DOUBLE REFRACTION IN ISOMORPIIOUS SUBSTANCES. 259 



It would indeed seem as if the true problem to be solved is, 

 whether geometrical and chemical isomorphism may not in 

 reality exist independently and separately. 



Moreover, the physical conditions of isomorphism appear to 

 be no less obscure than the chemical conditions ; and if we are 

 in the first instance led to believe that the interior resemblance 

 is always a consequence of the exterior geometrical resemblance 

 and the chemical analogies of composition, certain properties, 

 which we ought to regard as closely connected with the essential 

 constitution of substances, will at once present themselves to us 

 as sometimes almost constant, sometimes essentially variable in 

 the same class of compounds ; indeed the curious observations 

 of M. Pasteur have made us acquainted with substances which 

 are not merely analogous, but positively identical in elementary 

 composition and chemical characters, nearly identical in every 

 part of their forms, but nevertheless incapable of associating in 

 crystallization, on account of a molecular difference indicated 

 solely by an optical peculiarity, and by specialities of form which 

 were considered almost accidental. 



It is therefore evident that isomorphism supposes certain con- 

 ditional analogies, and is at the same time consistent with cer- 

 tain differences of internal organization, of which no indications 

 are recognizable, except in those characters which are most im- 

 mediately dependent upon that organization. In order to detect 

 these delicate shades of difference, it is necessary to have recourse 

 to every means of investigation which have been applied in any 

 manner whatever to the study of the molecular constitution of 

 substances. It was from such a motive that I have been in- 

 duced to undertake the comparative study of the optical charac- 

 ters presented by isomorphous substances. 



Since the time when Brewster discovered the distinctive phae- 

 nomena of double biaxial refraction, and thus divided crystals into 

 two principal classes distinguished by their optical characters, 

 many physicists have occupied themselves in studying the phae- 

 nomena of double refraction; but they have directed their attention 

 to the fact itself, for the purpose of ascertaining its laws, or the 

 general relations of position between the form and the optical 

 axes. It is thus that they have established the fact, that in all 

 crystalline substances an axis of symmetry is always an axis of 

 ordinary elasticity for different colours, and that every axis of 



