322 On the Colouring Principle of Lapis Lazuli. 



which is deftroyed by acids. The hydro-fulfure of ammonia 

 occafions in it a green precipitate incHning to black. When 

 the nitric acid is ufed diluted with water, there is a difengage- 

 mentof a little fulphurated hydrogen. Pruffiats then form in 

 the liquor only a bright green precipitate, which acids caufe 

 immediately to difappear. With the hydro-fulfure of am- 

 monia the precipitate is a beautiful green. If the lapis lazuli 

 has been previoufly fubje£led to calcination, diluted nitric 

 acid difengages from it a little fulphureous acid gas. 



N. Thefe fails prove, not only that the lapis lazuli holds 

 a little fulphur, but they demonftrate alfo the identity of the 

 colouring principle of this ftone, with every compofition in 

 Tivhich earthy fubftances are made to enter in combination 

 vith the fulfure of iron, fince we have feen (F, G,) the ful- 

 phat of lime, holding iron, carried to the ftate of fulfure by 

 carbon, and the fulfure of iron, prepared in a dirc£t manner, 

 prefent the fame phcnouK-na under the fame circumftances. 



O. Before I terminate this memoir I fhall add fome ob- 

 fervations, which will be of utility to thofe who may be dif- 

 pofed to repeat thefe experiments. 



The compofition of every kind of lapis lazuli is not eflen- 

 tially the fame : one may perceive very difiinilly in feveral 

 fpecimens, even of that called the oriental, fulfure of iron in 

 cryftals which have metallic brilliancy. Sometimes it is dif- 

 feminated in fmall particles ; and it is this, no doubt, which 

 has hitherto prevented the moft accurate chemifts from de- 

 tefting the real colouring principle of this fubftance. They 

 have feen the fulphur as an accidental production foreign to 

 the fubjeft of the analyfis, without fufpefting that there ex- 

 iilcd a blue fulfure of iron. It may readily be conceived 

 thjit, in experiments of this kind, the greatetl care muft be 

 taken to chopfe fragments abfolutcly free from every particle 

 of pyrites or fulfure of iron, of a yellow metallic appearance. 



The prefence of this Jaft fulfure is not the only difference 

 found betweeri fpecimens of lapis lazuli. Of three kinds fub- 

 jfiftcd to c.\u.mination, que contained, with cryftallifcd fulfure 

 pf iron and blue fulfure of iron, only fulphat of lime and (ilex. 

 'J'i>e f^pond, bcfides thefe, contained barytes: and the third, 



^bfplwtely 



