On the Colouring "Principle of Lafis Lazuli. 321 



But the termination of this labour opened a field for new 

 refearches, of which it is time I fliould give an account alfo. 



I. Obferving the blue colour which the fulphat of lime of 

 Montolier had aflumed with pot-afli, the etfefts of the re- 

 agents on the acid holding its iron in folution ; and, above 

 all, feeing the green precipitate formed by the prufliats dif- 

 appear by the addition of acid, C. Deformes recollefted that 

 he had obferved fimilar phenomena during fome experiments 

 which he undertook laft year with C. Clouet on lapis lazuli. 

 This was already a ftrong indication that this ftone con- 

 tained fome other colouring metallic oxyd than that of iron. 

 The queftion then was, to follow the coincidence of fafts in 

 all its circumftances, to determine the particular fiate in 

 which this metal produces that beautiful blue compofition. 

 Some experiments added to the obfervations already known 

 refpefting the properties of lapis lazuli will give an idea of 

 this coincidence. 



K. Lapis lazuli may be brought to a red heat, and lofe as 

 much aso"2 of its weight without any fenfible alteration in its 

 colour; but at a ftronger heat, fuch as that of an enameller's 

 furnace, its colour pafles to grey. By ftill increafing the in- 

 tcnfity of the hea*, it is reduced to a brownifli vitreous fcoria, 

 and there is a decreafe of from 10 to 11 hundred parts of its 

 weight. 



L. When lapis lazuli is pulverifed, one fometimes per- 

 ceives a fmell fimilar to that of muflc, and which argil and 

 magnefia emit alfo when they are united with a little fulphur. 



IV[. Lapis lazuli is deprived of its colour by the three mi- 

 neral acids, but more or lefs fpeedily. This effeft, by the 

 nitric acid, is produced almofl; inftantaneoufly : the muriatic 

 acid is the next in order: the atSlion of fulphuric acid is the 

 (loweft. The fame odour is often difengaged from it by thefe 

 acids as by trituration. 



If the nitric acid is concentrated, there is a produftion of 

 nitrous gas; and, accidentally, of carbonic acid gas when the 

 lapis lazuli contains carbonat of lime. The liquor tried by 

 prulTials gives a precipitate, the colour of which approaches 

 that of Pruffiau blue, but inclining I'cnfibly to green, and 



Vol. \7. Tt . which 



