On the Nature of the colouring Principle of eke Lapis-lazuli. %tt 



which was poured upon it. In this (late the fame re-agents, which had been employed 

 in the former experiments prefented the fame phenomena. It might be imagined that 

 fulphate of unoxide<} iron would, in the fame circumftances, afFord fimilar elFecSts ; but 

 pruffiate of iron afforded only a white precipitate, as had been announced by Profeflbr 

 Prouft. 



H. . It was now eafy to draw a conclufion refpe£ting the analyfis of the red fulphate of 

 lime of Montolier, It confirmed the obfervation I had made that it contained nothing but 

 oxide of -iron, and determined befides the quantities of its conftituent parts, which arc 

 as follows : 



Sulphuric acid - - , - 2,9.1 



Lime - - - 16. , 



Oxide of iron - - - 21.9 



Siliceous earth - - » _ 8.6 



"Water driven off by the firft calcination - 22.3 



Lofs - - - • - a.i 



100.0 



The termination, however, of this fubjeft opened a path to new enquiries, of which 

 it is now time to give an account. 



I. From obferving the blue colour which the fulphate of lime of Montolier had 

 afiumed by the addition of pot-afli, the efFe£ts of the re-agents upon the acid, in which 

 its iron is held in folution, and more particularly the difappearance of the green precipitate 

 formed by prufliates by the addition of an acid, M. Deformes immediately recolledted that 

 he had obferved phenomena perfectly fimilar in fome experiments performed during lad 

 year with Citizen Clouet upon the lapis-lazuli. This fa£t already afforded a ftroiig 

 indication that the ftone in effedi: contains no other colouring metallic oxide than iron. 

 It remained therefore only to purfue the coincidence of the facts in all their circumftances, 

 to determine the particular flate in which this metal is found when it produces this 

 beautiful blue compofition. Some experiments added to the obfervations already known 

 upon the properties of the lapis-lazuli will form this connedlion. 



The lapis-lazuli .may be urged to a red heat, and even lofe 0.2 of its weight, without 

 any vifible alteration in its colour ; but with a ftronger heat, fuch as that of the furnace of 

 an enameller, its colour changes to grey. By ftill encreafing the intenfity of the fire, it is 

 reduced to a brownifli glalTy fcoria, with a diminution of from lo to i\ hundredths of 

 its weight. 



L. When the lapis is pulverized, a fmell of mufk is fometimes perceived, which 

 alumine and magnefia equally afford when they are united with a little fulphur. 



M. The lapis is difcoloured with more or lefs quicknefs by the three acids which are 

 called mineral. , The nitrous acid a£ls almofl: inftantaneoufly ; the muriatic acid is lefs 

 fpeedy ; and the fulphuric acid afls the mod flowly. Thefe acids often difengage the 

 fame fmell as is produced by trituration. 



If 



