.1822.] Separation of Iran from other Metals. 97 



perfectly neutral). In the cases of cobalt and cerium, this 

 rc-dissolution of the recent precipitate formed by carbonate of 

 ammonia is very considerable, and a solution of either of these 

 metals, thus impregnated with the metallic carbonate, becomes 

 a test of the presence of peroxide of iron, of a delicacy surpass- 

 ing most of the reagents used in chemistry, the minutest trace 

 of it being instantly thrown down by them from a boiling solu- 

 tion, provided no marked excess of acid be present. To be 

 certain, however, that we have not gone too far, it is advisable, 

 after separating the ferruginous precipitate, to test the clear 

 liquid, while hot, with a drop of the alkaline carbonate. If the 

 cloud which this produces be clearly re-dissolved on agitation,, 

 we may be sure that only iron has been separated. If otherwise, 

 a little acid must be added, the liquor poured again through the 

 filter, so as to wash the precipitate, and the neutralization per- 

 formed anew. 



The precipitation of iron above described seems at first sight 

 to result from a double decomposition. Were it so, the princi- 

 ple of the method would be merely a difference of solubility in 

 the carbonates of iron and the other metals, and as such would 

 have no claim to be regarded as rigorous. Such, however, is 

 not the case. The iron is not separated in the state of a carbon- 

 ate, but of a subsalt, or a simple peroxide, the whole of the 

 carbonic acid escaping with effervescence at each addition of 

 the alkali. The phsenomenon turns on a pecuharity in the per- 

 oxide of this metal, in virtue of which it is incapable of existing 

 in a neutral solution at the boiling temperature. If we add an 

 alkaline, earthy, or metaUic carbonate by little and little to a 

 cold solution of peroxide of iron, the precipitate formed is redis- 

 . solved with effervescence, readily at first, but gradually more 

 and more slowly, till at length many hours, or even days, elapse 

 before the liquid becomes quite clear. Meanwhile it deepens in 

 colour till (unless much diluted) it becomes dark brown or red. 

 if the addition of the carbonate be carried as far as possible with- 

 out producing a permanent precipitate, the solution is perfectly 

 ■neutral, and continues clear at a low temperature for any length 

 . of time. In this state it may be evaporated to dryness in vacuo, 

 .and the residue (which does not effervesce with acids) is still 

 soluble in water without letting any iron fall, and so on as often 

 as we please. 



The compound thus formed is, however, far from permanent. 

 It is in fact in a state of tottering equilibrium, which a very 

 slight cause is sufficient to overset. Supposing the point of 

 saturation to have been exatitly attained, the addition of an 

 extremely small quantity ?wo;(? of the alkaline solution is sufficient 

 to determine the separation of the whole, or nearly the whole, 

 ine,taUic contents; and if the solution operated on be pretty 

 concentrated, it fixes after a longer or shorter time into a stiff 

 New Series, vol. hi. h 



