MOLECULAR ARCHITECTURE 353 



with similar acid radicles. In the sulphates and selenates of 

 the metals potassium, rubidium and caesium we have a typical 

 isomorphous group, which has been most carefully studied by 

 Tutton.^ The crystals of two isomorphous substances, unless 

 they belong to the cubic system, are dimensionally never 

 identical but [the differences are smaller the more nearly the 

 substances be allied chemically. Thus potassium and rubidium 

 sulphates exhibit axial ratios which are more nearly equal than 

 are those of potassium and caesium sulphates, apparently for 

 the reason that potassium and rubidium are nearer relatives in 

 the periodic family than are potassium and caesium. In organic 

 substances the displacement of equivalent elements by one 

 another is frequently observed to be unattended by any marked 

 change in crystalline form. The halogen elements, chlorine, 

 bromine and iodine exhibit this property of being isomor- 

 phously interchangeable in a marked degree. 



When it is known that a particular unit can assume two 

 types of arrangement and that a second similar unit can assume 

 one of these types, it might be expected to be able also to 

 assume the other. That is to say, the two substances, the units 

 of which we are considering, would be doubly isomorphous or 

 in other words the two forms of the one would be respectively 

 isomorphous with the two forms of the other. Instances of this 

 phenomenon, known as isodimorphism, are indeed well known. 

 The general phenomenon of bodies being isomorphous in more 

 than one form is termed isopolymorphism. It frequently hap- 

 pens that this property of related bodies can only be inferred. 

 For example two similarly constituted substances A and B may 

 occur in totally distinct forms. In such cases it may be 

 observed that B will form mixed crystals with A in the form 

 characteristic of A until the mixed crystals contain a certain 

 proportion of B. On the other hand A will form mixed crystals 

 with B in the form characteristic of B until the proportion of 

 A reaches a certain maximum. Thus although when alone each 

 substance crystallises in but one form, it may be induced to 

 assume the second form in presence of the other substance. 

 The two substances may therefore be regarded as isodimorphous. 



Certain relationships subsist between the crystalline forms 

 of more or less related bodies which could not have been 

 foreseen from the results of the work of the earlier crystallo- 

 > /.C.S. Trans. 1897, p. 846. 



