ELASTICITY. 



12. Air has another characteristic and highly important quality, 

 called ELASTICITY, which, like its compressibility, is unlimited 

 and uniform. 



Let us suppose that the plug p in the tube A B, fig. 1, Fig. 3. 

 instead of being pressed down, is drawn upwards, the 

 tube being long enough to allow it all the necessary play, 

 as in fig. 3. If, in that case, water had filled the tube 

 under the plug, a void space would remain between the 

 surface of the water and the plug. In short, the ele- 

 vation of the plug would be followed by no sensible 

 change in the space occupied by the water. But when 

 the tube contains air, the result is quite otherwise. In 

 that case, when the plug is drawn, upwards, the air 

 which before filled the tube between the plug and the 

 bottom now expands, and swells so as still to fill the 

 increased space left open to it by the elevation of the 

 plug, and this expansion will go on without any practical 

 limit, to whatever height the plug may be elevated. 



This capability of swelling without limit into aug- 

 mented dimensions when relieved from the conditions 

 which confine it is called ELASTICITY. Like COMPBJESSI- 

 BILITY, it is a characteristic property in which no other 

 form of matter participates. Liquids are for all practical 

 purposes inelastic. Some solid bodies possess a certain 

 elasticity, but not at all identical in its character or laws 

 with the elasticity of air above described. 



13. It has been explained that air in its common state 

 exercises a pressure of 15 Ibs. on each square inch of 

 surface with which it is in contact. It exercises this 

 pressure equally whether it is in communication with 

 the external atmosphere or not. In the case of the tube 

 A B and plug P, the air in the tube, before the introduc- 

 tion of the plug, pressed on the surface of the tube with 

 a force of 15 Ibs. per square inch, because it sustained 

 that pressure from the incumbent weight of the atmo- 

 sphere, and transmitted that pressure freely and undimin- 

 ished to the inner surface of the tube. But when the plug 

 is introduced, all communication with the external air B 



is cut off, and nevertheless the included air still presses on the 

 surface of the tube with the same force. As this pressure cannot 

 arise from the incumbent weight of the external air, all com- 

 munication with which is intercepted by the piston, it is due 

 altogether to the elasticity of the air confined within the tube. 



The piston inserted in the tube in this case is therefore subject 

 to the action of two equal forces. The WEIGHT of the external air 



7 



