94 ANSWERS TO PRACTICAL QUESTIONS 



Cannon are condemned and recast after being fired a cer- 

 tain number of times, even though they show no flaw, as the 

 jarring to which they are exposed causes the iron to take on a 

 crystalline form, and become less fibrous and tough. A cast- 

 iron gun of 10-inch bore or less, ought to stand 1,000 rounds ; 

 larger calibers, a smaller number. 



5. IVliy is " chilled iron" used for safes ? 



The iron being cooled so instantaneously, the crystals are 

 exceedingly small, and the metal is correspondingly harder than 

 when cast in the ordinary way. 



6. Docs a blacksmith plunge his ivork into water 

 merely to cool it? 



The metal is harder when cooled quickly, and therefore re- 

 sists wear longer. 



7. What causes the white coating made ^vhen we spill 

 water on zinc ? 



The oxide of zinc which is formed on the surface of the 

 metal through the favoring influence of the water. 



8. Is it well to scald pickles, make sweetmeats, or fry 

 cakes in a brass kettle ? 



(See Chemistry, p. 161.) 



9. WJiat danger is there in the use of lead pipes ? Is a 

 lining of Zn or Sit a protection ? 



(See Chemistry, p. 162, and Fireside Science, p. 149.) 

 Zinc and tin are corroded by oxygen, though less readily 

 than Pb, and, while their salts are poisonous, the lead is soon 

 laid bare, and this also oxidizes. 



10. Is ivater which It as stood in a metal-lined ice- 

 pitcher healthful ? 



(See Chemistry, p. 159.) 



The dissimilar metals fastened with solder, which corrodes 



in the presence of water, develop a voltaic current which, 



hastens the oxidation. The salts thus formed are very dan 

 gerous. 



