ELASTICITY. 



153 



Bodies the form of which has thus been changed by 

 an external force tend in a greater or less degree to 

 recover their previous form if the force ceases to act. 

 A wire of lead, about l m long and l mm thick may be 

 stretched by the hand so as to increase its length by a 

 few centimetres, and if left to itself, it will not return 

 to its former length. A copper or brass wire, after 

 being made red-hot and cooled, may be bent into any 

 shape; but a brass wire which has been hardened by 

 hammering will, if bent, tend to return to its previous 

 form. Bodies which tend to recover their form and 

 dimensions, when these are forcibly changed, are called 

 elastic^ the property itself is called elasticity. India-rubber 

 and watch-spring are examples of highly elastic bodies. 

 Glass cannot be much altered in form without breaking, 

 it is therefore called brittle] but if not broken, it imme- 

 diately recovers its previous form, when left to itself; 

 brittleness and elasticity are therefore by no means 

 opposite properties, but a body may possess both 

 at the same time. The elasticity 

 of glass is well shown by a narrow 

 long strip of window -glass, but 

 much better by a spiral cut in the 

 |form of a spring, out of a glass 



cylinder. Such a spiral, tig. 107, 



nay be pulled at both ends with 



humb and forefinger, and the 



'ingle coils will separate as far 



is 2 mm from one another without 



)reaking; if the strain ceases, they 



lose again completely together, 

 The spiral is best cut from a wide cylinder. One end of a piece 



