ELASTICITY OF AI1L 



53 



Such a bladder placed under several heavy weights will raise them by the ex- 

 pansion of the air. 



Fig. 8. 

 Co, 





Let a close glass vessel, A, B, fig. 8, be partially filled with water B, and 

 Jet the tube C D be inserted through its neck, the end D being below the sur- 

 face of the water ; the air above the surface will thus be confined. If such a 

 vessel be placed under a receiver, and the air be withdrawn, the elastic force 

 of the air confined in A, B, above the surface of the water, will press the water 

 up in the tube D, C, from which it will issue in a stream at C, when the pres- 

 sure of the atmosphere is removed by rarefaction. 



By means of an air-pump, we are enabled to demonstrate that the power 

 which causes water to follow the piston in a pump is the atmospheric pressure, 

 by showing that the water will not follow the piston when that atmospheric 

 pressure is removed. Let a small exhausting-syringe, with its lower end in a 

 vessel of water, be placed on the plate of the air-pump, and let a glass re- 

 ceiver, open at the top, be placed over it. On the top of this receiver let a brass 

 cap fitting it air-tight be placed, through a hole in the centre of which a metal 

 rod, terminating in a hook, passes air-tight. Let the hook be attached to the 

 end of the piston-rod, so that by drawing the rod up through the air-tight col- 

 lar, the piston may be drawn from the bottom of the cylinder toward the top. 

 If this be done before the air has been exhausted from the receiver, the water 

 will be found to rise after the piston as in the common pump ; but as soon as 

 the air in the receiver has been highly rarefied, it will be found that although 

 the piston may be drawn up in the syringe, the water will not follow it. This 

 effect may be rendered visible by constructing the barrel of the pump or syringe 

 of glass, through which the water will be seen to rise in the one case and not 

 in the other. If an air-tight piston be placed in close contact with the bottom 

 of a syringe not furnished with a valve, any attempt to draw it up will be re- 

 sisted by the atmospheric pressure ; and if it be forced to the top of the cylin- 

 der and there discharged, it will be immediately urged with considerable force, 

 to the bottom. The atmospheric pressure above the piston, acting with a force 

 of about fifteen pounds on the square inch, produces this effect, for the space 

 between the piston and the bottom of the cylinder not containing any air, this 

 pressure is unresisted. Now if this piston be introduced under the receiver 

 of an air-pump, and be drawn up as already described, it will be found that in pro- 

 portion as the air is withdrawn from the receiver, less and less force will be re- 

 quired to produce the effect ; and at length, the rarefaction will become so great, 

 I that the pressure of the remaining air is incapable of overcoming the friction 



