1814.] On the Cause of Chemical Proportions, 253 



certain metals, as copper or bismuth ; but that when alloyed with 

 gold or silver it was insolublf. Hence it appears, that to l)e <^ ..^ 

 to oxydize rhodium by nitromuriatic acid, it must he co ,:^ 

 with another substance, whose propensity to toim a u* .It 



with rhodium increases its attraction for oxygen. 



We know tliat chromium, notwithstanding its great coml'usti- 

 bility, is scarcely attacked by nitromuriatic acid ; but that it is 

 readily oxydated when exposed to heat, especially if in contact with 

 an alkaH. 1 resolved therefore to treat rhodium in a similar 

 manner. I mixed rhodium in fine powder with caustic potash and 

 a little saltpetre, and exposed it to heat in a platinum crucible. As 

 soon as the crucible became red hot, a strong eftervescence took 

 place. The metallic powder increased in volume, and became a 

 blackish mass. Water removed the excess of alkali, and left a 

 flea-brown powder, similar to the peroxide of lead. Neither the 

 alkaline lye, nor the water employed to wash the powder, contained 

 rhodium : so that the oxide formed was quite insoluble in water, 

 both cold and hot. Muriatic acid did not dissolve this oxide'; but 

 much oxymuriatic acid gas was disengaged when the two substances 

 were heated together. 



To examine this oxide with more accuracy, I dried it in a pla- 

 tinum crucible over tlie fiame of a spirit lamp. 148 parts of the 

 oxide thus dried, being treated with muriatic acid, disengaged 

 oxymuriatic gas. After some hours' digestion, I separated the 

 liquid from the undissolved portion. The liquid was evaporated to 

 dryness, and the residue exposed to a slightly red heat. I then 

 dissolved it in water. It left as a residue a little muriate of rho- 

 dium, formerly held in solution by the excess of acid. This I 

 added to tlie undissolved portion. The aqueous solution had a 

 slightly red colour, and yielded by evaporation a quantity of muriate 

 of potash weighing 37 parts. It dissolved completely in water, 

 still preserving its red shade of colour, which was owing to a trace 

 of triple muriate of rhodium. The insoluble portion of oxide thus 

 treated weighed 143'8 parts. 



127 parts of this mass being exposed to the heat of a spirit of 

 wine lamp, lost 1*4 parts of moisture. The remaining 125'6 parts 

 were put into a platinum crucible, exactly weighed, and exposed to 

 the strongest heat that I could raise by means of a pair of bellows. 

 The crucible being taken occasionally out of the fire, was found to 

 emit the smell of oxymuriatic gas, which did not stop till the cru- 

 cible had been exposed an hour to the fire. The brown powder 

 had diminished much in volume, and was now a grey metallic 

 mass exactly similar to platinum, obtained in a similar way from 

 the animonio-muriate. The metallic rhodium obtained from 125*6 

 of muriate weighed 97 parts. 



To verify this experiment, I decomposed in the same way 100 

 parts of muriate of rhodium, and obtained 77'3 of reduced metal. 

 These experiments mutually confirm each other, for 126*5 : 97 " 

 100 : TJ'2'6. We know from chemical proportions iiow much 



