RHU 
in this order, upon being cut, yield plen- 
tifully a juice wliich is white in the poppy, 
and yellow in others. See Poppy. 
RHOMB sjiar, in mineralogy, a species 
of the Calx genus, of a greyish colour pass- 
ing to yellow : it is never massive, but al- 
ways in regular, middle-sized rhombs ; the 
lustre is splendent, and between vitreous 
and pearly ; it is brittle, easily frangible ; 
specific gravity 2.5 ; it is infusible, without 
addition. With acids, it produces very lit- 
tle effervescence, even when pulverized. 
Constituent parts : 
Carbonate of lime 52 
Carbonate of magnesia 45 
Oxide of iron and manganese S 
100 
It is found in Switzerland, Sweden, and 
in chlorite rocks, on the banks of Loch 
Lomond in Scotland. 
RHOMBOIDES, in geometry, a quadri- 
lateral figure, whose opposite sides and an- 
gles are equal, but is neither equilateral nor 
equiangular. 
RHOMBUS, in geometry, an oblique- 
angled parallelogram, or a quadrilateral fi- 
gure, whose sides are equal and parallel, 
but the angles unequal, two of the opposite 
ones being obtuse, and the other two acute. 
To find the area of a rhombus, upon the 
base, let fall the perpendicular, which is 
the altitude of the figure ; then multiply 
the base by the altitude, the product will 
be the area. 
RHUBARB. This is the root of the 
rheum palmatum, and perhaps also of some 
other species of rheum, brought chiefly 
from the northern parts of China, by the 
way of Russia, though of late it has been 
cultivated also in Britain. The root is 
large, of an oblong or roundish shape ; of a 
dark-brown colour externally, with black 
and reddish streaks ; internally it is reddish- 
yellow, and, when fresh, contains a juice of 
the same colour. No accurate chemical 
analysis of rhubarll has yet been made ; but, 
from the experiments of Neumann, it ap- 
pears that nearly one-half of it is soluble in 
water, and that alcohol scarcely takes up 
any thing from the residue. From the pro- 
perties of the watery extract, enumerated 
by that laborious chemist, we may infer, 
with some probability, that it consists 
chiefly of an extractive and bitter principle, 
and that it contains some tannin. A small 
quantity of greenish-yellow, resinous mat- 
ter, seems also to be present. Scheele se-. 
RIC 
parated from the root about one-sixth of 
its weight of oxalate of lime. But this salt 
is not taken up by water. See Materia 
Medica, and Pharmacy. 
RHUMB, in navigation, a vertical circle 
of any given place, or the intersection of 
such a circle with the horizon j in which 
last sense rhumb is the same with a point 
of the compass. 
Rhomb line, is also used for the line 
which a ship describes when sailing in the 
same collateral point of the compass, or ob- 
lique to the meridians. 
RHUS, in botany, sumach, a genus of 
the Pentandria Trigynia class and order. 
Natural order of Diunosae. Terebintaceae, 
Jussieu. Essential character ; calyx five- 
parted ; petals five ; berry one-seeded. 
There are thirty-four species. 
RIBES, in botany, the currant and goose~ 
berry, a genus of the Pentandria Monogy- 
nia class and order. Natural order of Po- 
raaceae. Cacti, Jussieu. Essential charac- 
ter’; petals five, inserted with the stamens 
into the calyx ; style bifid ; berry many- 
seeded, inferior. There are seventeen spe- 
cies, sir. ten of the currant, and seven of 
the gooseberry ; all these shrubs are too- 
well known to need a particular descrip- 
tion in this work. ' I 
RICCI A, in botany, so named in honour 
of Pietro Francisco Riccio; a genus of the 
Cryptogamia Hepatic® class and order. 
Generic character : male, flowers sessile on 
the surface of the frond ; calyx and corolla 
none : female, flowers on the same, or, ac- 
cording to Micheli, on a distinct plant ; ca- 
lyx none, except a vesicular cavity, within 
the substance of the leaf ; corolla none. 
Linn®ns has five species, natives of Europe. 
Withering reckons the same number, all 
natives of Britain. 
RICHARDIA, in botany, so named from 
Richardson ; a genus of the Hexandria Mo- 
nogynia class and order. Natural order of 
Stellat®. Rubiace®, Jussieu. Essential 
character : calyx six-parted ; corolla one- 
petalled, sub-cylindric j seeds three. There 
is only one species, viz. R. scabra, a native 
of Vera Cruz. 
RICHERIA, in botany, so named in 
memory of Pierre Richer de Belleval, 
Professor of Botany, at Montpelier ; a ge- 
nus of the Dioecia Pentandria class and or- 
der. Essential character : capsule corti- 
cate, six-valved, three celled j seeds soli- 
tary, pendulous, below the tip of the colu- 
mella ; style trifid. There is but one spe- 
