50 ON THE PRIMITIVE FOB MS OF CR YSTALS. 



II. On the Connection between the Primitive 

 Forms of Crystals and the Number of their 

 Axes of Double Refraction. 



By David Brewster, LL. D. F. R. S. Lond. and Edin. 



(Read 20th March I8I9.) 



In examining the structure of crystallised bodies, 

 mineralogists do not appear to have observed the 

 slightest connection between their primitive forms 

 and any of their physical or chemical properties *. 

 Without the guidance of a general principle, their 

 determinations have often been at variance, and 

 even the same observer has at different times as- 



* When speaking of doubly refracting crystals, La Place 

 makes the following observation : — " L'Ellipsoide qui leur est 

 " relatif, doit etre determines par l'experience ; et sa position 

 " par rapport aux faces naturelles du crystal, pent repandre un 

 " grand jour sur la nature des molecules integrantes des substances 

 " crystallisees ; car ces molecules doivent, chacune, avoir les 

 " memes proprietes que le crystal entier." — Mem. de I'Institut. 

 xii. p. S02. It will be seen from the following paper, that the 

 sagacious conjecture of this illustrious mathematician has been 

 fully verified, but in a very different way from that which he 

 anticipated. The existence of more than one axis had not been 

 ascertained when his paper was published. 



