S8 CONDENSING SYRINGli. 



refied space is formed between the upper and lower valve m 

 one cylinder ; then the air which is contained in the receiver 

 rushes through the brass tube and by its elasticity forces 

 up the lower valve and enters the cylinder ; then the valve 

 closes and prevents the air from returning into the receiver. 

 When the motion is reversed, the other piston ascends, and 

 the first is depressed ; in its depression, the elasticity of the 

 air contained between the two valves, forces open the up 

 permost valve, and it escapes into the upper part of the cy- 

 linder ; then the valve closes and prevents its return. Whilst 

 one piston, therefore, exhausts the air from the receiver, the 

 other is discharging it from the top of the cylinder. Thus 

 by continued exhaustion, the density of the air keeps de- 

 creasing in the receiver, till its elasticity is no longer able to 

 force up the lower valves, which terminates the effect of the 

 machine. The air is admitted into the receiver again by 

 unscrewing a small nut which is so situated as to comniuni* 

 cate with the air channel. 



If the air be exhausted from a receiver, it will be held 

 fast by the pressure of the external air. If a small receiver 

 be placed under a larger, and both exhausted, the larger 

 will be held fast, while the smaller will be easily moved. If 

 a guinea and a feather be dropped from the top of the re- 

 ceiver, they will reach the bottom at the same instant, be- 

 cause there is then no resisting medium. Animals cannot 

 live in an exhausted receiver, and the continuance of life 

 varies according to the strength or size of the animal. A 

 man requires a gallon of fresh air every minute. If a lighted 

 candle be covered with a receiver containing a gallon of air, 

 the candle will burn a minute ; and then the flame, after 

 having gradually decayed, will go out. A constant supply 

 of fresh air, therefore, is as necessary to feed flame as to 

 support life. If two brass hemispheres of three or four 

 inches in diameter be put together, and the internal air ex- 

 hausted, the pressure from without will require one hundred 

 and fifty pounds to separate them ; but if the external air 

 fee taken away, they wilr separate of themselves. 



The Condensing Syringe has a solid piston, and a valve 

 in the lower part of its barrel which opens downwards. By 

 thrusting down the piston the air is forced through the valve, 

 which is afterwards held close by the elasticity of the con- 

 densed air. When the piston is raised up a vacuum is pro- 



