LEAD AND TIN 505 



into cakes of white lead. The vapor of acetic acid arising from 

 the vinegar may be regarded as a catalytic agent. 



In Mild's process melted lead is atomized by a jet of steam, and 

 the lead dust is beaten with vinegar, air, and carbon dioxide in a 

 vat for about seven days. In the French process white lead is 

 precipitated by a stream of carbon dioxide from a solution of the 

 Jasic acetate. 



Other Compounds of Lead. Lead chromate PbCr0 4 is 

 ^precipitated by adding potassium chromate solution to a solution 

 of a salt of lead. It is used as a pigment (chrome-yellow). Lead 

 ihloride PbCl 2 (white) is very little soluble in cold water and the 

 iodide PbI 2 (yellow) is insoluble. Both are formed by precipita- 

 tion. Lead sulphide PbS (black) is precipitated by hydrogen 

 sulphide, even from acid solutions (see p. 253). The sulphate 

 PbSO 4 is a very insoluble salt. On this account, the workmen 

 in white lead works add a little sulphuric acid to the water they 

 drink. 



Zinc, or any of the metals more active than lead, when placed 

 in a solution of a soluble salt of lead, will displace the metal, and 

 deposit it in a mossy form (" lead-tree ") : 



Zn + Pb++ -> Zn++ + Pb J, . 



The Storage Battery. In the ordinary lead accumulator the 



plates consist of leaden gratings. The openings in these gratings 

 are filled with finely divided lead in one plate and with lead dioxide 

 in the other. These, and the dilute sulphuric acid in the cell, are 

 the active substances when the cell is charged. When the battery 

 is used, the SO 4 = ions migrate towards the plate filled with the 

 lead (Fig. 115), and convert this lead into a mass of the insoluble 

 lead sulphate: SOr + Pb -> PbS0 4 + 20. This plate there- 

 fore becomes negatively charged. Simultaneously, the H + ions 

 move towards the other plate and there reduce to monoxide the 

 lead dioxide with which it is filled. 



Pb0 2 + 2H+ -> H 2 O + PbO + 2 . 



