Air 219 



end. (See Fig. 70.) Open the tap and connect the 

 glass tube by means of stout rubber tubing to an air- 

 pump or a good water-pump. Exhaust the air from the 

 hard glass tube as completely as the efficiency of the 

 pump permits. Close the tap. and disconnect the rubber 

 tubing. Fix the hard glass tube in a clamp attached to 

 a retort stand and strongly heat the tube and its contents 

 with a powerful Bunsen flame for about five minutes. 

 Remove the flame and note the appearance of the 

 copper wire as a result of this treatment. If the 

 evacuation of the tube has been thoroughly carried out 

 the wire should still be bright. 



Open the tap, thus causing the tube to be again 

 filled with air. Again heat the tube strongly and watch 

 the change in the appearance of the copper. After a 



mmmaum 



Fig. 70. 



short time it will entirely lose its brightness and the 

 exterior of the wire will be covered by a black film. 

 This is copper oxide, formed by the combination of the 

 copper with oxygen from the air. 



This experiment emphasises the important part 

 played by the air in many of the chemical changes 

 which take place under ordinary conditions. The 

 absence of the active constituent of the air oxygen is 

 sufficient entirely to prevent chemical change in the 

 copper as a result of heating it. 



EXPERIMENT 85. Hold a length of bright copper 

 wire, by means of the crucible tongs, in the flame of a 

 Bunsen burner till it is red-hot. Remove it from the 

 flame, and note the behaviour of the copper while 

 cooling and the appearance of its surface when cold. 



