254 Dr. Brewster on the laws of polarisation, &c. 
Sect. V. On the polarising structure of crystals that have the 
cube, the regular octahedron, and the rhomboidal dodecahedron 
for their primitive form . 
In examining the structure of doubly refracting crystals, 
it will be found that there are thirteen bodies which seem to 
be entirely destitute of the polarising structure, and other 
nine which sometimes exhibit distinct traces of two opposite 
structures.* And it is a very curious circumstance, that 
the crystals which have this character are those which 
crystallize in the form of a cube, a regular octohedron, 
and a rhomboidal dodecahedron, as appears from the following 
table. 
Names of crystals. Primitive form. 
Garnet Rhomboidal dodecahedron 
Blende Rhomboidal dodecahedron 
Diamond Regular octohedron 
Spinelle ruby Ditto 
Ceylanite Ditto 
Alum Ditto 
Muriate of ammonia Ditto 
Fluor spar Ditto 
Ruby copper Ditto 
Nitrate of lead Ditto 
Nitrate of strontian Octohedral crystals 
Names of crystals. Primitive form. 
Boracitef Cube 
Muriate of soda Cube 
Muriate of potash Cube 
Leucite Cube 
Analcime Cube 
Nitrate of barytes 
Sulphate of alumine and ammonia 
Nitrite of lead 
Cinnamon stone 
Essonite 
Uranite 
These crystals, as will be seen by a comparison with 
Hauy’s table of primitive forms, J include all the transparent 
and translucent crystals which he has arranged under the 
preceding forms, excepting tungstate of lime ; but this ex- 
* See the Edinburgh Transactions, vol. vm. p. 157, where I have given an ac- 
count of the discovery of this property. 
f Boracite has an axis in every direction, like the waxen partitions of the honey- 
comb. 
% Traiti de Mineralogie, tom. i. p. 273. 
