290 SENARMONT ON THE OPTICAL CHARACTERS OF 



to these tartrates. These are points which cannot be determined 

 except by very careful and delicate experiments with crystals of 

 large size and perfect purity. I must therefore postpone their 

 consideration altogether until a future occasion. 



The experiments described in this memoir tend to the con- 

 clusion, that the mechanical causes which determine the geo- 

 metrical form are of another order than those which determine 

 the optical characters, because the form remains the same in 

 whole groups of isomorphous substances, while the optical cha- 

 racters experience in their essential elements not only modifica- 

 tions in quantity, but a complete inversion of relative magnitude. 

 One and the same cause could not manifest itself at the same 

 time by similar geometrical effects and opposite optical effects. 



The very remarkable relations discovered by Sir David Brew- 

 ster and the physicists who have followed him, between the 

 optical characters of crystals and geometrical types, may be in 

 reaUty nothing more than a result of symmetry. It is true, we 

 do not possess any definite ideas as to the reciprocal relations 

 which may exist between luminous agency and ponderable 

 matter ; but it is sufficiently evident, that in every molecular 

 network, constituted with the conditions of a regular arrange- 

 ment in reference to certain directions, the general resultants of 

 the partial forces emanating from each molecule act, whatever 

 may be their nature and cause, in the same directions, and thus 

 become axes of symmetry for all the phaenomena dependent upon 

 these forces. Such a character of coordination, expressed more- 

 over in very different characters, in the geometrical form, the 

 optical, thermic and other characters, does not therefore by any 

 means prove the identity of the forces themselves, but merely 

 an identity in the direction of their resultants. To prove the 

 identity of the forces, something more is necessary ; their actions 

 in these compulsory directions ought at least to preserve the 

 same order of relative magnitude among each other ; but in the 

 case of light the precise contrary obtains ; and differences of a 

 similar kind are met with among the phaenomena of heat, elec- 

 tricity, magnetism, and in short all the varieties of physical 

 agency, when crystals are submitted to the test of experiments 

 sufficiently varied and delicate. 



The manner in which complex media, formed by the crystal- 



