255 0)1 the Cause of Chemical Proport'mis. [April, 



protoxide is slowly formed. Its csloiir is black. When rubbed T"ith a polished 

 beinatitc, it does not exhibit any appearance of inelallic brilliancy. When 

 slishtlv lieatrd ■with tallow or sugar, it is reduced with detonation; but, unless it 

 b? noiv removed from the fire, it is oxidized again. The protoxide of rhodium 

 thiw oi)ia;ned is insoluble in acids. It must be employed in its nascent state to 

 forma coinl)inatiGn -.viih them. 



2. Dculoxida of Rhoduim. — [ call this oxide oxidum rhodeum, Just as in another 

 Jncmoir 1 ha^e ralleu the inlcvniedi.ite oxide of tin oxidum sUinnaum. We obtain 

 it by calcining rhodium .u powder with caustic potabh and a little saltpetre. The. 

 pefash is removed by ualer; and if any partof ihe metal remains, it is separated 

 from the oxide by levijration. The oxide ih'js gained is ligli', flea-coloured, and 

 contains betwei-ii 15 ami 16 per cent, of potash. If in j)l.ice of caustic potash we 

 use thesubcarhoiiatc, the oxide combines witii a portion of the subcarbonate which 

 water is not able to sepaiale^ The same thing happens wlien the couipound of the 

 oxide and polash is long exposed while nfoist to the air. Diluted sulphuric and 

 nitric acids unite with the potui.h, but they leave the oxide witiiout forming any 

 combination with if. Muriatic acid forms muriate of rhodium and oxymuriatic 

 acid. This oxide has a strong affinity for alkalinesubstances. If tiie caustic potash 

 employed contain lime, the oxide combines with the lime ; and if the experiment 

 be made in an earthen vessel, it combines lilvinvise with alumina. 



3. Pcrojinc of Rlwdiwn (oxidum rhodicum). — We obla n it by precipitating the 

 sodamuriate of rliodium with caublic potash. Tlie reddish precipitate is the 

 hydrate of rhodium. When iieated it gives out its water, and becomes darker 

 coloured ; and at a iieat below redness takes fire, gives out part of its oxygen, 

 and is reduced to thi- st;ite of protoxide. We have liere the same eftect as takes 

 place wlien cuclihriuf disengages its excess of oxygen and becomes oxymuriatic 

 fas ; and tlie only proliabic explanation of the c\irious ph nomenon that muriatic 

 aid, instead of dividing the deutoxidc in(o piotoxide ;ind peroxide, gives out 

 owuiuriatic ga:-, and forms muriate of rhodium — the most probable explanation 

 of this phenomenon is, that in the dcutoxide the oxygen exists in a state more 

 neutializcd, or less electronegative, than in the protoxide ; and on this account, 

 wlien part of the oxygen is disengaged from the deuloxide, the other part com- 

 bines nuire intimately with the metal, and produces the ptienomenon of fire. On 

 the other side, to form peroxide from the deutoxide, tlie oxygen of this last ought 

 to be brougiit to a higher electronegative stale, which cannot be done. If we 

 precipitate sodamuriate of rhodium by an excess of caustic ammonia, the precipi- 



. tate contains ammonia ; but it docs not explode when heated, and is only decom- 

 posed with decrepitation, leaving metallic rhodium. I acknowledge that these 

 curious properties of t!ii« o\ide made me at first believe tliat the metallic rhodium 

 which Dr. Wollaston had the goodness to give me was not the ^ame substance 

 w iiieh is found in the sodamuriate ; but having reduced a portion of ihodiuoi 

 fro.i) the sodauiuriate, I was s(ion convinced that my su?picions were unfounded. 



Muriate of Rhodium is obtained by treating the deutoxide of rliodium with mu- 

 riatic acid. It furms an amber-coloured powder, similar to muriate of platinum, 

 which I have described in a preceding memoir, it is insoluble in water, but 

 somewhat soluble in cimcentrated muriatic acid, comiviunicating to it a reddish 

 colour. The alkalies precipitate it with a colour at first grey, but which after- 

 wards becomes brownish, as the precipitate becomes denser. JMo acid, not even 

 the nitromuriatie, decomposes this salt. I treated it with nitromuriatic acid con- 

 taining muriate of soda in solution, in order to convert it into sodamuriate; but it- 

 was not alteied. Caustic potash does not decompose this muriate, though they be 

 riigestcd tiigeiher. Il is capable of enduring a red heat without decomposition, 

 and can only be reduced to the metallic state by an intense heat long continued. 



Sulphate II f Rhodium is obtained wlien the persulphate, to be mentioned below, 

 is exposed to a cherry red heat. It swells up, ar.d disengages sulphuric acid and 

 oxygen gas, leaving for residue a black powder in^ol.iblc in water and in acids. 

 Caiisiic potu'h separates from it a portion of sulphuiic acid. This salt is obtained 

 wiiei; sulphuret of rhodium is exposed to a moderate heat. 



Pasxi'yhale of Rhudium. — This salt is obtained in a way analogous to that by 

 which Mr. iidmuiid Davy prepared sulphate of platinum. A solution of sodamu- 

 riate of iltodium is mixed with hydrosulp'iurct o<' amnipiiia. ^fo precipitate 

 apnrars at first ; but v. hen heat is applied suljihiiret of rhidiiim precipitate.-. It 

 ha-* the same property with the sulpiiuret of platinum to acidifj in the air while 

 dr>inj ; but the property is not so striking in this sulph'iret as in that of platinum. 

 Xlic d: i!:d tulphuret, being treated wiib fuming nitric acid, is converted into per-^ 



