6N0 
when creeping. In appearance it very 
much resembles a limax, but differs prin- 
cipally in wanting the shield and lateral 
pore, and in being furnished with a vent 
behind. Body above convex; head small 
and placed beneath, which when the ani- 
mal is in motion is peipetiially cl;anging 
its form and size, and drawn in when at 
rest; mouth placed lengthways and con- 
tinually varying its shape froni circular to 
linear ; feelers retractile resembling those 
of the slug, and apparently tipt with eyes ; 
arms dilatable, solid, compressed, and pal- 
mate when fully expanded. 
ONION in botany, see Ai.liom. Con- 
sidered chemically it may be observed that 
as it possesses most of the properties of 
Garlic (which see) though not in so large 
proportions, a volatile oil, on which its ac- 
tivity depends, might be expected, but this 
has not been found. Water distilled from 
it yeilds no oil ; if therefore there is any 
oil it must be in very small quantities and 
soluble in w'ater. The active principle of 
the onion acts upon the tin of the alembic 
in which experiments have been made. 
ONISCU8, in natural history, a genus 
of insects of the order Aptera. Jaw trun- 
cate denticnlate ; lip bifid ; antennas from 
two to four, setaceous ; body oval consist- 
ing of about fourteen transverse segments ; 
fourteen legs. These insects feed on ani- 
mal and 'vegetable matter, and they cast 
their skin. There are nearly fifty species 
divided into sections. A. without feelers ; 
four antennaj, sessile. B. feelers unequal, 
the hind ones longer ; antenme filiform. 
The most common species is the O. asellus, 
or common wood-louse, found in great 
quantities under the bark of decayed trees, 
beneath stones in damp situations. It 
preys on minuter insects. O. armadillo, 
the medical wood-louse, is of a darker 
colour than the former, but found in similar 
situations. When suddenly disturbed or 
touched, it rolls itself up into a round form 
in the manner of the armadillos ; frequently 
remaining in that state for a considerable 
length of time, This insect was formerly 
considered as a specific in many disorders, 
but is now rarely used. Among marine 
insects of this genus, is the O, entomon, 
measuring two inches in length. It is a 
native of the European seas, and is found 
among rocks ; it is of a strong fabric, the 
divisions of the upper part being of an al- 
most calcareous nature. . 
ONOCLEA, in botairy, a genus of the 
Cryptogamia Eilices class and order. Na- 
OPA 
tural order of Eilices or Ferns. Generic 
character: capsules uitder the recurved 
and contracted pinnules of the fi-ond, re- 
sembling pericarps. There are two-species, 
vis. O. sensibilis and O. polypodioides, the 
former is a native of Virginia, the latter 
was found by Koenig, in the fissures of the 
rocks near the top of the Table Mountain 
at the Cape of Good Hope. 
ONONIS, in botany, reslharrow, a gentis 
of the Diadelphia Decandria class and order. 
Natural order of Papilionace® or Legumi- 
nos®. Essential character : calyx, five- 
parted, with linear segments; banner stri- 
ated ; legume turgid, sessile ; filaments con- 
nate without a fissure. There are thirty- 
eight species ; these are herbaceous plants 
or under shrubs ; leaves ternate, with the 
leaflets often serrulate ; stipules fastened 
to the bottom of the petiole ; flowers yel- 
low or purple, one or many flowered. 
ONOPORDUM, in botany, a genus of 
the Syngenesia Polygamia jEqualis class 
and order. Natural order of Compositae 
Capitatm. Cinarocephal®, Jussieu. Es- 
sential character : calyx scales mucronate ; 
receptacle honey combed. There are 
seven species. 
ONOSMA in botany, a genus of the 
Pentandria Monogynia class and order. 
Natural order of Asperifoliae. Borraginese, 
Jussieu. Essential character : corolla, bell- 
shaped, with the throat pervious; seeds 
four. There are three species. 
ONYX. See Chalce DOXY. 
OPACITY, in philosophy, a quality of 
bodies which renders them impervious to 
the rays of light. It has been supposed 
that opacity consists in this, that the pores 
of the body are not all straight. This doc- 
trine, however, is deficient: for though to 
have a body transparent, its pores must be 
straight, or rather open everyway; yet it 
is inconceivable how it should happen, that 
not only glass and diamonds, but even 
water, whose parts are so very moveable, 
should have all their pores oj)en and per- 
vious every way ; while the finest paper, 
or the thinnest gold leaf, should exclude 
the light, for want of such pores. 
So that another cause of opacity must be 
sought for. Now all bodies have vastly 
more pores or vacuities than are necessary 
for an infinite number of rays to pass freely 
through them in right lines, without striking 
on any of the parts themselves. For since, 
water is nineteen times lighter or rarer 
than gold ; and yet gold itself is so very 
rare that magnetic effluvia pass freely 
