Pneumatics, called also Pneumatology and Pneumatophy, among schoolmen, the doctrine and contemplation of spirits and spiritual substances, as God, angels, and the human soul; in which sense pneumatics are the same with what we otherwise call metaphysics. See the article Metaphysics.
Pneumatics is more commonly used among us for that branch of natural philosophy which treats of the weight, pressure, and elasticity of the Air, and the effects arising from it.
Sect. I. Of the Properties of Air.
The air is that thin transparent fluid body in which we live and breathe. It encompasses the whole earth to a considerable height; and, together with the clouds and vapors that float in it, is called the atmosphere. The air is justly reckoned among the number of fluids, because it has all the properties by which a fluid is distinguished. For, it yields to the least force impressed, its parts are easily moved among one another, it presses according to its perpendicular height, and its pressure is every way equal.
That the air is a fluid, consisting of such particles as have no cohesion betwixt them, but easily glide over one another, and yield to the slightest impression, appears from that ease and freedom with which animals breathe in it, and move through it without any difficulty or sensible resistance.
But it differs from all other fluids in the three following particulars. 1. It can be compressed into a much less space than that which it naturally possesses. 2. It cannot be congealed or fixed, as other fluids may. 3. It is of a different density in every part, upward from the earth's surface; decreasing in its weight, bulk for bulk, the higher it rises; and therefore must also decrease in density. 4. It is of an elastic or springy nature, and the force of its spring is equal to its weight.
That air is a body, is evident from its excluding all other bodies out of the space it possesses: for if a glass jar be plunged with its mouth downwards into Properties of Air.
A vessel of water, there will but very little water get into the jar, because the air of which it is full keeps the water out.
As air is a body, it must needs have gravity or weight; and that it is weighty, is demonstrated by experiment. For, let the air be taken out of a vessel by means of the air-pump, then having weighed the vessel, let in the air again, and upon weighing it, when re-filled with air, it will be found considerably heavier. Thus, a bottle that holds a wine quart, being emptied of air and weighed, is found to be about 17 grains lighter than when the air is let into it again; which shews that a quart of air weighs 17 grains. But a quart of water weighs 14625 grains; thus divided by 17, quotes 860 in round numbers; which shews, that water is 860 times as heavy as air near the surface of the earth.
As the air rises above the earth's surface, it grows rarer, and consequently lighter, bulk for bulk. For since it is of an elastic or springy nature, and its lowermost parts are pressed with the weight of all that is above them, it is plain that the air must be more dense or compact at the earth's surface, than at any height above it; and gradually rarer the higher up. For the density of the air is always as the force that compresseth it; and therefore the air towards the upper parts of the atmosphere being less pressed than that which is near the earth, it will expand itself, and thereby become thinner than at the earth's surface.
Dr Cotes has demonstrated, that if altitudes in the air be taken in arithmetical proportion, the rarity of the air will be in geometrical proportion. For instance,
| Altitude of Earth | Air's Weight | |------------------|-------------| | 7 | 16 | | 14 | 64 | | 21 | 256 | | 28 | 1024 | | 35 | 4096 | | 42 | 16384 | | 49 | 65536 | | 56 | 262144 | | 63 | 1048576 | | 70 | 4194304 | | 77 | 16777216 | | 84 | 67108864 | | 91 | 268435456 | | 98 | 1073741824 | | 105 | 4294697296 | | 112 | 17179569184 | | 119 | 68719476736 | | 126 | 274877906944| | 133 | 1099511627776|
And hence it is easy to prove by calculation, that a cubic inch of such air as we breathe would be so much rarefied at the altitude of 500 miles, that it would fill a sphere equal in diameter to the orbit of Saturn.
The weight or pressure of the air is exactly determined by the following experiment.
