368 . OXIDATION. 



The colour oblained from It furpafles all others iii duratioili 

 fince nothing le(s than combuftion can deftroy it. It is necef- 

 fary, however, to leave the fi!k a long time in this dye; and 

 the depth of the fhade will be in proportion to the number of 

 times the article is dipped ; it muft be well wrung, rinced, 

 and dried, between each immerfion. 

 The g^aaano^ The gradation of colours produced by mixture of the nilro- 

 offhades already muriatic, and muriatic folutions of tin, being much weakened 



noticed are in- rj .-,. /-,.- ,.^.. 



dications of a "V dropping loliition of gold m a great excefs of acid, con- 

 gradual oxida- fiderably diluted with water, into the mixture, feems to me 

 "^"* to indicate a gradual oxidation. The acetic folution of iron 



proves the fame truth ; for on being expofed to the atmof- 

 pheric air, or to the contad of oxigen gas, it gradually 

 Sulphate of iron ^^^"g^s ^om a fea green to a reddiQi yellow colour. I have 

 lofes its excefs of fliewn, in a memoir on the alkaline Tin<aure of Mars of Stahl, 

 *'ofure tothe' ^''*^ Sulphate of iron may be fuper-oxigenated, and alfo lofe 

 Jight. J's excefs of oxigen by the adion of light. On mixing con- 



centrated fulphuricacid with nitric folution of iron, I oblained, 

 after the nitric acid was evaporated, by leaving the refiduum 

 to imbibe the moitiure of tlie air for feveral months ; cryftals 

 of fuper-oxigenated fulphate of iron, which were at firft 

 diftinguilliable by their whilenefs from fulphate of alumine ; 

 but the adion of the light gradually tinged their furface with 

 a yellow colour; their original whitenefs, however, might, 

 by a gentle waftiing, bereftoredat plealure. Super-oxigenateui 

 fulphate of iron, of nearly an equal degree of whitenefs, 

 may in like manner, be obtained by precipitating nitrate of 

 iron, and diflblving the precipitate, edulcorated and freed 

 from water, gradually in fulphuric acid, which, if well con- 

 centrated, will produce cryftals of fuper-oxigenated fulphate of 

 iron without evaporation. This fait ponTefles an incomparable 

 degree of aftringency. 

 The faft that The progrefs of the tranhniflion of oxigen is more manifeft 



lintns printed on linen fimply printed with. acetate of iron and madder. 



With acetate of , . , n i " i • /- . . 



iron are liable to which mult be a long time expofed in the air to bleach, un- 

 become rotten, jefs the arliticial means of bleaching be adopted. The prinf- 

 gralTudLnf! ^^ P^""' ^^ ^^*^ ''"^^ frequently periflies, bearing the appear- 

 miffion of ance of having been cut with 3 fliarp inftrument, or burned 



axigen. ^j^j^ concentrated acid ; this, it ftiould feem, muft proceed 



from the adion of the oxigen contained in the coloured oxide 

 of iron, continually repleniflied from the atmofpheric air. 



It 



