that it gradually
blackens when exposed to sulphur compounds, which are often
present in the air, forming black lead sulphide (PbS).

~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