~Technical preparation of white lead.~ Different methods are used in the preparation of white lead, but the old one known as the Dutch process is still the principal one employed. In this process, earthenware pots about ten inches high and of the shape shown in Fig. 89 are used. In the bottom A is placed a 3% solution of acetic acid (vinegar answers the purpose very well). The space above this is filled with thin, perforated, circular pieces of lead, supported by the flange B of the pot. These pots are placed close together on a bed of tan bark on the floor of a room known as the corroding room. They are covered over with boards, upon which tan bark is placed, and another row of pots is placed on this. In this way the room is filled. The white lead is formed by the fumes of the acetic acid, together with the carbon dioxide set free in the fermentation of the tan bark acting on the lead. About three months are required to complete the process. [Illustration 1: 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(NO_{3})_{2} + H_{2}S = PbS + 2HNO_{3}. It is insoluble both in water and in dilute acids. ~Other insoluble salts.~ Lead chromate (PbCrO_{4}) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus: K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2 KNO_{3}. It is used as a yellow pigment. Lead sulphate (PbSO_{4}) is a white substance sometimes found in nature and easily prepared by precipitation. Lead chloride (PbCl_{2}) 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. EXERCISES 1. How could you detect lead if present in tin foil? 2. Stannous chloride reduces gold chloride (AuCl_{3}) to gold. Give equation. 3. What are the products of hydrolysis when stannic chloride is used as a mordant? 4. How could you detect arsenic, antimony, or copper in lead? 5. Why is lead so extensively used for making water pipes? 6. What sulphates other than lead are insoluble? 7. Could lead nitrate be used in place of barium chloride in testing for sulphates?