Fig. 89
Lead sulphide (PbS). In nature this compound occurs in highly crystalline condition, the crystals having much the same luster as pure lead. It is readily prepared in the laboratory as a black precipitate, by the action of hydrosulphuric acid upon soluble lead salts:
Pb(NO3)2 + H2S = PbS + 2HNO3.
It is insoluble both in water and in dilute acids.
Other insoluble salts. Lead chromate (PbCrO4) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus:
K2CrO4 + Pb(NO3)2 = PbCrO4 + 2 KNO3.
It is used as a yellow pigment. Lead sulphate (PbSO4) is a white substance sometimes found in nature and easily prepared by precipitation. Lead chloride (PbCl2) is likewise a white substance nearly insoluble in cold water, but readily soluble in boiling water.
Thorium and cerium. These elements are found in a few rare minerals, especially in the monazite sand of the Carolinas and Brazil. The oxides of these elements are used in the preparation of the Welsbach mantles for gas lights, because of the intense light given out when a mixture of the oxides is heated. These mantles contain the oxides of cerium and thorium in the ratio of about 1% of the former to 99% of the latter. Compounds of thorium, like those of radium, are found to possess radio-activity, but in a less degree.