112 METALLIC SULPHURETS. 



Natural produc- The pyrites undergo a flow decompofition, by which oxf-* 

 tion of red ochre. geil i s .f u bftituted in the place of the fulphur, without their 

 lofing any thing of their dimenfions ; they are then nothing 

 more than red oxide. When the waters tranfport, divide, 

 and attenuate the oxided pyrites, they become changed into 

 red ochre: of this kind is that which in Spain is termed Al- 

 magnc, from the name of the village where it is found. It is 

 employed for painting houfes, marking fheep, colouring the 

 Seville muff, pol idling glafs, &c. This earth, when diftilled, 

 yields fulphureous acid in abundance : it announces its origin 

 by the remnant of fulphur which it contains. 

 Yellow ochre. As to the beautiful yellow ochres, which acquire a red co- 

 lour by merely lofing their humidity, they proceed from the 

 fpontaneous decompofition of the fpathofe ores ; we therefore 

 find them to contain lime and manganefe. I have obferved 

 this conversion of the fpathoie ore into the ftate of yellow ochre 

 in a very diftinct manner in certain veins ; that is to fay, the 

 oxides of iron and manganefe, when both at the minimum, 

 and faturated with carbonic acid, raife themfelves to the max- 

 imum, by abandoning the acid, which can no longer remain 

 united with them in this flate. 



Umber. 

 Analyfis of um- If tills earth be a vegetable refidue, as cannot be doubted, 

 be vegetable. %*> °"g* n 1S we ^ confirmed by its analyfis, for it is a compound 

 of oxide of iron and manganefe, both at the maximum, of 

 argill, of fand, &c. As to the argill, it abounds in the allies 

 of certain kinds of wood ; I have found it in large quantity, in 

 that of the olive and careb trees ; in that of the holm (encina 

 in Spanifli) I have found the calcareous phofphate, which I 

 neglected to announce at the proper time. In addition to my 

 fecond memoir on Pruflian blue, I have to add, that ox's blood 

 certainly contains manganefe. 



Sulphuret of Mercury. 



Mercury in cin- Artificial cinnabar constantly yields 85 per cent, of mer- 

 mbar 1S not cury on the one hand ; on the other it relinquifhes 14 and 14^. 

 of fulphur. to the antimony which is ufed for effecting its de- 

 compofition.. ..The. mercury is not therefore oxiginated in cin- 

 nabar. 



In 



