DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 291 



line association of two isomorphous substances possessing oppo- 

 site optical characters, behave towards light, indicates the exist- 

 ence of a uniform and continuous mode of aggregation, a homo- 

 geneity of structure as perfect as that of their elementary con- 

 stituents. It appears therefore in the highest degree probable, 

 that this union — ^^in which each of the constituents, by a gradual 

 and mutual concession, merges a part of its individual characters, 

 so as to form a whole capable of acting in its turn in the same 

 manner as a simple system — takes place in the crystalline mole- 

 cule between elementary particles less gross than those of which 

 the sensible crystal is formed. This line of induction would 

 therefore, as in many other instances, lead us to regard crystals 

 as complex structures consisting of molecular systems of different 

 orders. It would therefore be necessary to ascertain what mem- 

 bers of this ascending series produce the determining conditions 

 of the isomorphism and of each physical character ; but all spe- 

 culation on a subject with regard to which the most natural 

 induction has always been a false guide, is at least premature. 

 Consequently I shall more directly arrive at some positive in- 

 ferences from these experiments, by showing that they furnish 

 a clue to certain hitherto inexplicable optical properties of 

 minerals. 



The crystalline associations of several isomorphous compounds 

 of dissimilar or even opposite optical characters, must have their 

 representatives in the mineral kingdom ; in fact, few compound 

 minerals present in different specimens optical characters which 

 are absolutely invariable ; and there are some instances in which 

 they habitually present considerable discrepancies in this par- 

 ticular. 



I do not here speak of irregularities which are obviously con- 

 sequences of defective homogeneity in the crystalline medium, 

 and manifest themselves as partial alterations of colour, striae and 

 interior reflexion ; without taking into account these accidental 

 phaenomena, it is certain that an extreme variability of optical 

 characters appears to be in some sort essential to many natural 

 families of minerals, the sensible characters of which indicate a 

 homogeneity of structure, and which are shown to possess a cry- 

 stalline continuity by the far more delicate test of polarized 

 light. Such, for instance, are the topazes, in which the angle 