Take a glass tube about three feet long, and open at one end; fill it with quicksilver, and, putting your finger upon the open end, turn that end downward, and immerse it into a small vessel of quicksilver, without letting in any air: then take away your finger, and the quicksilver will remain suspended in the tube 29½ inches above its surface in the vessel; sometimes more, and at other times less, as the weight of the air is varied by winds and other causes. That the quicksilver is kept up in the tube by the pressure of the atmosphere upon that in the basin, is evident; for, if the basin and tube be put under a glass, and the air be then taken out of the glass, all the quicksilver in the tube will fall down into the basin; and if the air be let in again, the quicksilver will rise to the same height as before. Therefore the air's pressure on the surface of the earth is equal to the weight of 29½ inches depth of quicksilver all over the earth's surface, at a mean rate.
A square column of quicksilver, 29½ inches high, and one inch thick, weighs just 15 pounds, which is equal to the pressure of air upon every square inch of the earth's surface; and 144 times as much, or 2160 pounds, upon every square foot; because a square foot contains 144 square inches. At this rate, a middle-sized man, whose surface may be about 14 square feet, sustains a pressure of 30240 pounds, when the air is of a mean gravity: a pressure which would be intolerable, and even fatal to us, were it not equal on every part, and counterbalanced by the spring of the air within us, which is diffused through the whole body, and reacts with an equal force against the outward pressure.
Now, since the earth's surface contains (in round numbers) 200,000,000 square miles, and every square mile 27,878,400 square feet, there must be 515756800000000 square feet on the earth's surface; which multiplied by 2160 pounds (the pressure on each square foot) gives 12,043,468,800,000,000 pounds for the pressure or weight of the whole atmosphere.
When the end of a pipe is immersed in water, and the air is taken out of the pipe, the water will rise in it to the height of 33 feet above the surface of the water in which it is immersed; but will go no higher: for it is found, that a common pump will draw water no higher than 33 feet above the surface of the well; and unless the bucket goes within that distance from the well, the water will never get above it. Now, as it is the pressure of the atmosphere on the surface of the water in the well that causes the water to ascend in the pump, and follow the piston or bucket, when the air above it is lifted up; it is evident, that a column of water 33 feet high, is equal in weight to a column of quicksilver of the same diameter, 29½ inches high; and to as thick a column of air, reaching from the earth's surface to the top of the atmosphere.
In serene calm weather, the air has weight enough to support a column of quicksilver 31 inches high; but in tempestuous stormy weather, not above 28 inches. The quicksilver thus supported in a glass tube, is found to be a nice counterbalance to the weight or pressure of the air, and to shew its alterations at different times. And being now generally used to denote the changes in the weight of the air, and of the weather consequent upon them, it is called the barometer or weather-glass. See Barometer.
The pressure of the air being equal on all sides of a body body exposed to it, the softest bodies sustain this pressure without suffering any change in their figure; and so do the most brittle bodies without being broke.
**Sect. II. Experiments with the Air-pump, Shewing the Resistance, Weight, and Elasticity of the Air.**
The Air-pump being in effect the same as the water-pump, whoever understands the one will be at no loss to understand the other.
Having put a wet leather on the plate LL of the air-pump, place the glass-receiver M upon the leather, so that the hole i in the plate may be within the glass. Then turning the handle F backward and forward, the air will be pumped out of the receiver; which will then be held down to the plate by the pressure of the external air or atmosphere: for as the handle (fig. 9.) is turned backwards, it raises the piston d in the barrel BK, by means of the wheel F and rack Dd; and as the piston is leathered so tight as to fit the barrel exactly, no air can get between the piston and barrel; and therefore all the air above d in the barrel is lifted up towards B, and a vacuum is made in the barrel from e to b; upon which, part of the air in the receiver M (fig. 8.), by its spring, rushes through the hole i, in the brass plate LL, along the pipe GCG, (which communicates with both barrels by the hollow trunk IHK (fig. 9.), and pushing up the valve b, enters into the vacant place b e of the barrel BK: for wherever the resistance or pressure is taken off, the air will run to that place, if it can find a passage.—Then as the handle F is turned forward, the piston d will be depressed in the barrel; and as the air which had got into the barrel cannot be pushed back through the valve b, it will ascend through a hole in the piston, and escape through the valve at d, and be hindered by that valve from returning into the barrel when the piston is again raised. At the next raising of the piston, a vacuum is again made in the same manner as before, between b and e; upon which more of the air which was left in the receiver M, gets out thence by its spring, and runs into the barrel BK, through the valve B. The same thing is to be understood with regard to the other barrel AI; and as the handle F is turned backwards and forwards, it alternately raises and depresses the pistons in their barrels; always raising one whilst it depresses the other. And as there is a vacuum made in each barrel when its piston is raised, every particle of air in the receiver M pushes out another, by its spring or elasticity, through the hole i, and pipe GG, into the barrels; until at last the air in the receiver comes to be so much dilated, and its spring so far weakened, that it can no longer get through the valves; and then no more can be taken out. Hence there is no such thing as making a perfect vacuum in the receiver; for the quantity of air taken out at any one stroke, will always be as the density thereof in the receiver: and therefore it is impossible to take it all out; because, supposing the receiver and barrels of equal capacity, there will be always as much left as was taken out at the last turn of the handle.
