Hartley — A Study in Thermo- Chemistry . 41 



JReduction of Mercury from Cinnabar by Air : — 



HgS + O2 = Hg + SO2 + (69-2 - 19-8) heat-units. 

 Heat evolved = 49*4 units. 



The reaction takes place easily because it is strongly exothermic. 



The S?nelting of Galena {Lead Sulphide). — A series of four 

 •equations is necessary to explain the process : — 



(1.) First Process. — The Roasting of Galena to Lead Oxide : — 



PbS + 3 = PbO + SO2 + (51 + 69-2 - 17-8) heat-units. 

 Heat evolved = 102*4 units. 



(2.) Second Process. — The Reduction of Galena by Lead Oxide: — 



2 PbO + PbS = 3 Pb + SO3 + (69-2 - 51 x 2 - 17-8). 

 Heat absorbed = - 50*6 units. 



This reaction being endothermic, a large amount of extraneous 

 Tieat must be applied, equal to 50*6 units. Hence the necessity 

 for increasing the temperature of the furnace, vyhich proceeding is 

 invariable at this stage of the process. 



But a third and a fourth change are also effective in lead 

 smelting : — 



(3.) The Roasting of Galena to Lead Sulphate : — ' 



PbS + 2 O2 = PbSOi + (214 - 17-8) heat-units. 

 Heat evolved = 196'2 units. 



(4.) The Reduction of Galena by means of Lead Sulphate : — 



PbS + PbSOi = 2 Pb + 2 SO2 + (138-4 - 214 - 17-8) heat-units. 

 Heat absorbed = - 127 units. 



This change, being very strongly endothermic, requires a great 

 increase of temperature. 



