RIIO 



HHU 



RHIZOPHORA, in botany, a genus of 

 1<he Uodecandria Monogynia class and 

 wder. Natural order of Holoracea:. Ca- 

 prifolia, Jussieu. Essential character: 

 calyx four-parted ; corolla four-parted ; 

 seed one, very long 1 , fleshy at the base. 

 There are six species, of which R. man- 

 gle, or mangrove tree, commonly attains 

 the height of fifty feet ; it is generally 

 found on the borders of the sea, in whose 

 waters alone it seems to thrive, and there 

 only in such places as have a soft and 

 yielding bottom ; its larger branches fre- 

 quently emit soft and weakly appendicles, 

 having the appearance of so many slen- 

 der, leafless branches, always bending 

 downwards ; but as these are softer, and 

 furnished each with a large column of a 

 lax, spongy pith in the centre, they grow 

 more luxuriantly than the other parts 

 of the tree, and reach the mud in a short 

 time, where they throw out a numberless 

 series of slender fibres, which in time be- 

 come roots, to supply the stem more co- 

 piously with nourishment, whilst they be- 

 come so many props or limbs to the pa- 

 rent tree ; the trunk seldom grows to any 

 considerable thickness ; the bark is excel- 

 lent for tanning leather ; it performs this 

 operation more perfectly in six weeks, 

 lhan oak bark will do in ten. The man- 

 grove is a native both of the East and 

 West Indies, of the Society and Friend- 

 ly Islands, the New Hebrides, and New 

 Caledonia, in the South Seas. 



RHODTOLA, in botany, a genus of the 

 Dioecia Octandria class and order. Na- 

 tural order of Succulents. Sempervivre, 

 Jussieu. Essential character : male, ca- 

 lyx four-parted ; corolla four petalled ; 

 nectary four : female, calyx four-parted ; 

 corolla four-petalled ; nectary tour ; pis- 

 tils four ; capsules four, many-seeded. 

 There are two species, viz. R. rosea, 

 common, or yellow rose- wort, and R. bit- 

 ternata. 



RHODODENDRUM, in botany, a ge- 

 nus of the Dodecandria Monogynia class 

 and order. Natural order of Bicornes. 

 Rhododendra Jussieu. Essential charac- 

 ter: calyx five parted; corolla funnel-form; 

 stamina declined ; capsule five-celled. 

 There are nine species. 



RHODORA, in botany, a genus of the 

 Decandria Monogynia class and order. 

 Natural order of Bicornes. Rhododen- 

 dra, Jussieu. Essential character ; calyx 

 five-toothed ; petals three, unequal ; sta- 

 mina declined ; capsule five-celled. 

 There is only one species, viz. R. cana- 

 densis, a native of Newfoundland, from 



which place it was introduced by Sir Jo- 

 seph Banks. 



RHOEADE^E, in botany, the name of 

 the twenty-seventh order in Linnaeus' 

 Fragments of a Natural Method, consist- 

 ing of the poppy, and a few genera which 

 resemble it in habit and structure. The 

 plants, in this order, upon being cut, 

 yield plentifully a juice which is white in 

 the poppy, and yellow in others. See 

 POPPY. 



RHOMB spar, in mineralogy, a species 

 of the calx genus, of a greyish colour 

 passing to yellow : it is never massive, 

 but always in regular middle-sixed 

 rhombs ; the lustre is splendent, and be- 

 tween vitreous and pearly ; it is brittle, 

 easily frangible : specific gravity 2.5 ; it is 

 infusible without addition. With acids, it 

 produces very little effervescence, even 

 when pulverized. Constituent parts : 



Carbonate of lime 52 



Carbonate of magnesia .... 45 

 Oxide of iron and manganese 3 



100 



It is found in Switzerland, Sweden, and 

 in chlorite rocks, on the banks of Loch 

 Lomond in Scotland. 



RHOMBOIDBS, in geometry, a quadri- 

 lateral figure, whose opposite sides and 

 angles are equal, but is neither equilate- 

 ral nor equiangular. 



RHOMBUS, in geometry, an oblique- 

 angled parallelogram, or a quadrilateral 

 figure, whose sides are equal and paral- 

 lel, but the angles unequal, two of the 

 opposite ones being obtuse, and the other 

 two acute. To find the area of a rhom- 

 bus upon the base, let fall the perpendi- 

 cular, 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 rhubarb has yet 

 been made ; but, from the experiments of 

 Neumann, it appears that nearly one-half 

 of it is soluble in water, and that alcohol 

 scarcely takes up any thing from the resi 