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(a) Such is the common construction. But there is another invented by Mr Smeaton; by which a purer vacuum is obtained, and which also acts as a condensing engine. There is, moreover, what they call a portable air-pump, which is placed on a table, and may be easily conveyed from one place to another. See the article (Air)-Pump.
There is a cock k below the pump-plate, which being turned, lets the air into the receiver again; and then the receiver becomes loose, and may be taken off the plate. The barrels are fixed to the frame Eee by two screw-nuts ff, which press down the top-piece E upon the barrels; and the hollow trunk H (in fig. 9.) is covered by a box, as GH in fig 8.
There is a glass tube lmmnn open at both ends, and about 34 inches long; the upper end communicating with the hole in the pump-plate, and the lower end immersed in quicksilver at n in the vessel N. To this tube is fitted a wooden ruler mm, called the gage, which is divided into inches and parts of an inch, from the bottom at n, (where it is even with the surface of the quicksilver), and continued up to the top, a little below l, to 30 or 31 inches.
As the air is pumped out of the receiver M, it is likewise pumped out of the glass tube lmn, because that tube opens into the receiver through the pump-plate; and as the tube is gradually emptied of air, the quicksilver in the vessel N is forced up into the tube by the pressure of the atmosphere. And if the receiver could be perfectly exhausted of air, the quicksilver would stand as high in the tube as it does at that time in the barometer: for it is supported by the same power or weight of the atmosphere in both (a).
The quantity of air exhausted out of the receiver in each turn of the handle, is always proportionable to the ascent of the quicksilver on that turn; and the quantity of air remaining in the receiver, is proportionable to the defect of the height of the quicksilver in the gage from what it is at that time in the barometer.
1. There is a little machine, consisting of two mills, iv. a and b, which are of equal weights, independent of each other, and turn equally free on their axes in the frame. Each mill has four thin arms or sails fixed into the axle: those of the mill a have their planes at right angles to its axis; and those of b have their planes parallel to it. Therefore, as the mill a turns round in common air, it is but little resisted thereby, because its sails cut the air with their thin edges; but the mill b is much resisted, because the broad sides of its sails move against the air when it turns round. In each axle is a pin near the middle of the frame, which goes quite through the axle, and stands out a little on each side of it: upon these pins the slider d may be made to bear, and so hinder the mills from going, when the strong spring c is set on bend against the opposite ends of the pins.
Having set this machine upon the pump-plate LL, (fig. 8.) draw up the slider d to the pins on one side, and let the spring c at bend upon the opposite ends of the pins: then push down the slider d, and the spring acting equally strong upon each mill, will set them both a-going with equal forces and velocities: but the mill a will run much longer than the mill b, because the air makes much less resistance against the edges of its sails than against the sides of the sails of b.
Draw up the slider again, and let the spring upon the pins as before; then cover the machine with the receiver M (fig. 8.) upon the pump-plate; and having exhaust- ed the receiver of air, push down the wire PP (thro' the collar of leathers in the neck q) upon the slider; which will disengage it from the pins, and allow the mills to turn round by the impulse of the spring: and as there is no air in the receiver to make any sensible resistance against them, they will both move a considerable time longer than they did in the open air; and the moment that one stops, the other will do so too.—This shows, that air resists bodies in motion; and that equal bodies meet with different degrees of resistance, according as they present greater or less surfaces to the air, in the planes of their motions.
