Place in the receiver a small bladder, such as are sold in the streets for a few cents. Wet it a little to make it more supple. Now, in the ordinary way the air inside the bladder exerts the same pressure on the skin of the bladder as does the air on the outside. Now work the pedal so that the air in the receiver is gradually exhausted. The bladder will be seen to gradually swell and finally burst. It bursts because as the air in the receiver is exhausted by the pump, the air outside the bladder exerts a less force than the air inside. But the air inside is confined by the bladder skin, a not very strong material, as you know, so as soon as the difference between the inside and outside pressures is greater than the strength of the bladder, the latter bursts. This experiment also shows the expansible power of air.