O N O 
O P A 
292 
Body above convex, beneath flat and smooth; 
head small, and placed beneath, which, when 
the animal is in motion, is perpetually chan- 
ging its form and size, and drawn in when at 
rest ; mouth placed lengthways, and conti- 
nually varying its shape from circular to 
linear ; feelers retractile, resembling those of 
a slug, and apparently tipt with eyes; arms 
dilatable, solid, compressed, and somewhat 
palmate when fully expanded. 
ONION. See Allium. 
ONISCUS, a genus of insects of the order 
aptera: the generic character is ; legs four- 
teen ; antennae setaceous ; body oval. Of this 
genus, which consists of more than 40 species, 
the best known is the oniscus asellus, popu- 
larly known by the name of the woodlouse. 
It is a very common insect in gardens, fields, 
&c. and is observed in great quantities under 
the barks of decayed trees, beneath stones in 
damp situations, '&c. Its general length is 
about halt an inch, or rather more, and its 
colour livid brown, the larger specimens 
often exhibiting a double series of pale spots 
down the back: like the rest of the genus, it 
preys on the minuter insects. 
2. Oniscus armadillo, or the medical wood- 
louse, is of somewhat larger size than the pre- 
ceding, much darker colour, and of a polish- 
ed surface : it is equally common with the 
preceding species, and is found in similar 
situations; when suddenly disturbed or han- 
dled, it rolls itselt up into a completely glo- 
bular form, in the manner of the curious 
quadrupeds called armadillos, frequently re- 
maining in this state for a very considerable 
length of time, or so long as it is any ways 
disturbed. Swammerdam relates a ludicrous 
mistake of a servant-maid, who, finding in 
the garden a great many in this globular state, 
imagined she had discovered some handsome 
materials for a necklace, and betook herself 
to stringing. them with great care; but on 
suddenly perceiving them unfold, was seized 
with a panic, and ran shrieking into the 
house. 
'1 hough considered as of but slight im- 
portance in the present practice of physic, 
these animals once maintained a very respect- 
able station in the materia medica, under the 
title of millepedes. 
3. Oniscus aquations is a native of the 
clearer kind of stagnant waters, and is of the 
general size and colour of the oniscus asellus, 
but of a more lengthened form, and with 
longer limbs in proportion ; the two last legs 
being bifid. This species is viviparous, and 
of a considerably prolific nature. 
Among the marine insects of this genus the 
largest is the oniscus entomon, measuring two 
inches in length : its general form and colour 
resemble that of the oniscus asellus, but the 
lour lower pair of legs are longer in propor- 
tion, the three first pair being very small and 
short ; the tail is long and pointed. It is a 
native of the European seas, and is found 
about rocks, &c. It is of a strong fabric, the 
divisions of the upper part being of an almost 
calcareous nature. This animal is capable of 
living several days in fresh water. 
ONOCLEA, a genus of the class and order 
cryptogamia filices. The capsules are under 
the recurved and contracted pinnules of the 
frond, resembling pericarps. There are two 
species. 
ONONIS, or An on is, rest-harroxv , in 
botany. SeeANONis. 
O P A 
ONOPOPvDUM, a genus of the class and 
order syngenesia polygamia aequalis. The 
essential character is, calyx scales mucronate; 
recept. honey-combed. There are seven spe- 
cies, one of them well-known under the name 
of cotton-thistle or pig-leaves. 
ONOSMA, a genus of the monogynia or- 
der, in the pentandria class of plants, and in 
the natural method ranking under the 4lst 
order, asperifoliai. The corolla is cam pan u- 
lated, with the throat pervious : there are 
four seeds. There are three species, rock 
plants of the South of Europe. 
ONYX, in natural history, one of the se- 
mipel lucid gems, with variously-coloured 
zones, but none red; being composed of 
crystal, debased by a small admixture of 
earth, and made up either of a number of flat 
plates, or of a series of coats surrounding a 
central nucleus, and separated from each 
other by veins of a different colour, resem- 
bling zones or belts. We have four spe- 
cies of this gem: 1. A blueish-white one, 
with broad white zones. 2. A very pure 
onyx, with snow-white veins. 3. The jasp- 
onvx, or homy onyx, with green zones. 4. 
The brown onyx, with blueish-white zones. 
'I’he antients attributed wonderful properties 
to the onyx, and imagined that it worn on 
the finger it acted as a cardiac ; they have 
also recommended it as an astringent, but at 
present no regard is paid to it. The word in 
the Greek language signifies nail; the poets 
feigning this stone to have been formed by 
the Parcae from a piece of Venus’s nails, cut 
off by Cupid with one of his arrows. See 
Chalcedony. 
