420 Scientific Intelligence. 
lrised appearances and colored bands are produced by this kind 
cleavage. In all cases images are polarized in directions parallel to 
the diagonals of the faces of the rhombohedron 
. Cleavage parallel to the smaller diagonal of the primary faces. 
n one direction. Very rare. . Produces a species of colored rings. 
M. Baudrimont gives details shdwing also that the lustre and trans- 
parency vary like the cleavage, and that the plane angles at the sum- 
mit are often unequa 
n Different Cr ystals and Fluids in Topaz ; by Sir Davin Brew- 
eres, Trans. Roy. Soc., xvi, 11; Phil. Mag., xxxi, 497. )—This valu- 
able memoir by Sir David Brewsteg, may be considered a continuation 
of two former ‘papers of his, communicated in 1823 and 1826 to the 
Royal Society of Edinbur: h. In these earlier papers, he described 
two remarkable fluids occurring in topaz and other minerals. In the 
ent memoir a peculiar gaseous substance is described (in addition to 
the fluids Se discovered), and also, at least, five distinct crystallized 
saben anc 
- The ca ovine are at times isolated, but often exist in _— 
tensive layers; and the layers, though sometimes confor 
tion to the primary or secondary planes, often have every ool 
sometimes in radiations ; <a different layers, though very clos 
another, have striking differe ces of character. 
he crystals observed have the following forms, the tetrahedron ; 
be; cube with truncated edges and angles; the rhombohedron ; 
octahedron, truncated on its edges and angles; long rectangular 
plates. Besides these, there are amorphous crystallized masses. He 
observes :— 
“In the examination of the individual crystals, many interest ing 
facts present themselves to our notice. The crystals of the tessular 
class, which are taped of the cube, are very numerous, ap 
In like manner, some 2 of the sonbty rae andes melt readily, 
others with very great difficulty, and others at all; so that there 
must ree different substances, which belong to the classes of forms 
different seieentary forms which [ have already enumerated. 
_ T have seldom found any crystals in these cavities ‘which depolarize 
_ white light, or the highest order of colors. I have found some tha 
— four orders of colors; and when the css which y does this 
isa flat hexagonal plate, it is highly interesting to see it pass through 
all the tints which these orders include, while lowly melting, 
in reproducing them durin - recrystallization 
Onin ext whit wee tied aced ia to be entirely black from the 
total reflexion of the light which fell upon it, I observed thie eo 
gs. hese appeared to be fixed crystals, or rather paris of 
topaz, surrounded by a cavity. I found, however, that the hexagonal 