2. Take off the receiver M and the mills; and having put the guinea a and feather b upon the brass flap e, turn up the flap, and shut it into the notch d. Then putting a wet leather over the top of the tall receiver AB (it being open both at top and bottom) cover it with the plate C, from which the guinea-and-feather tongs e d will then hang within the receiver. This done, pump the air out of the receiver, and then draw up the wire f a little, which by a square piece on its lower end will open the tongs e d; and the flap falling down as at c, the guinea and feather will descend with equal velocities in the receiver, and both will fall upon the pump-plate at the same instant. N.B. In this experiment, the observers ought not to look at the top, but at the bottom, of the receiver, in order to see the guinea and feather fall upon the plate; otherwise, on account of the quickness of their motion, they will escape the sight of the beholders.
1. Having fitted a brass cap, with a valve tied over it, to the mouth of a thin bottle or Florence flask, whose contents are exactly known, screw the neck of this cap into the hole i of the pump-plate; then having exhausted the air out of the flask, and taken it off from the pump, let it be suspended at one end of a balance, and nicely counterpoised by weights in the scale at the other end; this done, raise up the valve with a pin, and the air will rush into the flask with an audible noise: during which time, the flask will descend, and pull down that end of the beam. When the noise is over, put as many grains into the scale at the other end as will restore the equilibrium; and they will show exactly the weight of the quantity of air which has got into the flask and filled it. If the flask holds an exact quart, it will be found, that 17 grains will restore the equipoise of the balance, when the quicksilver stands at 29½ inches in the barometer: which shows, that when the air is at a mean rate of density, a quart of it weighs 17 grains: it weighs more when the quicksilver stands higher, and less when it stands lower.
2. Place the small receiver O (fig. 8.) over the hole i in the pump-plate; and upon exhausting the air, the receiver will be fixed down to the plate by the pressure of the air on its outside, which is left to act alone, without any air in the receiver to act against it; and this pressure will be equal to as many times 15 pounds as there are square inches in that part of the plate which the receiver covers; which will hold down the receiver so fast, that it cannot be got off until the air be let into it by turning the cock k; and then it becomes loose.
3. Set the little glass AB (which is open at both ends) over the hole i upon the pump-plate LL, and put your hand close upon the top of it at B: then, upon exhausting the air out of the glass, you will find your hand pressed down with a great weight upon it; so that you can hardly release it until the air be re-admitted into the glass by turning the cock k; which air, by acting as strongly upward against the hand as the external air acted in pressing it downward, will release the hand from its confinement.
4. Having tied a piece of wet bladder b over the open top of the glass A, (which is also open at bottom,) set it to dry, and then the bladder will be tight like a drum. Then place the open end A upon the pump-plate over the hole i, and begin to exhaust the air out of the glass. As the air is exhausting, its spring in the glass will be weakened, and give way to the pressure of the outward air on the bladder; which, as it is pressed down, will put on a spherical concave figure, which will grow deeper and deeper, until the strength of the bladder be overcome by the weight of the air; and then it will break with a report as loud as that of a gun.—If a flat piece of glass be laid upon the open top of this receiver, and joined to it by a flat ring of wet leather between them, upon pumping the air out of the receiver, the pressure of the outward air upon the flat glass will break it all to pieces.
5. Immerse the neck cd of the hollow glass-ball e b Fig. 14., in water, contained in the phial a z; then set it upon the pump-plate, and cover it and the hole i with the close receiver A; and then begin to pump out the air. As the air goes out of the receiver by its spring, it will also by the same means go out of the hollow ball e b, through the neck d c, and rise up in bubbles to the surface of the water in the phial; from whence it will make its way, with the rest of the air in the receiver, through the air-pipe GG and valves a and b, into the open air. When it has done bubbling in the phial, the ball is sufficiently exhausted; and then, upon turning the cock k, the air will get into the receiver, and press upon the surface of the water in the phial, as to force the water up into the ball in a jet, through the neck c d, and will fill the ball almost full of water. The reason why the ball is not quite filled, is because all the air could not be taken out of it; and the small quantity that was left in, and had expanded itself so as to fill the whole ball, is now condensed into the same state as the outward air, and remains in a small bubble at the top of the ball, and so keeps the water from filling that part of the ball.
