PNEUMATICS. 145 



pump. This shows that smoke does not ascend on 

 account of its being positively light, or having no 

 weight, but because it is lighter than air ; and its 

 falling to the bottom when the air is taken away 

 shows that it is not destitute of weight. So most 

 sorts of wood ascend or swim in water; yet we all 

 know that wood has gravity or weight. 



21. Set a receiver which is open at top on the 

 air-pump, and cover it with a brass-plate and wet 

 leather, and having exhausted it of air, let the air 

 in again at top through an iron pipe, making it 

 pass through a charcoal flame at the end of the 

 pipe ; and when the receiver is full of that air, lift 

 up the cover, and let down a mouse or bird into 

 the receiver, and the burnt air will immediately 

 kill it. If a candle be let down into that air, it 

 will go out directly ; but by letting it down gently, 

 it will drive out the impure air, and good air will 

 get in. 



22. Set a bell on the pump-plate, having a con- 

 trivance so as to ring it at pleasure, and cover it 

 with a receiver ; then make the clapper strike 

 against the bell, and the sound will be very well 

 heard ; but exhaust the receiver of air, and then, 

 if the clapper be made to strike ever so hard 

 against the bell, it will make no sound; which 

 shows that air is absolutely necessary for the pro- 

 pagation of sound. 



OF CONDENSED AIR. 



It has been shown, that air can be rarefied, or 

 made to expand : we now proceed to show that it 

 can also be condensed, or pressed into less space 

 than what it generally occupies. The instrument 

 used for this purpose is called a condenser. 



VOL. i. l 



