CHAPTER VII. 



MIMETIC FORMS AND APPARENT SYMMETRY. 



Observed Disagreements between Crystallographical and Phy- 

 sical Symmetry. Crystal-Aggregates in General. Twinning : 

 general Remarks. --Repeated Twinning: Cyclic and Polysyn- 

 thetic Twins. - - Forms of Approximate Symmetry. Pseudo- 

 Symmetry. Mimetic Forms. Mimicry and Pseudo-Symmetry. 

 Compound Twinning of Microscopical Lamellae of Approximate 

 Symmetry. - - The Explanation of Optical Anomalies. Exam- 

 ples. - - Mallard's Theory of the Optical Behaviour of crossed 

 Lamellae. - - The Rotatory Power of Crystals. Pseudo-sym- 

 metry and Polymorphism. Mallard's Views about the Pseudo- 

 symmetrical Character of all Crystalline Matter. Final Remarks. 



^Cette tendance vers la Sy metric est une des grandes 

 lots de la nature inorganique .... Elle n'est d'ailleurs 

 qu'une manifestation de la tendance plus gtnerale de 

 la Nature vers la stabilitf, c'est-d-dtre vers le repos, 

 tendance, qui est une des grandes forces antagonistes 

 de I' Univers^ 



E. Mallard, 1880. 



1. In the previous chapters we have repeatedly had occasion 

 to observe, how the symmetry-principle and its laws find application 

 in all considerations regarding crystalline matter and its inherent 

 properties. Indeed, with respect to their external appearance no 

 less than with respect to their internal structure, crystals are objects 

 whose behaviour is chiefly governed by the laws of symmetrical 

 configuration. In general it may appear that no essential discordances 

 exist between the external forms of each crystalline individual and its 

 molecular structure ; and the world of crystals appeared from this 

 to be rigorously ruled by stubborn laws which do not allow any ex- 