6. Pour some quicksilver into the jar D, and set it Fig. 15., on the pump-plate near the hole i; then set on the tall open receiver AB, so as to be over the jar and hole i; and cover the receiver with the brass-plate C. Screw the open glass tube f g (which has a brass top on it at b) into the syringe H; and putting the tube through a hole in the middle of the plate, so as to immerse the lower end of the tube e in the quicksilver at D, screw the end b of the syringe into the plate. This done, draw up the piston in the syringe by the ring I, which will make a vacuum in the syringe below the piston; and as the upper end of the tube opens into the syringe, the air will be dilated in the tube, because part of it by its spring gets up into the syringe; and the spring of the undilated air in the receiver acting upon the surface of the quicksilver in the jar, jar, will force part of it up into the tube; for the quicksilver will follow the piston in the syringe, in the same way, and for the same reason, that water follows the piston of a common pump when it is raised in the pump-barrel; and this, according to some, is done by suction. But to refute that erroneous notion, let the air be pumped out of the receiver AB, and then all the quicksilver in the tube will fall down by its own weight into the jar, and cannot be again raised one hair's-breadth in the tube by working the syringe: which shows that suction had no hand in raising the quicksilver: and to prove that it is done by pressure, let the air into the receiver by the cock k (fig. 8.), and its action upon the surface of the quicksilver in the jar will raise it up into the tube, although the piston of the syringe continues motionless.—If the tube be about 32 or 33 inches high, the quicksilver will rise in it very near as high as it stands at that time in the barometer. And if the syringe has a small hole, as m, near the top of it, and the piston be drawn up above that hole, the air will rush through the hole into the syringe and tube, and the quicksilver will immediately fall down into the jar. If this part of the apparatus be air-tight, the quicksilver may be pumped up into the tube to the same height that it stands in the barometer; but it will go no higher, because then the weight of the column in the tube is the same as the weight of a column of air of the same thickness with the quicksilver, and reaching from the earth to the top of the atmosphere.
7. Having placed the jar A, with some quicksilver in it, on the pump-plate, as in the last experiment, cover it with the receiver B; then push the open end of the glass tube d e through the collar of leathers in the brass neck C (which it fits so as to be air-tight) almost down to the quicksilver in the jar. Then exhaust the air out of the receiver, and it will also come out of the tube, because the tube is close at top. When the gauge m m shows that the receiver is well exhausted, push down the tube so as to immerse its lower end into the quicksilver in the jar. Now, although the tube be exhausted of air, none of the quicksilver will rise into it, because there is no air left in the receiver to press upon its surface in the jar: but let the air into the receiver by the cock k, and the quicksilver will immediately rise in the tube, and stand as high in it as it was pumped up in the last experiment.
Both these experiments show, that the quicksilver is supported in the barometer by the pressure of the air on its surface in the box, in which the open end of the tube is placed: And that the more dense and heavy the air is, the higher does the quicksilver rise; and, on the contrary, the thinner and lighter the air is, the more will the quicksilver fall. For if the handle F be turned ever so little, it takes some air out of the receiver, by raising one or other of the pistons in its barrel; and consequently, that which remains in the receiver is so much the rarer, and has so much the less spring and weight; and thereupon the quicksilver falls a little in the tube: but upon turning the cock, and re-admitting the air into the receiver, it becomes as weighty as before, and the quicksilver rises again to the same height.—Thus we see the reason why the quicksilver in the barometer falls before rain or snow, and rises before fair weather; for, in the former case, the air is too thin and light to bear up the vapours, and in the latter, too dense and heavy to let them fall.
[N.B. In all mercurial experiments with the air-pump, a short pipe must be screwed into the hole i, so as to rise about an inch above the plate, to prevent the quicksilver from getting into the air-pipe and barrels, in case any of it should be accidentally spilt over the jar; for if it once gets into the pipes or barrels, it spoils them, by loosening the folder, and corroding the brass.]
