Mg 2 SiO 4 and Al 2 BeO 4 , and even in 

 compounds which contain unequal numbers of atoms in the 

 molecule, e.g. PbS and Ag 2 S or (NH 4 )Al(SO 4 ) 2 ,i2H 2 O and 

 KAl(SO 4 ) 2 ,i2H 2 O. A remarkable collection of cases of isomor- 

 phism (in which fluorine replaces oxygen, water replaces chlorine, 

 &c., without producing any change of crystalline form) has been 

 made by T. V. Barker (Trans. Chem. Soc., 1912, 101, 2484). 



D. FORMUL/E OF SALTS 



In the case of salts it is usually possible to give only an em- 

 pirical formula, which expresses its composition, but not its 

 molecular weight. Such a formula can be deduced readily from 

 the analytical data by methods indicated on pp. 373 to 380. 



