REPORT ON MINERALOGY. 
327 
of light which remains undestroyed when we apply to the sur¬ 
face a lamina of liquid' differing but slightly from the mineral 
in its refractive power: the differences of lustre and colour in 
minerals, may thus become much more sensible than when the 
whole effect is compared. 
The different kinds of lustre,—glassy, fatty, pearly, adaman¬ 
tine, metallic,—undoubtedly depend upon optical differences in 
the surfaces, which differences have not however as yet been 
clearly explained. Professor Breitliaupt is in the habit of 
showing, by the superposition of a number of watch-glasses, 
that the pearly lustre results from the lamellar structure of a 
transparent mass. The very curious difference between the 
optical properties of the surfaces of metals, and of transparent 
bodies, has been traced, on different roads, by Sir David 
Brewster and by Professor Airy ; and both agree in considering 
the optical properties of the diamond as intermediate between 
the transparent and the metallic character ; though they do not 
agree in their representation of the peculiar laws which the 
diamond discloses. When the connexion of these properties 
with those of other bodies is clearly made out, we shall pro¬ 
bably learn more distinctly than we now can, what is the pre¬ 
cise distinction of metallic, adamantine, and vitreous lustre. 
The more distinct cleavages of minerals are among their most 
important characters, and the less distinct are also of value. 
Sir David Brewster has suggested a method of obtaining 
cleavages too indistinct to be made visible in any common way, 
by tearing the surface of the mineral with a dry file. In this 
manner he made obvious a cleavage of calc spar in the direction 
of the long diagonal of each of the rhombic faces. 
We may notice here, also, the ingenious mode of mechanical 
analysis described by M. Cordier, and successfully employed 
by him in the examination of rocks of various kinds which had 
been considered as homogeneous substances, but which are in 
fact aggregates of small crystalline portions of various simple 
minerals. The specimen is reduced to minute fragments, 
rather by pressure than by trituration*, and the particles of 
different kinds, being separated by differences of specific gra¬ 
vity or appearance, are examined in various ways, and especially 
by means of the blowpipe. This method was found to be par¬ 
ticularly applicable to the discrimination and discussion of rocks 
of a trappean character. 
2. Crystallography . 
Though no change has since been made with reference to 
the crystalline forms of minerals which has excited so much 
* Journ. Phys. 1816, pp. 82, 83. 