8. Take the tube out of the receiver, and put one end of a bit of dry hazel-branch, about an inch long, tight into the hole, and the other end tight into a hole quite through the bottom of a small wooden cup: then pour some quicksilver into the cup, and exhaust the receiver of air; and the pressure of the outward air on the surface of the quicksilver will force it through the pores of the hazel, from whence it will descend in a beautiful shower into a cup placed under the receiver to catch it.
9. Put a wire through the collar of leathers in the top of the receiver, and fit a bit of dry wood on the end of the wire within the receiver; then exhaust the air, and push the wire down, so as to immerse the wood into a jar of quicksilver on the pump-plate. This done, let in the air; and upon taking the wood out of the jar, and splitting it, its pores will be found full of quicksilver, which the force of the air, upon being let into the receiver, drove into the wood.
10. Join the two brass hemispherical cups A and B Fig. 17. together, with a wet leather between them, having a hole in the middle of it; then screw the end D of the pipe CD into the plate of the pump at i, and turn the cock E, so as 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 a quarter round, which will shut the pipe CD, and keep out the air. This done, unscrew the pipe at D from the pump; and screw the piece F h upon it at D; and let two strong men try to pull the hemispheres asunder by the rings g and h, 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 188 pounds. And to shew that it is the pressure of the air that keeps them together, hang them by either of the rings upon the hook P of the wire in the receiver M (fig. 8.), and upon exhausting the air out of the receiver they will fall asunder of themselves.
11. Place a small receiver O (fig. 8.) near the hole i on the pump-plate, and cover both it and the hole with the receiver M; and turn the wire so by the top P, that its hook may take hold of the little receiver by a ring at its top, allowing that receiver to stand with its own weight on the plate. Then, upon working the pump, the air will come out of both receivers; but the large one M will be forcibly held down to the pump by the pressure of the external air; whilst the small one O, having no air to press upon it, will continue loose, and may be drawn up and let down at pleasure, by the wire PP. But, upon letting it quite down to the plate, and admitting the air into the receiver M, by the cock k, the air will press so strongly upon the small receiver O, as to fix it down to the plate; and at the same time, by counterbalancing the outward pressure on the large receiver M, it will become loose. This experiment ment evidently shows, that the receivers are held down by pressure, and not by suction; for the internal receiver continued loose whilst the operator was pumping, and the external one was held down; but the former became fast immediately by letting in the air upon it.
12. Screw the end A of the brass pipe ABC into the hole of the pump-plate, and turn the cock e until the pipe be open; then put a wet leather upon the plate cd, which is fixed on the pipe, and cover it with the tall receiver GH, which is close at top: then exhaust the air out of the receiver, and turn the cock e to keep it out; which done, uncrew the pipe from the pump, and set its end A into a bason of water, and turn the cock e to open the pipe; on which, as there is no air in the receiver, the pressure of the atmosphere on the water in the bason will drive the water forcibly through the pipe, and make it play up in a jet to the top of the receiver.
13. Set the square phial A (fig. 21.) upon the pump-plate, and having covered it with the wire-cage B, put a close receiver over it, and exhaust the air out of the receiver; in doing of which, the air will also make its way out of the phial through a small hole in its neck under the valve b. 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.
14. Tie up a very small quantity of air in a bladder, and put it under a receiver; then exhaust the air out of the receiver; and the small quantity which is confined in the bladder (having nothing to act against it) will expand itself so by the force of its spring, as to fill the bladder as full as it could be blown of common air. But upon letting the air into the receiver again, it will overpower the air in the bladder, and press its sides almost close together.
15. If the bladder so tied up be put into a wooden box, and have 20 or 30 pounds weight of lead put upon it in the box, and the box be covered with a close receiver; upon exhausting the air out of the receiver, that air which is confined in the bladder will expand itself so, as to raise up all the lead by the force of its spring.
