gta On the Nature of tkt colouring Principle of the Lapis-lazuH. 



If the nitrous acid be concentrated, it afFords nitrous gas, and fometimes when the lapis 

 contains carbonate of calx, carbonic acid gas. The liquid, when tried by the prufliates, 

 afFords a precipitate whofe colour refembles that of Pruflian blue, but vifibly inclining 

 towards a green which is deftroyed by acids. The hydro-fulphuret of ammoniac occafions 

 a precipitate inclining to black. 



When nitrous acid, diluted with water, is ufed, there is a difengagement of a fmall 

 quantity of fulphurated hydrogen. The prufliates then form only a bright green precipitate 

 in the liquid, which is inftantly deftroyed by acids : with hydro-fulphuret of ammoniac the 

 precipitate is of a fine green. 



When the lapis has been previoufly fubje£led to calcination, the dilute nitrous acid 

 difengages a little of the fulphureous acid gas. 



N. Thefe faGs not only prove that the lapis-lazuli contains a fmall portion of fulphur, but 

 they alfo demonftrate the idenity of the colouring principle of this ftone with every com- 

 polition in which earthy fubftances enter into combination with fulphuret of iron. For we 

 have feen (F. G.) the fulphate of lime containing iron, and converted by charcoal to the 

 ftate of fulphuret of iron ; and the fulphuret of iron, prepared in a direft manner, prefcnt 

 the fame phenomena under the fame circumftances. 



O. Before I conclude this memoir, I (hall make a few obfcrvations, to guide thofe 

 perfons who may be difpofed to repeat thefe experiments. 



The compofition of every fpecimen of lapis-lazuli is not eflentially the fame. In many 

 pieces, and even in what is called the oriental, we diftin£tly perceive the fulphuret of 

 iron in cryftals with the metallic brilliancy ; fometimes it is difleminated in fmall portions; 

 and this undoubtedly is the caufe which has hitherto prevented the moft; accurate chemifts 

 from afcertaining the true colouring principle of this fubftancc. They have confidcred 

 the fulphur merely as an accidental produft foreign to the fubjeft of analyfis, without 

 fufpefting that there exifted a blue fulphuret of iron. It is obvious that in experiments of 

 this kind the greateft attention (hould be paid to the choice of fragments abfolutely exempt 

 of all pyritous admixture, or fulphuret of iron of a metallic yellow. 



The prefence of this laft fulphuret is not the only difference which is found in the 

 fpecimens of the lapis. Out of three kinds which were fubjefted to experiment, one con- 

 tained fulphate of lime and filiceous earth, with cryftalized fulphuret of iron, and the blue 

 fulphuret of the fame metal : the fecond alfo contained barytes: the third, which was 

 abfolutely free from pyritous admixture, contained alfo alumine and carbonate of lime in 

 its compofition, like that which Was analyfed by M. Klaproth. 



The fa£t of cryftals of alum being rapidly formed upon a fine plate of the lapis, as 

 before remarked, proves beyond all doubt that fome fpecimens contain accidentally a fmall 

 quantity of pot-afh. It will be proper, therefore, to feck the colouring principle amongft 

 thofe component parts which are efTential to the ftone, and not to be diverted too much by 

 thefe accidental variations. We muft not, however, fuppofe that the affinity of the earths to 

 each other, or to the colouring principle, is without their influence upon the nature of the 



compound. 



