and a remarkable Structure in Analcime. 193 



that enters into the combination, retains its own character, and, 

 considered by itself, possesses the ordinary properties of double 

 refraction. 



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 pheno- 

 mena related to planes of no double refraction ; but the resem- 

 blance goes no farther, as the structure of the glass depends 

 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 

 this case the isinglass retains a fixed doubly refracting struc- 

 ture, related to the axis of compression or dilatation ; and if it 

 were cut into pentahedrons, similar to those of the Analcime, we 

 might combine them together, so as to imitate, at least in the 

 direction of one of the axes, the phenomena exhibited by the 

 mineral. 



The property which has now been described becomes an in- 

 falh'ble and easily applied mineralogical character for Analcime. 

 However shapeless be the fragment, and however much obliter- 

 ated be its external faces, its action upon polarised light will in- 

 stantly determine whether or not it belongs to this species. 



HAUY first observed in Analcime the singular circumstance of 

 its yielding no electricity by friction, and he even derived its spe- 

 cific name from its want of this property. If we consider that 



VOL. x. P. i. B b 



