OF THE AIR-PIMP. 225 



cessary to feed flame ; and so it also is for animal life. LECT. 

 For a bird kept under a close receiver will soon die, v^-s, -x. 

 although no air be pumped out ; and it is found that, in 

 the diving-bell, a gallon of air is sufficient only for one 

 minute for a man to breathe in. a 



The moment when the candle goes out, the smoke 

 will be seen to ascend to the top of the receiver, 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 pump. This shews, 

 that smoke does not ascend on account of its being 

 positively light, but because it is lighter than air ; and 

 its falling to the bottom when the air was taken away, 

 shews, that it is not destitute of weight. So most sorts 

 of wood ascend or swim in water ; and yet there are 

 none who doubt of the wood's having gravity or weight. 



29. Set a receiver, which is open at top, upon 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 char- 

 coal flame ; at the end of the pipe ; and when the re- 

 ceiver is full of that air, lift up the cover, and let down 

 a mouse or bird into the receiver, and the burned air will 

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

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

 it will purify the air so far as it goes ; and so, by letting 

 it down more and more, all the air in the receiver will 

 be purified. 



30. Set a bell upon a cushion on the pump-plate, and 



\ote 61. it has already been stated that oxygen gass is essential to 

 combustion a fact fully proved by the above experiment ; and it might, 

 at first vie:v, be supposed that an excess of this important ingredient 

 of our atmosphere, might be an additional comfort to those who inhale 

 it. The contrary, however, is the case; as Infinite Wisdom has so ap- 

 portioned the compound, that the slightest addition to the vivifying 

 principle would lessen, if not entirely destroy the advantages arising 

 from artificial illumination by rendering combustion too rapid. 

 16. Q 



