258 Dr Brewster on a New Species (^Double Refraction. 
form, and every fragment of a crystal, however minute, possesses 
this axis, and all the optical properties of the original crystal, 
however large. The propert}^ of double refraction, in short, in 
regularly crystallised substances, resides in the ultimate particles 
of the body, and does not depend upon the mode in which they 
are aggregated to form an individual crystal. 
In Analcime, on the contrary, we have planes of no double 
refraction, having a definite and invariable position, and we may 
even extract a portion of each separate pentahedron which has 
no axis at all. 
Nor has the doubly refracting structure of Analcime any rela- 
tion to that of composite crystals, such as the hipyramidal sul- 
phate f potash *5 which consists of several individual rhomboidal 
prisms, beautifully combined to form a regular geometrical solid, 
or that still more complicated mineral apophyllite^ where an in- 
dividual crystal with one axis is symmetrically united with seve- 
ral individual crystals with two axes, so as to constitute a regu- 
lar crystal *[•, In these, and other cases, each individual crystal 
that enters into the combination, retains its own character, and, 
considered by itself, possesses the ordinary properties of double 
yefraction. 
The Analcime partakes of the character of other composite 
minerals, in so far as it is made up of twenty-four individual 
pentahedrons ; but each pentahedron possesses a new species of 
double refraction, which has been found in no other crystal. 
This structure resembles, to a certain degree, that of rectangular 
plates of glass, while in the act of being heated, in having the 
phenomena related to planes of no double refraction ; but the 
resemblance goes no farther, as the structure of the glass de- 
pends upon its external form, and the planes of no polarisation 
change their position with the outline of the plate. In Analcime, 
on the other hand, the structure is permanently fixed, and has 
no relation whatever to the external shape of the fragment. 
In the absence of more striking analogies, we may consider 
this structure as resembling that which is produced by harden- 
ing Isinglass, when in a state of compression or dilatation. In 
* See this Journal, vol. i. p. 0. 
Sec this Journal, vol. i, p, 1 j and Edin, Trans, vol. ix, p. 317. 
