122

Of the Air-Pump.

syringe and lead will descend upon the piston-rod by their natural gra-vi ty; and, upon admitting the air into the receiver, they will bedrove upward again, until the piston be at the very bottom of thesyringe.

27. Let a large piece of cork be suspended by a thread at one endof a balance, and counterpoised by a leaden weight, suspended in thefame manner, at the other. Let this balance be hung to the inside ofthe top of a large receiver ; which being set on the pump, and theair exhausted, the cork will preponderate, and fhew itself to be hea-Vier than the lead; but upon letting in the air again, the equilibriumwill be restored. The reason of this is, that since the air is a fluid,and all bodies lose as much of their absolute weight in it, as is equalto the weight of their bulk of the fluid, the cork being the largerbody, loses more of its real weight than the lead does; and thereforemust in fact be heavier, to balance it under the disadvantage of losingforne of its weight: which disadvantage being taken ofF by removingthe air, the bodies then gravitate according to their real quantities ofmatter, and the cork, which balanced the lead in air, shews itself tobe heavier when in vacuo.

28. Set a lighted candle upon the pump, and cover it with a talireceiver. If the receiver holds a gallon, the candle will burn a mi-nute, and then, aster having gradually decayed from the first instant,it will go out: which shews, that a constant supply of frelh air is ne-ccflary to feed flame; and fo it also is for animal life. For a bird keptunder a dose receiver will soon die, although no air be pumped out jand it is found that, in the diving-bell, a gallon of air is sufficient onlyfor one minute for a man to breathe in.

The moment when the candle goes out, the smoke will be seen toafcend to the top of the receiver, and there it will form a fort of cloud :but upon exhausting the air, the smoke will sali down to the bottomof the receiver, and leave it as ciear at top as it was before it was setupon the pump. This shews, that smoke does not afcend on accountof its being positively light, but because it is lighter than air; and itsfalling to the bottom when the air was taken away, shews, that it isnot destitute of weight. So most sorts of wood afcend or swim inwater; and yet there are none who doubt of the wood ! s having gra-vity or weight.

29. Set a receiver, which is open at top, upon the air-pump, andcover it with a brass piate, and wet leather; and having exhausted itof air, let the air in again at top through an iron pipe, making it passthrough a charcoal flame at the end of the pipe; and when the re-

2 ceiver