OOLITE. See Pisolite. 
OPACITY, in philosophy, a quality of 
bodies which renders them impervious to the 
rays of light. 
Plie cause of opacity in bodies does not 
consist, as was formerly supposed, in the want 
of rectilinear pores, pervious every way ; but 
either in the unequal density of the parts, in 
the magnitude of the pores, or in their being 
filled with a matter, by means of which the 
rays of light in their passage are arrested by 
innumerable refractions and reflections, 
become extinct, and are absorbed. 
OPAL, in mineralogy: this stone is found 
m many parts of Europe, especially in Hun- 
gary, in the Crapacks near the village of 
Czennizka. When first dug out of the earth 
it is soft, but it hardens and diminishes in bulk 
by exposure to the air. The substance in 
which it is found is a ferruginous sand-stone. 
The opal is always amorphous. Its frac- 
ture is conchoidal. Commonly somewhat 
transparent. Specific gravity from 1.9.18 to 
2.540. The lowness of its specific gravity, 
in some cases, is to be ascribed to accidental 
cavities which the stone contains. These are 
sometimes tilled with drops of water. Some 
specimens of opal have the property of emit- 
ting various-coloured rays, with a particular 
effulgency, when placed between the eye and 
the light. The opals which possess this pro- 
perty are distinguished by lapidaries by the 
epithet Oriental ; and often by mineralogists 
by the epithet nobilis. This property ren- 
dered the stone much esteemed by the an- 
tients. Opals acquire it by exposure to the 
sun. Werner has divided this species into 
five subspecies: 
1. Noble opal. Lustre internal, glassy. 
Colour, usually light bluish-white. When its 
position is varied, it reflects the light of va- 
rious bright colours. Brittle. Specific gra- 
vity 2.1 14. Does not melt before the blow- 
pipe. When heated it becomes opaque, and 
sometimes is decomposed by the action of 
the atmosphere. Hence it seems to follow 
that water enters essentially into its composi- 
tion. A specimen of this variety, analysed 
by Klaproth, contained 
90 silica, 
10 water 
100 . 
2. Common opal. Fracture imperfectly 
conchoidal. Lustre external and internal, 
glassy or greasy. Its colours are very va- 
rious; milk-white, yellows, reds, greens of 
different kinds. Infusible by the blowpipe. 
Specimens of this variety sometimes occur 
with rifts : these readily imbibe water, and 
therefore adhere to the tongue. Some opals 
gradually become opaque, but recover their 
transparency when soaked in water by im- 
bibing that fluid. They are then called hy- 
drophanes, or oculi mundi. The constituent* 
ot the common opal, as ascertained by Klap- 
roth, are 
Opal of Kosemutz. 
98.75 
0.1 
0.1 
0.0 
98.95 
Opal ofTelkobanya. 
93.5 silica 
1.0 oxide of iron 
0.0 alumina 
5.0 water. 
99.5 
3. Semi-opal. Colours, various shades of 
white, grey, yellow, red, brown, often mixed' 
together. Lustre glassy, sometimes greasy. 
Fracture imperfectly conchoidal Brittle. 
Sometimes adheres to the tongue. Specific 
gravity 2.540. Infusible before the blow- 
pipe. Its constituents, as ascertained by Klap- 
roth, are. 
pal ofTelkobanya Of Menal-montant, 
43.5 
- 
85.5 silica 
47.0 
- 
0.5 oxide of iron. 
7.5 
- 
1 1.0 water 
— 
1.0 alumina 
98.0 
0.5 lime. 
98.5 
4. Hotz-opal or wood-opal. Colours, va- 
rious shades of white, grey, brown, yellow, 
red. Found in large pieces, which have the 
form of wood. Lustre glassy, sometimes 
greasy. Fracture in one direction conchoidal, 
in another exhibiting the texture of wood. 
Usually opaque. Brittle. Considered as 
fragments of wood impregnated with semi- 
opal. 
5. Under the opal maybe placed also the 
mineral known by the name of cat’s-eye. It 
comes from Ceylon, and is seldom seen by 
European mineralogists till it has been po- 
lished by the lapidaiy. Mr. Klaproth has 
described a specimen which he received in 
its natural state from Mr. Greville of Lon- 
don. Its figure was nearly square, with sharp 
edges, a rough surface, and a good deal of 
brilliancy. Its texture is imperfectly foliated. 
Lustre greasy. Specific gravity 2.625 to 
2.66. Colour grey, with a tinge of green, 
yellow, or white ; or brown, with a tinge of 
yellow or red. In certain positions it re- 
