702 RHODIUM 



RESIST. A paste used in calico-printing to keep portions white when the cloth 

 is dipped into the dye. See CALICO-PRINTING. 



RETEHTE. A hydrocarbon, similar to benzine, obtained from the resinous matter 

 found on pine-stems in peat-bogs. It has also been obtained by the dry distillation 

 of resinous pine-wood. 



RETXNXTE. A mineral resin found in rounded or irregular lumps. It is soluble 

 in alcohol, leaving an unctuous residue. It is found in the lignite beds of Bovey 

 Tracey, in similar deposits in Hanover, and in the coal-mines of Moravia. 

 RETIlffOXiE. A hydrocarbon, obtained from the turpentine-resins. 

 RETORT. Retorts may be of various shapes, and made of very different ma- 

 terials, according to the requirements. Some are of glass ; others of clay. They 

 may be made of any of the metals. Eetorts are employed to effect the decom- 

 position of compound bodies by the action of heat ; sometimes alone, and sometimes 

 aided by the action of other furnaces. They vary in shape ; but generally may 

 be regarded as consisting of a bulb and a beak. For producing coal-gas there are 

 many modifications, varying in dimensions and shape with the caprice of the con- 

 structor. 



RETURN AIR. In coal-mining, the air which ascends after having passed 

 through the workings of a colliery. 



RETURNS. A light-coloured and mild kind of tobacco. 



REVAXiENTA ARABIC A. The commercial name for the flour obtained from 

 the lentil, Ervum lens. It was first called Ervalenta, then by transposing the letters 

 of the first portion of the name, revalenta was obtained. 



REVERBERATOR? FURNACE. A furnace in which the flame passes over 

 a bridge, and plays down or reverberates against the hearth on which the materials 

 are placed. See METALLURGY, COPPER, IKON, SODA, &c. 



RHATANY ROOT. Krameria triandra, a native of Peru. The root is hori- 

 zontal, very long and branched. It is used as an astringent, and in powder as a 

 dentifrice. 



RHODIUM. A metal discovered by Dr. Wollaston in 1803, in the ore of 

 platinum. It is contained to the amount of 3 per cent, in the platinum-ore of 

 Antioquia in Columbia, near Barbacoas ; it occurs in the Ural ore, and alloyed with 

 gold in Mexico. The palladium having been precipitated from the muriatic solution 

 of the platinum-ore previously saturated with soda by the cyanide of mercury, 

 muriatic acid is to be poured into the residuary liquid, and the mixture is to be 

 evaporated to dryness, to expel the hydrocyanic acid, and convert the metallic salts 

 into chlorides. The dry mass is to be reduced to a very fine powder, and washed 

 with alcohol of specific gravity 0-837. This solvent takes possession of the double 

 chlorides which the sodium forms with the platinum, iridium, copper, and mercury, 

 and does not dissolve the double chloride of rhodium and sodium, but leaves it in the 

 form of a powder of a fine dark-red colour. This salt being washed with alcohol, 

 and then exposed to a very strong heat, affords the rhodium. But a better mode of 

 reducing the metal upon the small scale consists in heating the double chloride gently 

 in a glass tube, while a stream of hydrogen passes over it, and then to wash away 

 the chloride of sodium with water. 



Rhodium resembles platinum in appearance. According to Wollaston, the specific 

 gravity of rhodium is 11. It is insoluble by itself in any acid ; but when an alloy of 

 it with certain metals, as platinum, copper, bismuth, or lead, is treated with aqua 

 regia, the rhodium dissolves with the other metals ; but when alloyed with gold 

 or silver, it will not dissolve along with them. It may, however, be rendered 

 very soluble by mixing it in the state of a fine powder with chloride of potassium or 

 sodium, and heating the mixture to a dull red-heat, in a stream of chlorine gas. It 

 thus forms a triple salt, very soluble in water. The solutions of rhodium are of a 

 beautiful rose colour, whence its name. Its chief use at present is for making the 

 unalterable nibs of the so-named rhodium pens. 



The following remarks from a recent paper by Deville and Debray, ' On some pro- 

 perties of the so-called Platinum Metals' are full of interest. These chemists prepare 

 rhodium by fusing platinum-residues with an equal weight of lead and twice its weight 

 of litharge. When the crucible has attained a bright red heat, and the litharge is 

 thoroughly liquid, the crucible is shaken once or twice, and is then allowed to cool 

 slowly. The button of lead, which contains all the metals in the residue less oxidisable 

 than lead, is treated with nitric acid, diluted with an equal volume of water, which 

 . removes, besides the lead, the copper and the palladium. The insoluble powder which 

 remains is mixed with five times its weight of binoxide of barium, weighed exactly, 

 and is heated to redness in a clay crucible tor one or two hours. After this it 

 is first treated with water, and then with aqua regia to remove the osmic acid. 

 When the liquor has lost all smell, sufficient sulphuric acid is added to exactly 



