21S On Palladium and Rhodium. [Skpt, 



certain quantity of platinum ; if that volume lie reduced to one, it 

 will be much more capable of producing the effect than it was 

 before. Thus an aqua regia composed of two parts of muriatic at 

 22°, and one part of nitric acid at 3i°, and which marks anout 25° 

 on the areometer, will dissolve about ±th of its wehjit of platinum, 

 while an aqua regia composed of the same proportion oi muriatic 

 acid at 22°, and of nitric acid at 44°, and marking 285° on the 

 areometer, will dissolve the fourth of its weight of platinum. Hence 

 one half of the quantity of this last aqua regia would dissolve the 

 same weight of platinum as the first ; but one part of this last acid 

 costs less" than two parts of the first. We have this advantage 

 besides, that the solution with the strong acid is much more rapid 

 than with the weak, and hardly requires any other heat than what 

 is naturally developed during the process. 



It will be easy to understand the reason of this when I observe that 

 nitromuriatic acid of 1»° or 20° does not attack platina without the 

 assistance of heat, and even acts upon it very slowly at a boiling 

 heat; but at such a temperature a great proportion of the acid is 

 evaporated before it has time to act upon the metal at all. Hence 

 the reason that the acid collected during the solution of platinum, 

 even when reduced to the consistence of a thick syrup, does not act 

 upon platinum again. Hence also the reason that by means of 

 weak aqua regia we may dissolve the gold mixed with platinum 

 without touching the platinum itself. 



The third observation relates to the state in which the solution 

 of platinum ought to be in order that the metal may be completely 

 precipitated by sal ammoniac. It ought to be sufficiently concen- 

 trated by evaporation to assume a crystalline state on cooling ; for if 

 it contains a great excess of acid, it retains much more triple salt 

 in solution after the addition of the sal ammoniac. If into such a 

 solution we add a portion of alkali to saturate a portion of the 

 superabundant acid, a new precipitate of triple salt takes place. It 

 is well known too that ammonio-muriate of platinum is much more 

 soluble in water acidulated by aqua regia than in pure water. I 

 discovered this by putting into the liquid from which ammonio-mu- 

 riate of platinum had been separated iron plates to precipitate the 

 metals which still remained in solution. The first effect produced 

 was the precipitation of a considerable quantity of triple salt of 

 platinum almost pure ; but what embarrassed me at first respecting 

 ths true cause of this precipitation was that sulphate of iron pro- 

 duced the same effect as metallic iron. 



This made me suspect that in the solution of platinum that metal 

 night be in two states of oxidation ; that the protoxide alone was 

 precipitated by sal ammoniac, while the other that remained in 

 solution was precipitated in its turn by yielding a portion of its 

 oxygen to the iron. To verify this conjecture, I passed a quantity 

 of oxymuriatic acid through a solution of triple salt of platinum to 

 peroxidize the whole metal, if that should be possible. 



The gas dissolved in the liquid very readily, heat was evolved, 



5 



