254 Proceedings of the Royal Physical Society. 



angle between the optic axes produced by isomorphous 

 replacement, conclusions which led him to deny the existence 

 of such a connection. His words are:* "Les experiences 

 detaillees dans ce Memoire tendent a cette conclusion, que 

 les causes mecaniques determinantes de la forme geometrique 

 sont d'un autre ordre que les causes mecaniques determinantes 

 des proprietes optiques birefringentes, puisque cette forme 

 demeure la menie dans des groupes entiers de substances 

 isomorphes, tandis que les proprietes optiques eprouvent, 

 dans leurs elements essentiels, non- seulement des modifica- 

 tions de quantite, mais une inversion complete de grandeur 

 relative. Une meme cause ne saurait se manifester en meme 

 temps par des effets geometriques semblables et par des effets 

 optiques opposes." 



In reply to De Senarmont, it is only necessary to remark, 

 that the crystalline form does not " demeure la meme dans 

 des groupes entiers de substances isomorphes," nor are the 

 " effets geometriques semblables" As before observed, no 

 two isomorphous substances have exactly the same crys- 

 talline form ; and their angular elements, even in the most 

 well-known groups, sometimes differ as much as 2° 30'. 

 This variation in form is amply sufficient to account for the 

 variation in the angle between the optic axes of so-called 

 isomorphous substances. De Senarmont here has been 

 plainly misled by the derivation of the word, forgetting that 

 the only sense in which substances are said to be isomorphous 

 is in being able to replace each other in all proportions with- 

 out essentially altering the crystalline form, As an expe- 

 rimental fact, substances capable of replacing each other in 

 this manner are found to possess approximately the same 

 crystalline form. 



Impressed by the facts now adduced of the dependence of 

 the optical properties of a crystal on its form, that is, on the 

 arrangement of the material molecules therein, I have for a 

 long time attempted to discover formulae expressing directly 

 some of the optical properties in terms of the angles between 

 the faces or the angular elements. Having seen how essen- 



* Annales de Chimie 1851, vol. xxxiii. page 433. 



