On the Colouring Principle oj Lapis Lazuli. 333 



abfolutely free from all mixture of pyrites, had in its com- 

 pofition argil and carbonat of lime fimilar to that analyfed 

 by Mr. Klaproth. 



The cryftals of alum, which were formed very fpeedily^ 

 as already mentioned, on a beautiful plate of lapis lazuli, 

 leave no doubt that there are fome kinds in which a little 

 pot-afli may be accidentally contained. It will be proper, 

 therefore, to fearch for the colouring principle in what con- 

 llitutes its real effence, without paying too much attention to 

 thefe accidental varieties* We muft not, however, believe 

 that the affinity of earths, either with each other, or with 

 the colouring principle itfelf, has any influence on the na- 

 ture of the compound. A proof of the contrary here pre- 

 fcnted itl'elf, in a fenlible manner, in one of the fynthetic 

 operations. Sulphat of lime charged with iron having been 

 treated with charcoal-duft, and then held in digeftion in the 

 nitric acid, the prufliat of pot-afli at flrft only coloured the 

 liquor greeu, without giving a precipitate: the addition of a 

 folution of argil immediately produced a green precipitate, 

 which was taken up by acids, and, under all circumftances, 

 exhibited the fame phenomena as that obtained from the 

 decompofition of lapis lazuli. 



The confequences which appear to me to refult from the 

 fadls announced in this paper, are as follows : 



1. The fulphat of lime of Montolier is coloured by a red 

 oxyd of iron, which adhefes fo ftrongly to filex as to refill 

 the a6lion of acids. 



2. This fulphat, treated with carbon, gives birth to a ful- 

 fure of iron, in which that metal is lefs oxydated ; which 

 being diffolved in acids, does not by prufliats give Pruffiaa 

 blue, but a green precipitate, which acids d( l:roy inftead of 

 heightening, and which retains the blue colour peculiar to 

 it even in pot-afh, and at that heat which is required for its 

 fufiyii in the dry way. 



3. By operating on the fulfure of iron prepared in a direft 

 manner, you obtain a product which manifefts the fame pro- 

 perties in the fame acids, and bv the fame re-acents. 



4. Thefe phenomena are exaftly fimilar to thofe exhibited 

 hy lapis lazuli ful^'ettc'l to the fame operations. 



T t i5 5. \\> 



