4.5« Piujlnn Blue ; Blaci Dye ,- Itii. 



The white prufliate is n fait which is afle£ted by atmofpheric air in no Other mamier than 

 the green fulphalcs, muruites and carbonates, or, in a word, moft of tlic faline combinationt 

 which contain iron oxided to the minimum. There is no other dificrence with refpcft to 

 tlicir fuper-oxidation, but the prcater or lefs time thefe metallic falts demand -, for he ob- 

 fcrveU that the muriates and pholpliatcs of which tlie oxide is at the minimum are not per- 

 ceptibly altered by expofurc to the air. 



Tlie pruQiatc of iron is not the only combination of this metal which owes its blue 

 colour to atmofpheric oxygcnc. Tliat which is called native Pruflian blue is merely 

 the phofphate of iron oxided to a certain point. Mr. Prouft intends to (hew artificial phof- 

 phates which are grey, blue, and white, according to the degree of oxidation. We are at 

 prefent well ac(|uainted, adds the author, with the caufe of thofe dull greenilh tinges which 

 frequently appear in the newly-made prulTiaies. It often happens that they are not bright- 

 ened well by acids, and do not acquire their peculiar lively colour but by expofure to the air. 

 This facSl is well known to manufaiilurers, who accordingly take care not to wade their 

 acid in attempting to render fuch blues perfe£l. 



It has been obferved, that the^pure green fulphate does not afford a black •with the gallic 

 acid. This is very true of it at the firft moment. But the contaft of air foon colours the 

 mixture at its furface. A few drops of the oxygenated muriatic acid immediately produce 

 the fame eftects throughout the fluid. AVe fee therefore that iron does not form ink with 

 the gallic acid, but in proportion as it is oxided. This black colour may likewife be de- 

 flroyed by including the black mixture in a bottle with a certain quantity of hepatic water. 



From thefe fafts it alfo very evidently appears why it is the eftabliflied praftice to expofc 

 ilufts to the air after the black dye ; and why ink newly made and pale becomes black 

 Tery fpecdily after it is fpread upon paper, &c. For in all thefe mixtures the ful- 

 phate of commerce is ufcd, which contains little of the red fulphate, and much of the green. 

 When the gallic acid is poured into folutions of the fulphate and muriate, containing the red 

 calx of iron, ink is inftantly produced. The bafis of ink and of every black dye is therefore 

 merely the gallate of iron, of which the iron is oxided to the maximum. Laftly, we cannot 

 but obferve in all thefe fafts that chemifts have hitherto erred with refpedl to the property of 

 the ordinary fulphate of iron to become black with the acid of galls, to afford a blue with 

 alkaline prufliate, &c. Thefe properties belong exclufively to the combinations of which the 

 oxide contains ,* ^ of oxygene, and not -fVo only. I fhall conclude, fays Mr. Proufl, by de- 

 ducing from thefe experiments the principle 1 have eftablilhed at the commencement of this 

 memoir; namely, that iron, like many other metals, is fubje£ted to the law of nature, which 

 prefidcs at every true combination ; that is to fay, that it unites with two conflant propor- 

 tions of oxygene. In this refpeft it does not differ from tin, mercury, lead, and, in a word, 

 almod every known combuftible. He intends fhortly to explain the nature of that kind 

 of oxide wliich refults from the union of oxygene with carbone, in a lefs proportion thja 

 is required to conftitute the carbonic acid. 



.VI. Jm 



