On the Colouring Principle of Lapis Lazuli. 317 



The means of analyfis having, for fome years part, been 

 carried to a degree of perfeftion which could fcarcely have 

 been expefted, it was natural to think that the chemifts moft 

 expert in this new art would not negle£l to apply it to the 

 folution of this important queftion. Among this number 1 

 fhall mention Mr. Klaproth, whofe labours have fo much 

 enriched the chemiftry of minerals, and who has paid par- 

 ticular attention to all the blue coloured f'lffils. In 1784 he 

 publidied experiments which demonftrate that what is called 

 native Pruffian blue, a fubftance found in turfy foil, and 

 which is often white before being expofed to the air, derives 

 its colour only from a combination of iron arid the phofphoric 

 acid *. 



Another mineral, remarkable for the fame colour, which 

 has been fucceflively taken for fmalt, or the native blue oxyd 

 of cobalt; for another kind of native Pruffian blue ; and for 

 a blue oxvd of copper, was difcovered at Vorau in Auftria. 

 But it refults, from the examination of it by the celebrated 

 chemift of Berlin, that the colour depends only on filex, 

 argil, and iron ; and though he difcovered that it refifted the 

 aftion of the fire lefs than lapis lazuli, he is of opinion that 

 it might be claffed among the number of its varieties, if it 

 flio'uld be found to contain alfo limef. This laft conclufion 

 fliows that Mr. Klaproth had previoufly determined, with 

 his ufual accuracy, the conftituent parts of the real lapis la- 

 zuli; and he indeed indicates them in the tenth article of his 

 Kefearches on Mineral Subftances; where we fee that the 

 pureft lapis lazuli, called oriental, confifts of 46 per cent, of 

 filex, 28 of carbonat of lime, I4"5 of argil, 6-c^ of fulphat of 

 lime, 3 of oxyd of iron, and 2 of water. 



He reftified, therefore, the analyfis of Margraf, by adding 

 argil; of which the latter made no mention, and which I 

 ihowed, fifteen years ago, to be contained in it, by touching 

 a polidicd piece of lapis lazuli with the fulphuric acid, which 

 at the eod of fome hours left very regular cryftals of alum, 

 that remained on it. But by what principle can the oxyd of 



'^ Chemlfche Annal. 17R4, p- 396. 



f See Bcylrage zur ittttnifs der miimal iorper, &c. Vol. I. p. 197, and 

 Aiinalii de Cbim'ic, Vol. XXI. p. 144. 



iron 



