IN CHEMISTRY. 127 



28. Why does the white paint around a sink turn black ? 



H 2 S is set free, which, acting on the paint, forms lead 

 sulphide the black sulphuret of lead. 



29. Why is aluminum, rather than platinum, used for 

 making the smallest weights ? 



Because of its bulk as compared with that of platinum. 



30. How would you detect the presence of iron particles in 

 black sand ? 



By a magnet. 



31. Which metals can be welded "t 



(See Philosophy, page 37.) 

 Iron and platinum. 



32. When the glassy slag from a blast-furnace has a dark 

 color, what does it show ? 



It might be anticipated that the appearance of the slag 

 would convey to the experienced eye some useful infor- 

 mation with respect to the character of the ore and the 

 general progress of the smelting operation. A good slag 

 is liquid, nearly transparent, of a light grey color, and has 

 a fracture somewhat resembling that of limestone. A 

 dark slag shows that much of the oxide of iron is escap- 

 ing unreduced. Streaks of blue are commonly found 

 when ores containing sulphur are being smelted, possibly 

 from the presence of a substance similar to ultramarine, 

 the constituents of which are all present in the slag. 

 Again, the slags obtained in smelting ores containing tita- 

 nium generally present a peculiar blistered appearance. 

 BLOXAM. 



