On (he Nature of the colouring principle of the Lapis.lazuU. 309 



his refearches into mineral fubftances, where it is obfervable that the pureft lapis, 

 which is called oriental, is a compofnion of 46 in the 100 of filiceous earth, 28 of 

 carbonate of lime, 14.5 of aluminc, 6.5 of fulphate of lime, 3 of oxide of iron, and 2 of 

 water. 



He therefore correfted the analyfis of Margraff by adding alumine to if, which the 

 other had not noticed, and which more than fifteen years ago I had demonftrated to be 

 contained in it, by touching a plate of the lapis with fulphuric acid, which, at the end of 

 feveral hours, left very regular cryftals of alum, which are preferred upon it. 



But by what principle can oxide of iron be tinged with blue without being combined 

 either with Pruffic acid, or the acid of phofphorus ? M. Kiaproth replies that he cannot 

 tell*. 



The experiments, of which I fhall now give an account, appear to me to folve this 

 important queftion ; but before I explain the proceffes and their refults, I muft, for 

 greater perfpicuity, take notice of fome operations which led the way to this concluGon. 



In the year 1780, whilft I was examining a well at Montolier, on the road from Dole 

 to Poligny, in fearch of coal, I found, at the depth of 35 metres, a bank of gypfum, con- 

 taining zones of a fine and very lively red. I gave a defcription and an analyfis of it 

 in the Journal de Phyfique for the month of December in that year, and I inferred from 

 my trials that it was a fulphate of lime coloured by the oxide of iron. 



The late difcovery of feveral new metallic fubftances, fome of which have the property 

 of affording colours of great intenfity, led me to think that the foffil of Montolier deferved 

 a re-examination with the view of fearching for one of thefe oxides. For this purpofe, 

 at the beginning of the prefent year, I appropriated a piece to thofe experiments of the 

 mineral analyfis which make a part of my courfe in the polytechnical fcliool. They have 

 been purfued with no lefs accuracy than intelligence by M. Deformes, formerly a pupil 

 and operator at the laboratory of the fecond clafs. 



After having afcertained that this mineral does not contain any carbonate of lime, ten 

 grammes of it, reduced to a powder, were ignited in a crucible, by which treatment the 

 colour became deeper, and changed to a yellow brown. There was a lofs in weight of 

 22.3 per cent. 



A. Muriatic acid was repeatedly digefted on the 77.7 parts remaining after calcination ; 

 they did not become difcoloured, and the acid only deprived it of a very minute portion 

 of iron. , 



B. The refidue was then boiled in a folution of carbonate of pot-afti, and a very fmall 

 part only has yet been decompofed. 



C. What remained was mixed with charcoal, and treated in the crucible. A fuiphuret 

 being formed, it was decompofed by the muriatic acid, which feized the lime and iron. 

 The filiceous earth remained mixed with the fuperabundant coal. 



• Beytrage, &c. I. 201, 



D. 



