320 METALS AND THEIR COMBINATIONS. 



gradually oxidized with the formation of a yellow powder, known 

 as massicot; at a high temperature this fuses, forming reddish-yellow 

 crystalline scales, known as litharge ; by heating still further in con- 

 tact with air, a portion of the oxide is converted into dioxide (or 

 peroxide), PbO 2 , and a red powder is formed, known as red lead (or 

 minium), which probably is a mixture (or combination) of oxide and 

 dioxide of lead, PbO 2 (PbO) 2 . 



Lead oxide is used in the manufacture of lead salts, lead plaster, 

 glass, paints, etc. 



Nitric acid when heated with red lead combines with the oxide, 

 while lead dioxide, PbO 2 , is left as a dark-brown powder, which, on 

 heating with hydrochloric acid, evolves chlorine (similar to man- 

 ganese 'dioxide). Lead dioxide is a conductor of electricity, differ- 

 ing thus from most oxides. 



Accumulator or storage battery. This consists of two sets of lead 

 plates made in the form of gratings. One set, which are all connected, have 

 the spaces in the gratings filled up with spongy lead ; the other set, likewise 

 connected, are filled up with lead dioxide. When the plates are dipped into 

 dilute sulphuric acid they show a difference of potential, and when connected 

 a current flows. When in action or discharging, the SO/' ions of the sul- 

 phuric acid are attracted to the plates filled with spongy lead, give up their 

 negative charges to the plate, and form lead sulphate. The H* ions of the acid 

 pass to the plates filled with PbO 2 , give up their positive charges, and reduce 

 PbO 2 to PbO, which combines with sulphuric acid and forms lead sulphate. 

 The current flows in the outside circuit from the plates filled with PbO 2 to the 

 plates filled with spongy lead, and has a voltage of about 2. Both plates ulti- 

 mately become filled with PbSO 4 , and the battery then is exhausted. Sul- 

 phuric acid is removed from the solution, and the specific gravity of the latter 

 falls. By this means one can tell when the battery is approaching exhaustion. 



Charging the battery consists in restoring the plates to their original state, 

 and is accomplished bypassing a current from a dynamo through it in a direc- 

 tion opposite to that of the current produced by the battery. By this action 

 electrical energy is stored up in the battery as chemical energy, which is given 

 back again as electrical energy when the battery is discharged. When the 

 dynamo current passes, H* ions of the acid solution pass to the plates originally 

 filled with lead, and form sulphuric acid with SO/ X ions of the lead sulphate, 

 leaving the plate finally filled with reduced spongy lead. At the same time, 

 SO 4 X/ ions of the acid solution pass to the other plates, where they are dis- 

 charged and enter into reaction with the lead sulphate in the plates, thus : 



PbS0 4 + SO, -f 2H 2 = PbO 2 + 2H 2 S0 4 . 



All the lead sulphate is ultimately converted into lead dioxide, and this set of 

 plates are restored to the original state. All the sulphuric acid is restored to 

 the liquid, and the battery is ready for use. The changes involved in the com- 

 plete cycle may be written in one equation, thus: 



discharging. 



2PbS0 4 -(- 2H 2 ^ Pb + 2H 2 S0 4 -f PbO 2 . 

 charging * 



