ocir 
the countries they border upon. For the 
saltness, tide.s, &c. of the ocean, see the ar- 
ticles Sea, Tides, &c. 
OCHRES, in cliemistry, combinations of 
earths with the oxide of iron: they are of 
various colours, and are principally employ- 
ed as pigments. 
OCHROIT, in chemistry, an earth dis- 
covered by Klaproth : the colour of the mi- 
neral in which the earth is found, and which 
is denominated ochroites, is between red 
and brown. It is compact, and breaks 
splintering in irregidar or angular pieces. 
It is perfectly opaque, and the powder is of 
a reddish grey. The specific gravity is 
about 4,6. The earth was called ochroit, 
from the Greek word account of 
the characteristic property which it pos- 
sesses of acquiring a light brown colour 
after being heated. The mineral consists of 
Ochroit earth 
Silex 
Oxide of iron 
5 
Loss 
97.5 
too 
Ochroit earth is capable of combining 
with carbonic acid, during its precipitation 
from acids by carbonated alkalies, and 
strongly consolidating a portion of water. 
It is observed in “ Nicholson’s Journal,” 
that the ochroit earth bears the nearest re- 
lation to ittria, and like that, it forms a 
connecting link between the earths and the 
metallic oxides. Like ittria, it has the pro- 
perty of forming a reddish-coloured salt 
with sulphuric acid, and is precipitable by 
prussiate of potash; but it differs from it- 
tria, in that it does not form sweet salts ; that 
it is not soluble, or at least very sparingly, 
in carbonate of ammonia ; and that, when 
ignited, it acquires a cinnamon-brown co- 
lour. It differs also from ittria, by not be- 
ing soluble in borax, or phosphate of soda, 
when urged upon charcoal before the blow- 
pipe, which salts easily effect a solution of 
ittria, and melt with it also into a pellucid 
pearl. See Ittria. 
OCHNA, in botany, a genus of the Po- 
lyandria Monogynia class and order. Na- 
tural order of Coadunat®. Magnoliae, Jus- 
sieu. Essential character : calyx five-leav- 
ed ; corolla five-petalled ; berries one-seed- 
ed, fastened to a large, roundish receptacle. 
There are three species. 
OCT 
OCHROMA, in botany, a genus of the 
Monadelphia Pentandria class and order. 
Natural order of Columniferae. Malvacese, 
Jussieu. Essential character : calyx dou- 
ble, outer three-leaved; anthers connate, 
anfractuose ; capsule five-celled, many-seed« 
ed. There is but one species, viz. O. lago- 
pus, a large tree, with divaricating branches ; 
the wood is white, tender, and sufficiently 
light to be used instead of corks for nets ; 
the bark is thick, fibrous, and ash-coloured ; 
leaves frequently a foot and half in diame- 
ter ; flowers on the upper branclilets, on 
thick, straight peduncles; calyx greenish 
red; petals white, fleshy ; capsule eight or 
ten inches long. It is a native of America. 
OCHROXYLUM, in botany, a genus of 
the Pentandria Trigynia class and order. 
Essential character : calyx five-cleft ; pe- 
tals five; nectary an annular three-lobed 
gland; capsule three, approximating, one. 
celled, two-seeded. 
OCIMUM, in botany, basil, a genus of 
the Didynamia Gymnospermia class and or- 
der. Natural order of Verticillatae. La- 
biate, Jussieu. Essential character : calyx 
with the upper lip orbiculate, the lower 
four-cleft ; corolla resupine, with one lip 
four-cleft, the other undivided ; filaments, 
the two outer putting forth a reflex process 
at the base. There are twenty-five spe- 
cies; these are either herbs or under- 
shrubs, possessing a sweet scent ; their 
flowers are in whorls, forming a loose spike, 
terminating and axillary. 
OCTAGON, in geometry, is a figure of 
eight sides and angles : and this, when all 
the sides and angles are equal, is called a 
regular octagon, or one which may be in- 
scribed in a circle. If the radius of a cir- 
cle, circumscribing a regular octagon, be 
— r, and the side of the octagon —y; then 
y = — r-v/^ 
Octagon, in fortification, denotes a 
place that has eight bastions. 
OCTAHEDRON, or Octaedron, in 
geometry, one of the five regular bodies, 
consisting of eight equal and equilateral tri- 
angles. See the article Body, The square 
of the side of the octahedron is to the square 
of the diameter of the circumscribing 
sphere, as 1 to 2. If the diameter of the 
sphere be 2, the solidity of the octahedron 
inscribed in it will be 1.33333, nearly. The 
octahedron is two pyramids put together at 
their bases, therefore its solidity may be 
found by multiplying the quadrangular base 
of either of them, by one-third of the per- 
