SULPHURET OF lEAD. 589 



rent of air upon its surface. Oxygen is absorbed, and an oxide 

 formed of a fine red colour, with a shade of yellow. It is not 

 constant in composition. The proportion of oxygen, when the 

 absorption is least considerable, approaches that of a compound 

 3PbO-fPbO 2 , and such was the composition of a crystallized 

 compound of a fine red colour, formed by accident in a minium 

 furnace, and analyzed by Houton-Labillardiere. But when the 

 absorption is favoured by time and most considerable, it ap- 

 proaches, but never exceeds 2.4 per cent of the original weight 

 of the protoxide, which gives a compound of 2PbO + PbO 2 , 

 according to the observations of M. Dumas. The proportion of 

 3 eq. of metal to 4 eq. of oxygen, which minium thus possesses, 

 is a very common one, but generally due to the combination of 

 a protoxide with a peroxide, the latter containing 2 atoms of 

 metal and 3 atoms of oxygen, as the peroxide of iron. Ber- 

 zelius considers the composition of minium to favour the exis- 

 tence of such an oxide of lead Pb 2 O 3 ; minium would then be 

 represented as Pb O + Pb 2 O 3 . The finest minium is obtained 

 by calcining oxide of lead from the carbonate, at about 600. 



Minium is not altered by being heated in a solution of ace- 

 tate of lead, which is capable of dissolving free protoxide of 

 lead. When heated to redness it loses oxygen, and the pro- 

 toxide remains. It does not combine with acids, but is resolved 

 by a strong acid into peroxide of lead, and protoxide, the last 

 combining with the acid. When minium is treated with con- 

 centrated acetic acid, it first becomes white, then dissolves en- 

 tirely in a new quantity of acid without colouring it. But the 

 solution gradually decomposes, and peroxide of lead separates 

 from it of a blackish brown colour. (Berzelius) . 



Protosulphuret of lead, Pb S, is thrown down from salts of 

 lead as a black precipitate, by sulphuretted hydrogen, which is 

 insoluble in diluted acids or in alkalies. It forms also the ore 

 galena, which crystallizes in the cube and other forms of the 

 regular system ; its density is 7585. Sulphuret of lead is de- 

 composed easily by nitric acid, and converted into nitrate and 

 sulphate of lead, with the separation of a little sulphur. Con- 

 centrated and boiling hydrochloric acid dissolves it, with disen- 

 gagement of sulphuretted hydrogen gas. Galena can be united 

 by fusion with more lead, and gives the subsulphurets Pb 4 S, 

 and Pb 2 S. When a solution of persulphuret of potassium is 

 added to a salt of lead, a blood red precipitate appears, which is 



