OP A 
tlirongh it, without any opposition, and 
quicksilver is readily received within its 
pores, and even water itself by compres- 
sion, it must have more pores than solid 
parts; consequently water must have at 
least forty times as much vacuity as solidity. 
The cause, therefore, why some bodies are 
opaque, does not consist in the want of rec- 
tilinear pores, pervious every way, but 
either in the unequal density of the parts, 
or in the magnitude of the pores, and to 
their being either empty, or tilled with a 
ditferent matter ; by means of which the 
rays of light, in their passage, are arrested 
by innumerable refractions and reflections, 
till at length falfing on some solid part, 
they become quite extinct, and are utterly 
absorbed. Hence cork, paper, wood, &c. 
are opaque ; while glass, diamonds, &c. are 
pellucid. For in the confines or joining of 
parts alike in density, such as those of glass, 
water, diamonds, &c. among themselves, 
no refraction or reflection takes place, be- 
cause of the equal attraction every way 
so that such of the rays of light as enter tlie 
first surface, pass straight through the body, 
excepting such as are lost and absorbed, 
by striking on solid parts : but in the bor- 
dering of parts of unequal density, such as 
those of wood and paper, both with regard 
to themselves, and with regard to the air, 
or empty space in their larger pores, the 
attraction being unequal, the reflections 
and refractions will be very great ; and thus 
the rays will not be able to pass through 
such bodies, being continually driven about, 
till they become extinct. 
That this interruption or discontinuity of 
parts is the chief cause of opacity. Sir Isaac 
Newton argues, appears from hence, that 
all opaque bodies immediately begin to be 
transparent, when their pores become filled 
with a substance of nearly equal density 
with their parts. Thus, paper dipped in 
water or oil, some stories steeped in water, 
linen cloth dipped in oil or vinegar, &c. 
become more transparent than before. 
OPAL, in mineralogy, a species of the 
Ottartz family, found in many parts of 
Europe, especially in Hungary. AVhen 
first dug out of the earth it is soft, but it 
hardens and diminishes in bulk by exposure 
to the air. The specific gravity varies 
from 1.9 to 2.6. There are four subspecies, 
viz. the precious, the common, the semi, 
and the wood opal. Some specimens have 
the property of emitting various coloured 
rays, with a particular effulgency when 
placed between the eye and the light. The 
OPE 
opals that possess this property are distin- 
guished by lapidaries by the epithet orien- 
tal or nobilis. It is esteemed the most 
beautiful of the gems by Eastern nations ; 
but in Europe it is not quite so highly 
valued on account of its liability to split on 
a sudden change of temperature: it is 
principally used for necklaces, ear-rings, 
and finger-rings. The most beautiful opals 
know'n are in the Imperial cabinet of 
Vienna; one is five inches long and two 
and a half in diameter : another is of the. 
size and nearly of the shape of a hen’s egg. 
The noble opal consists of silica and water 
in the proportion of 9 to 1. Specimens 
of the common and semi-opal have been 
analysed and found to consist as follows : 
Common Opal. Semi Opal, 
Silica 
. 98.75 
Alumina 
. 0 . 1 
0.0 
Oxide of iron.. 
. 0.1 
AVater 
,. 0.0 
7..5 
98.95 
Loss 
.. 1.05 
100 
100 
OPATRUM, in natural history, a genus 
of insects of the order Coleoptera. Antennae 
moniliform, thicker towards the tip ; head 
projecting from a cavity in the thorax ; 
thorax a little flattened, margined ; shells 
immarginate, longer than the abdomen. 
There are twenty-eight species, O. sabu- 
losum, is brown, shells with three indented 
raised lines ; thorax emarginate. Inhabits 
Europe and Ameriea, on sand. 
OPERA, a dramatic composition set to 
music, and sung on the stage, accompa- 
nied witli musical instruments, and enrich- 
ed with magnificent dresses, machines, and 
other decorations. 
OPERA-gZass, in optics, so called from its 
use in theatres, &c. it is sometimes called 
a “ diagonal perspective” from its conr 
struction. It consists of a tube about four 
inches long, in each side of which there is 
a hole exactly against the middle of a plane 
mirror which reflects the rays falling upon 
it, to the convex glass, through which they 
are refracted to the concave eye-glass, 
whence they emerge parallel to the eye 
at the hole in the tube. This instrument 
is not intended to magnify objects more 
than about two or three times. The pecu- 
liar artifice is to view a person at a small 
distance, so that no one shall know who 
is observed : for the instrument points to 
a different object from that which is view- 