16. Take the glass ball mentioned in the fifth experiment, which was left full of water all but a small bubble of air at top; and having set it with its neck downward into the empty phial aa, and covered it with a close receiver, exhaust the air out of the receiver; and the small bubble of air in the top of the ball will expand itself so as to force all the water out of the ball into the phial.
17. Screw the pipe AB into the pump-plate; place the tall receiver GH upon the plate cd, as in the 12th experiment; and exhaust the air out of the receiver; then turn the cock e to keep out the air, uncrew the pipe from the pump, and screw it into the mouth of the copper vessel CC (fig. 22.), the vessel having first been about half filled with water. Then open the cock e, (fig. 18.) and the spring of the air which is confined in the copper vessel will force the water up through the pipe AB in a jet into the exhausted receiver, as strongly as it did by its pressure on the surface of the water in the bason, in the 12th experiment.
18. If a fowl, a cat, rat, mouse, or bird, be put under a receiver, and the air be exhausted, the animal will be at first oppressed as with a great weight, then grow convulsed, and at last expire in all the agonies of a most bitter and cruel death. But as this experiment is too shocking to every spectator who has the least degree of humanity, some substitute a machine called the lungi-glass in place of the animal.
19. If a butterfly be suspended in a receiver by a fine thread tied to one of its horns, it will fly about in the receiver as long as the receiver continues full of air; but if the air be exhausted, though the animal will not die, and will continue to flutter its wings, it cannot remove itself from the place where it hangs in the middle of the receiver until the air be let in again; and then the animal will fly about as before.
20. Pour some quicksilver into the small bottle A, fig. 19; and screw the brass collar c of the tube BC into the brass neck b of the bottle, and the lower end of the tube will be immersed into the quicksilver, so that the air above the quicksilver in the bottle will be confined there, because it cannot get out about the joinings, nor can it be drawn out through the quicksilver into the tube. This tube is also open at top, and is to be covered with the receiver G and large tube EF; which tube is fixed by brass collars to the receiver, and is close at the top. This preparation being made, exhaust the air both out of the receiver and its tube; and the air will by the same means be exhausted out of the inner tube BC, through its open top at C; and as the receiver and tubes are exhausting, the air that is confined in the glass bottle A will press so by its spring upon the surface of the quicksilver, as to force it up in the inner tube as high as it was raised in the ninth experiment by the pressure of the atmosphere: which demonstrates, that the spring of the air is equivalent to its weight.
21. Screw the end C of the pipe CD into the hole of the pump-plate, and turn all the three cocks d, G, and H, so as to open the communications between all the three pipes E, F, DC, and the hollow trunk AB. Then cover the plates g and d with wet leathers, which have holes in their middle where the pipes open into the plates; and place the close receiver I upon the plate G: this done, shut the pipe F by turning the cock H, and exhaust the air out of the receiver I. Then turn the cock d to shut out the air, uncrew the machine from the pump, and having screwed it to the wooden foot L, put the receiver K upon the plate b; this receiver will continue loose on the plate as long as it keeps full of air; which it will do until the cock H be turned to open the communication between the pipes F and E, through the trunk AB; and then the air in the receiver K, having nothing to act against its spring, will run from K into I, until it be so divided between these receivers as to be of equal density in both; and then they will be held down with equal forces to their plates by the pressure of the atmosphere, though each receiver will then be kept down but with one half of pressure upon it that the receiver I had when it was exhausted of air; because it has now one half of the common air in it which filled the receiver K when it was set upon the plate; and therefore, a force equal to half half the force of the spring of common air, will act within the receivers against the whole pressure of the common air upon their outsides. This is called transferring the air out of one vessel into another.
22. Put a cork into the square phial A, and fix it in with wax or cement; put the phial upon the pump-plate with the wire cage B over it, and cover the cage with a close receiver. Then exhaust the air out of the receiver; and the air that was corked up in the phial will break the phial outwards by the force of its spring, because there is no air left on the outside of the phial to act against the air within it.
23. Put a shrivelled apple under a close receiver, and exhaust the air; then the spring of the air within the apple will plump it out, so as to cause all the wrinkles disappear; but upon letting the air into the receiver again to press upon the apple, it will instantly return to its former decayed and shrivelled state.
