194 SEVENTH REPORT—1837, 
but it does not seem to account for the fact that portions of the 
pentahedrons of analcime may be extracted which possess no 
double refraction, or for the properties of the several parts of 
the crystals of chabasie and diamond above referred to. The 
state of the elastic ether in these separate portions must de- 
pend on a difference either in the nature or mutual disposition 
of the ponderable molecules around which it exists; otherwise 
the optical properties could be of little value as indices either 
of chemical constitution or of crystalline form. In other words, 
if the optical properties observed in these minerals reside in the 
crystalline molecules, and not in the mass, the properties of 
the different parts must depend on a difference either in the 
chemical properties or in the mechanical arrangement of the 
ultimate molecules of which they are made up. 
I think it very likely that in some instances the former cause 
operates, in other cases, the latter. The introduction of an 
isomorphous substance of unlike chemical and optical relations 
may produce such differences as are observed in chabasie* ; a 
different arrangement of the, molecules, without change of com- 
position ; a dimorphism—in fact—may produce the singular dif- 
ferences of the several portions of analcime. The double re- 
fraction observed occasionally in alum and other regular cry- 
stals, points, as it appears to me, to an advanced period of our 
knowledge, when these and many other substances crystallizing 
similarly will be proved to be dimorphous. 
VI. 
40. Of epigene and pseudomorphous crystals.—In a former 
section I have adverted to the subject of pseudomorphous cry- 
stals, and to the possibility that some of the forms considered to 
be such may hereafter prove to be cases of dimorphismt. In 
connexion with the present subject, therefore, as well as in 
itself not void of interest, I shall here insert a list of the best 
known and most common cases of epigene, or changed crystals, 
and pseudomorphous crystals, or casts, which either occur in 
nature or can be formed artificially. 
* This principle I have illustrated in a short paper in the Lond. and Edinb. 
Phil. Mag. for Sept., 1836. , 
+ This opinion, in so far as regards the last substance (Serpentine) in the 
above list, has been recently supported by Dr. Tamnau, of Berlin, (Pog. Ann., 
xlii. p. 462,) who assigns several weighty reasons for considering the supposed — 
false forms of this substance from Snarum, in Norway, to be the true form of 
the mineral itself. 
