308 On the Nature of the colouring Principle of the Lapis-lazuli, 



IV. 



A Memoir on the Nature of the colouring Principle of the Lapis-lazuli. By 



Citizen Guytos*. 



HET)lue (lone known by the name of lapis-lazuli has long been an obje£t of chemical 

 refearch, with a view more particularly of difcovering that colouring matter which is the 

 caufe of its high price, from the luftre which it gives to works in which it is employed, 

 and on account of the preparation of ultramarine blue, which is fo highly efteemed as a 

 pigment. 



This colour was at firfl; attributed to copper. The celebrated Margraf demonftrated 

 the error of this opinion, but he found that it confided merely of filiceous earth, ful- 

 phate of lime, lime, and a fmall quantity of iron. Others, fince his time, have fuppofed 

 that oxide of cobalt formed a part of it, whilft others, like Rinman, imagined the fluoric 

 atid to be one of its ingredients. A more ftrid examination foon dellroyed thefe con- 

 jeiStures. 



The methods of analyfis having of late years been carried to a degree of perfection 

 beyond the moft fanguine expectations, it was natural to think that thofe chemifts who 

 were the moft flcilful in this new art, would not neglefl the application of them to the 

 folution of this important queftion. Amongft others, I fhall refer to Mr. Klaproth, whofe 

 works have fo much enriched the chemiftry of mineral bodies, and who has paid a par- 

 ticular attention to every kind of foflil of a blue colour. 



In 1784, he publilhed fome experiments, which demonftrated that what is called native 

 Prufftan Hue, which is found in peat grounds, and is often of a white colour before it 

 is expofed to the air, is indebted for its colour to nothing elfe than a combination of iron 

 and phofphoric acid f . 



At Vorau, in Auftria, another mineral remarkable for the fame colour was difcovered, 

 and was fucceffively taken for fmalt, or native blue oxide of cobalt, for another kind of 

 native Pruffian blue, and for a blue oxide of copper. It is afcertained by the examination 

 of the celebrated chemift of Berlin, that it contains only filiceous earth, alumine, and iron, 

 and though he found that it refifted the adtion of the fire in a lefs degree than the lapis- 

 lazuli, he thinks it might be clafled among its varieties if it likewife contained lime \. 



This laft conclufion proves that M. Klaproth had, with his ufual accuracy, determined 

 the conftituent parts of the true lapis, He, in fa£t, points them out in the tenth article of 



• Read before the philofophical and mathematical clafs of the National Inftitute of France 6 Pluviofe, 

 An. 8, and inferted in the Annales de Chimie, XXXIV, 54. 



+ Chemifche Annal. 1784, page 396. 



J Beytrage zur Kentnifs der Mineral Korper, etc. Tom. I. p. 1&7. -—Annales de Chimie, XXI« 

 p. 144. (or this Journal, I. 77.) 



his 



