352 SCIENCE PROGRESS 



symmetry (the common stable variety), two monosymmetric 

 and one anorthic. Dimorphous organic compounds are of 

 frequent occurrence. 



The difference between the two crystaUine forms of a sub- 

 stance is comparable in some aspects with that between the 

 solid and the liquid states. Passage from one form to another 

 is accompanied by change of energy content and each form 

 is stable only throughout a certain range of temperature and 

 pressure. At one particular temperature — the transition tem- 

 perature — corresponding to the melting point of a solid, both 

 forms are stable. It is clear then that the two crystalline 

 forms of a substance are not chemical isomers, for no transi- 

 tion temperature is in general observed between such related 

 compounds ; the difference between the tv/o forms must be 

 merely physical and due to a different homogeneous arrange- 

 ment of the same units, for the configuration of these units 

 in each modification must be the same, as any differences 

 here would be accompanied by chemical variations. Thus 

 although at present we cannot lay down any definite rules 

 governing the differences of arrangement of the structural 

 units in two such modifications of the same substance, yet in 

 consideration of the readiness with which in many cases one 

 form is transformed into the other, we can prophesy that the 

 types of architecture of the two forms are very intimately related. 



It may be conceived that two types of units more or less 

 similar to one another might assume similar homogeneous 

 arrangements. Examples of this have been multiplying ever 

 since the discovery of isomorphism among phosphates and 

 arsenates by Mitscherlich in 1822. 



It is reasonable to expect that only such substances which 

 contain very similar groupings would be isomorphous, as 

 isomorphism implies the same crystalline system with closely 

 approximating axial ratios ^ and also the possibilities of forming 

 mixed crystals of the isomorphous substances. As a rule 

 isomorphism is only found among such related substances as 

 salts of acids v^'ith equivalent metals or salts of the same metal 



^ Every crystalline substance is characterised (i) by its symmetry or the 

 crystal class to which it belongs, (ii) by stating the ratio a : b : c between the 

 lengths of the three edges of the parallelopipedal cell from which the crystal 

 structure may be regarded as built up. The ratios involved in the expression 

 a\b\c are termed the axial ratios of the crystal. 



