IN CHEMISTRY. 123 



duce a beautiful iridescent appearance, the color changing 

 with the thickness of the oxide. A soap-bubble exhibits 

 in the same way a play of variegated colors according to 

 the thickness of the film in different parts. 



3. How can the spectra of the metals be obtained? 



(See Astronomy, page 285.) 



By looking through a prism at a flame containing min- 

 ute portions of the volatilized metal. 



4. Ought cannon, car-axles, etc., to be used until they break 

 or wear out ? 



Cannon are condemned and recast after being fired a 

 certain 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 lo-inch bore or less, ought to 

 stand 1000 rounds ; larger calibres, a smaller number. 



5. Why 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. Does a blacksmith plunge his work into water merely 

 to cool it ? 



The metal is harder when cooled quickly and therefore 

 resists wear longer. 



7. What causes the white coating made when 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. 



