144 PNEUMATICS. 



that it will continue bubbling for near half an hour 

 together. 



18. Let a large piece of cork be suspended by 

 a thread at one end of a balance, and counter- 

 poised by a leaden weight, suspended in the same 

 manner, at the other. Let this balance be hung to 

 the inside of the top of a large receiver ; which 

 being set on the pump, and the air exhausted, the 

 cork will preponderate, and show itself to be hea- 

 vier than the lead ; but upon letting in the air 

 again, the equilibrium will be restored. The rea- 

 son of this is, that, since the air is a fluid, and all 

 bodies lose as much of their absolute weight in it 

 as is equal to the weight of their bulk of the fluid, 

 the cork being the larger body, loses more of its 

 real weight than the 4 lead does ; and, therefore, 

 must in fact be heavier, to balance it under the 

 disadvantage of losing some of its weight, which 

 disadvantage being taken off by removing the air, 

 the bodies then gravitate according to their real 

 quantities of matter, and the cork which balanced 

 the lead in air shows itself to be heavier when in 

 vacuo. 



19. Set a lighted candle upon the pump, and 

 cover it with a tall receiver. If the receiver holds 

 a gallon, the candle will burn a minute; and, 

 then, after having gradually decayed from the first 

 instant, it will go out ; which shows that a con- 

 stant supply of fresh air is as necessary to feed 

 flame as animal life. 



20. The moment when the candle goes out, the 

 smoke will be seen to ascend to the top of the re- 

 ceiver, and there it will form a sort of cloud ; but 

 upon exhausting the air, the smoke will fall down 

 to the bottom of the receiver, and leave it as clear 

 at the top as it was before it was set upon the 



