. Dr Brewster on Comptmite, 
a 
upon 
b 
137° 
56' 
e 
upon/ 
135° 
36^ 
b 

c 
132 
41 
f 
g 
135 
36 
a 
c 
90 
37 
e 
g 
91 
12 
c 
— 
d 
131 
24 
g 
— ^ h 
132 
51 
d 
— 
e 
138 
34 
h 
— a 
135 
36 
c 
— 
e 
89 
58 
g 
a 
88 
27 
In some crystals, the faces aye extinguished by the trun- 
cations b, d ; hyf; and in several crystals, I have observed with 
the microscope very slight but distinctly marked truncations on 
the terminal edges 6, d, f, whose incidence upon the summit 
mnh 99°, and upon the faces 171®. 
The summit plane w is often a little rounded, and some.? 
times composed of two planes tw, 7^, whose mutu^ inclinatioja 
is 179®, 178°, or 177°. 
The measurements of other crystals indicate very unequivocal- 
ly that the prism is rectangular ; but all of them make the inclis- 
nation of some of the truncations so high as 137^°. If the 
prism should turn out to have a rhombic base, its angles cannot 
exceed 90° 51' and 88° 9'* 
Comptonite belongs to the Prismatic System of Mohs. 
Optical Structure , — Comptonite has two axes of double re^p 
fraction, one of which is parallel, and the other perpendicular, 
to the axis of the prism. It consequently gives the double sys- 
tem of coloured rings. The inclination of the resultant axes, or 
diameters of no polarisation, is nearly 56° ; or they are inclined 
about 28° each to a line at right angles to the faces a, which 
is the principal axis of the crystal. The action of this axis is 
positive^ like that of Topaz. The plane passing through these 
resultant axes is perpendicular to the axis of the prism. The 
index of refraction is 1,553, when the ray passes through a and 
and no separation of the images can be seen ; but through a 
and m w, the images may be distinctly separated.. 
Chemical Character. — Comptonite is converted into a jelly, 
like all the mesotypes, by exposing it in the state of powder to 
the action of nitric acid. When a whole crystal is placed in niT 
trie acid, it does not swell out like Apophyllite and Auvergne 
niesotype, 
