OXIDATIONS OF IRON. ^QCf 



they being freed from this with more or less facility, accord- 

 ing- to their nature, and still more according- to the circum- 

 stances under which the precipitation has taken place, This 

 appears to be the case with the white precipitate of iron. 



The conditions requisite to throw down a white precipitate Conditions re- 

 from a green salt of iron, are: first, that the solution be quisite to pre- 

 highly concentrated ; second, that the precipitating alkai: w 'hite 

 be in some degree the same. This did not escape. the saga- 

 city of Mr. Thenard, who recommends boiling dilute sul- 

 phuric acid on an excess of iron filings, that the formation 

 of the white oxide may be more sure to succeed. In fact, 

 whenever a tolerably concentrated solution of alkali is added 

 to such a solution of iron, the alkali first seizes a portion of 

 the acid, and probably a little of the water, and precipitates 

 a white sulphate of iron, which frequently crystallizes, 

 though irregularly, at the moment of precipitation, and still 

 retains a sufficient quantity of acid, to be soluble in water. 

 It likewise constantly turns sirup of violets green, and forms Action on si- 



a red precipitate in water coloured with litmus, as is com- mp . ?. vlolcts 



, » . . - ancl htmus. 



moil to salts or iron with excess of oxide*. 



It is easy to verify this fact in a convincing manner, by Proofs of the 



letting fall a few drops of such a solution into an excess of fiict * 



alkali. If, after having left the white precipitate in it eight 



pr ten minutes, the fluid be decanted, or if it be drawn qif 



* I have obtained several salts of iron, which immediately produce a 



red precipitate with infusion of litmus, and at the same time turn sirup 



of violets green The property of precipitating infusion of litmus al- 



... , , , , „ . . , Infusion of lit- 



■ways indicates a salt that is at least neutral, and must not be confounded wives 



with the effect of simply changing its colour. In the mere change of precipitate in 



colour the small portion of alkali in the infusion combines with an ex- certain cases. 



cess of acid, and quits the colouring matter, which it modified. There 



can be no doubt, that a salt exhibiiing this phenomenon has an excess 



of acid, since there is sufficient to saturate the alkali in the infusion, 



without occasioning a precipitate ; and on this principle is founded the 



use of this reagent. When the change of colour is accompanied with a 



precipitate, the alkali, not finding an excess of acid to combine with, 



seizes part of that which held the oxide in solution, and the precipitated 



oxide carries down parr of the coloiuing matter with it. What appears a norrn i v f 



extraordinary is, that oxide of iron does not turn sirup of violets green, oxide of iron. 



though neutral salts of iron, or salts with an excess of oxide, have this 



effect. 



by 



