xvi ATOMIC WEIGHTS OF THE METALS 369 



those groups of elements which could take the place of one 

 another in compounds of identical crystalline form. In the 

 case of isomorphous elements, "The same number of 

 atoms combined in the same way produces the same crys- 

 talline form the crystalline form is independent of the 

 chemical nature of the atoms, and is determined only by 

 the number and relative position of the atoms " (ibid., p. 

 419). 



The law of isomorphism. The LAW OF ISOMORPHISM 

 as now enunciated states that : " Substances which are similar 

 in crystalline form and in chemical properties should be 

 represented by similar formula" 



This law is limited in two directions : (i) On one hand, 

 mere identity of crystalline form is not sufficient to prove 

 that the formulae of two compounds should be similar. 

 In the case of crystals of a high degree of symmetry, such as 

 the cube and octahedron, the angles can be calculated as 

 simple fractions or functions of a right angle, and are identical 

 for all the elements and compounds (iron, silver, iron pyrites, 

 salt, sal-ammoniac, etc.) which crystallise in these forms. 

 (2) On the other hand, if the crystalline form is less symmetri- 

 cal, the angular measurements may differ considerably, even 

 in the most closely-related compounds; thus, the mineral 

 carbonates crystallising in simple rhombohedra show the 

 following angles : 



Calcite, CaCO 3 7455' 



Magnesite, MgCO 3 7236' 



Chalybite, FeCO 8 73o' 



Calamine, ZnCO 3 722o' 



Rhodochrosite, MnCO 3 ... 72s8' 



Isomorphous mixtures or solid solutions. Some more 

 certain test than mere geometrical form is evidently 

 required before " isomorphism " can be used as trustworthy 

 evidence of analogous chemical composition. Such a test is 

 supplied by the fact (investigated by Mitscherlich) that many 



B B 



