RefieQlons on Prufflats. 7 



Stuff coloured by the oxyd of gold feems to altraft 

 the colouring particles of madder, and aflTumcs a reddifti- 

 brown tint, or a fort of carmelite, which is more and more 

 blackened by a continuation or increafe of the heat. The 

 other ingredients proper for dyeing alfo blacken gold by the 

 help of heat. Galls, by ebullition, have an analogous effeS, 

 and produce alnioll the fame greyifli-black colour as that 

 obtained by dipping in a muriatic folutiou of tin, probably 

 becaufe all thefe means de-oxygenate the oxyd of gold. The 

 change into blue, of ftuff coloured yellow by the oxyd of gol4 

 expofcd to the a6lion of diflblved pruffiat of pot-afh or of 

 lime, acidulated by the fulphuric or any other acid, takes 

 place only flowly. Three, four, and even five hours immer- 

 fion are necefTary to obtain a beautiful blue of equal intenfity 

 with the yellow produced by gold. By acidulating too itrongiy 

 the liquor of the prufliat, the oxyd of gold diflblves without 

 giving a blue. 



It is not uncommon to fee the experiment fail when the 

 folution of gold is direftly mixed with a folution of an alka- 

 line prufliat or a prufliat of lime. Every thing here depends 

 on the degree of acidulation ; but the experiment will gene- 

 rally fucceed when the oxyd of gold has been previoufiy pre- 

 cipitated by ammonia or fome other alkali. 



I for a long time imagined that thefe blues arofe from the 

 metal employed, and formed pruflliats. I was fo full of this 

 idea, that, when I made the difcovery in 1781, I communi- 

 cated it to the celebrated profefTor Spielmann, who returned 

 me the following anfwer : — " Wcllenberg, in a diflertstioa 

 maintained at Gottingen in 1772, and Marlin more amply 

 in a thefis printed here in 1775, remarked, that ley of blood 

 precipitated gold under a blue colour; but that, to obtaia 

 this colour, it was necefTary to make ufe of an acid. The 

 latter obferved chiefly, that alkali, faturatcd with prufllaa 

 blue, precipitated gold immediately of a blue colour; and 

 that, if ley of blood was deprived of all volatile alkali by 

 dilliHation, the refiduum, more concentrated, dyed ftill more 

 eafily the metals it precipitated from their menftrua." At 

 the time when I was employed in repeating my experiments, 

 ^y friend Charles Bartholdi objefted to nie, that all my 



biue:s 



