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 m^caniques determin antes de la forme geometrique 
sont d'un autre ordre que les causes m^caniques d^terminantes 
des proprietes optiques birefringentes, puisque cette forme 
demeure la memo 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 merne 
temps par des effets geometriques semhlahles 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 semhlahles" 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 formulee 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. 
