LEAD. 257 



this process, a portion of sulphuret is converted into 

 sulphate, by heating in a reverbaratory furnace. Both 

 parts of the process are in practice united ; a moderate 

 heat with abundant air being first supplied, a portion 

 of sulphate is produced. This is afterwards more 

 highly heated, with the undecomposed sulphuret which 

 remains. 



640. ACTION OF AIR AND HEAT. If 



What occurs i-i-i i i c .1-11 .1 



when lead is ^ ea( i is heated before the blow-pipe, in the 

 heated before oxidizing flame, it melts and disappears. 



the blow-pipe ? 



The charcoal support becomes at the same 

 time covered with yellow oxide of 

 lead or litharge. The grey coating 

 which at first forms upon the lead, is 

 an oxide containing less oxgen. If, 

 on the other hand, litharge is heated in the reducing 

 flame, it is converted into metal. 



641. ACTION OF WATER. Water, with 



Wh,at is the r . . 



action of water the help of the air which it always con- 

 on lead. tains, acts sensibly upon lead and becomes 

 in consequence poisonous. This action of water is 

 most decided when it contains no foreign matter. On 

 being conducted through leaden pipes it becomes 

 therefore more impure as a consequence of its very 

 purity. 



Whatprevents 642. The presence of sulphates and cer- 

 this action? tanl otner salts, such as are usually con- 

 tained in spring water, prevents this effect. The very 

 substances, whose presence in water we are accustomed 

 to regret as impurities, thus become our most efficient 

 protectors against the poisonous effects of lead. 



