IN POPULAR CHEMISTRY. 97 



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

 phide, which is black. 



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

 making the smallest weights ? 



Because of its low specific gravity as compared with that 

 of platinum. 



30. How would you detect the presence of iron particles 

 in Mack sand ? 



By a magnet. 



31. Which metals can be welded ? 



Iron and platinum, most easily ; others also by using a pow- 

 erful electric current to heat the ends of the pieces of metal. 



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

 dark color, ^vhat does it show ? 



It might be anticipated that the appearance of the slag 

 would convey to the experienced eye some useful information 

 with respect to the character of the ore and the general prog- 

 ress of the smelting operation. A good slag is liquid, nearly 

 transparent, of a light-gray color, and has a fracture somewhat 

 resembling that of limestone. A dark slag shows that much 

 of the oxide of iron is escaping 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 

 titanium generally present a peculiar blistered appearance. 

 BLOXAM. 



33. In welding iron the surfaces to be joined are some- 

 times sprinkled with sand. Explain. 



The silica acts as a flux with the oxide upon the surface, 

 and lays bare the metal for welding. 



34. Wliat is the difference between an alloy and an 

 amalgam ? 



An amalgam is composed of mercury and some other 

 metal. An alloy consists of any metals whatever. 



