Join the two brass hemispherical cups, A and B, together, (fig. 8) with a wet leather between them, having a hole in the middle of it ; then having screwed off the handle at C, screw both the he mispheres put together into the pump plate, and turn the cock E, so that the pipe may be open all the, way into the cavity of the hemispheres ; then exhaust the air out of them, and turn the cock ; unscrew the hemispheres from the pump, and having put on the handle C, let two strong men try to pull the hemispheres asunder by the rings, which they will find hard to do ; for if the diameter of the hemispheres be four inches, they will be pressed together by the external air with a force equal to 190 pounds ; and to show that it is the pressure of the air that keeps them together, hang them by either of the rings upon the hook of a wire in the receiver of the air-pump, and, upon exhausting the air out of the re ceiver, they will fall asunder of them selves.
Set a square phial upon the pump-plate, and having covered it with a wire cage, put a close receiver over it, and exhaust the air out of the receiver ; in doing which, the air will also make its way out of the phial, through a small valve in its neck. When the air is exhausted, turn the cock below the plate to re-admit the air into the receiver ; and as it cannot get into the phial again, because of the valve, the phial will be broken into some thousands of pieces by the pressure of the air upon it. Had the phial been of a round form, it would have sustained this pressure, like an arch, without breaking ; but as its sides are flat, it cannot.
Let a large piece of cork he suspended by a thread at one end of a balance, and counterpoised by a leaden weight, sus pended 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 heavier than the lead ; but upon letting in the air again, the equili brium will 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 equal to the weight of their bulk of the fluid, the cork, being the larger body, loses more of its real weight than the 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 re moving the air, the bodies then gravitate according to their real quantities of mat ter, and the cork which balanced the lead in air, skews itself to be heavier when in vacua.
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, having gradu ally decayed from the first instant, it will go out; which shows, that a constant supply of fresh air is as necessary to teed flame, as animal life.
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 bot tom of the receiver, and leave it as clear at the top as it was before it was set upon the pump. This shows, that smoke does not ascend on account of its being posi tively light, 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 word ascend or swim in water ; and yet there are none who doubt of the wood's having gravity or weight.
Set a receiver, which is open at top, on the air•purnp, and cover it with a brass plate and wet leather ; and having ex hausted it of air, let the air in again at top through an iron pipe, making it pass through a charcoal flame at the end ofthe 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 gent ly, it will drive out the impure air, and good air will get in.
Set a bell on the pump-plate, having a contrivance 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 absolute ly necessary for the propagation of sound.
It has been shown, that air can be ra refied, or made to expand; we now pro ceed to show, that it can also be condens ed, or pressed into less space than what. it generally occupies. The instrument used for this purpose is called a conden ser : (fig. 9) represents a machine of this kind; it consists of a brass barrel, con taining a piston, which has a valve open ing downwards ; so that as the piston is raised, the air passes through the valve ; but as the piston is pushed down the air cannot return, and is, therefore, furced through a valve at the bottom of the bar rel, that allows it to pass through into the receiver, 13, but prevents it from return ing. Thus, at every stroke of the piston, more air is thrown into the receiver, which is of very thick and strong glass. The receiver is held down upon the plate, C, by the cross piece, I), and the screws, EF. The air is let out of the receiver by the cock, U, which communicates with it.