24. Take a fresh egg, and cut off a little of the shell and film from its smallest end; then put the egg under a receiver, and pump out the air; upon which all the contents in the egg will be forced out into the receiver by the expansion of a small bubble of air contained in the great end between the shell and film.
25. Put some warm beer in a glass; and having set it on the pump, cover it with a close receiver, and then exhaust the air. Whilst this is doing, and thereby the pressure more and more taken off from the beer in the glass, the air therein will expand itself, and rise up in innumerable bubbles to the surface of the beer; and from thence it will be taken away with the other air in the receiver. When the receiver is nearly exhausted, the air in the beer, which could not disentangle itself quick enough to get off with the rest, will now expand itself so, as to cause the beer to have all the appearance of boiling; and the greatest part of it will go over the glass.
26. Put some warm water in a glass, and put a bit of dry wainscot or other wood into the water. Then cover the glass with a close receiver, and exhaust the air; upon which the air in the wood, having liberty to expand itself, will come out plentifully, and make all the water to bubble about the wood, especially about the ends, because the pores lie lengthwise. A cubic inch of dry wainscot has so much air in it, that it will continue bubbling for near half an hour together.
27. Screw the syringe H (fig. 15.) to a piece of lead that weighs one pound at least; and holding the lead in one hand, pull up the piston in the syringe with the other; then quitting hold of the lead, the air will push it upward, and drive back the syringe upon the piston. The reason of this is, that the drawing up of the piston makes a vacuum in the syringe; and the air, which presses every way equally, having nothing to resist its pressure upward, the lead is thereby pressed upward contrary to its natural tendency by gravity. If the syringe so loaded be hung in a receiver, and the air be exhausted, the syringe and lead will descend upon the piston-rod by their natural gravity; and upon admitting the air into the receiver, they will be drove upward again until the piston be at the very bottom of the syringe.
28. Let a large piece of cork be suspended by a thread at one end of a balance, and counterpoised by a leaden weight, suspended in the same manner, at the other end. 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 equilibrium 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 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.
29. 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 shews, that a constant supply of fresh air is necessary to feed flame; and so it also is for animal-life. For a bird kept under a close receiver will soon die, 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.
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 shows, 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, shows 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.
30. 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 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 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.
31. Set a bell upon a cushion on the pump-plate, and cover it with a receiver; then shake the pump to 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 at all; which shows that air is absolutely necessary for the propagation of sound.
32. Let a candle be placed on one side of a receiver, and viewed through the receiver at some distance; then, as soon as the air begins to be exhausted, the receiver will be filled with vapours which rise from the wet leather, by the spring of the air in it; and the light of the candle being refracted through that medium of vapours, vapours, will have the appearance of circles of various colours, of a faint resemblance to those in the rain-bow.
The elastic air which is contained in many bodies, and is kept in them by the weight of the atmosphere, may be got out of them either by boiling, or by the air-pump, as shewn in the 25th experiment: but the fixed air, which is by much the greater quantity, cannot be got out but by distillation, fermentation, or putrefaction.
If fixed air did not come out of bodies with difficulty, and spend some time in extricating itself from them, it would tear them to pieces. Trees would be rent by the change of air from a fixed to an elastic state, and animals would be burst in pieces by the explosion of air in their food.
Dr Hales found by experiment, that the air in apples is so much condensed, that if it were let out into the common air, it would fill a space 48 times as great as the bulk of the apples themselves; so that its pressure outwards was equal to 11,776 lb., and, in a cubic inch of oak, to 19,860 lb. against its sides. So that if the air was let loose at once in these substances, they would tear every thing to pieces about them with a force superior to that of gunpowder. Hence, in eating apples, it is well that they part with the air by degrees as they are chewed and ferment in the stomach, otherwise an apple would be immediate death to him who eats it.
The mixing of some substances with others will release the air from them, all of a sudden; which may be attended with very great danger. Of this we have a remarkable instance in an experiment made by Dr Slare; who having put half a dram of oil of caraway-seeds into one glass, and a dram of compound spirit of nitre in another, covered them both on the air-pump with a receiver six inches wide and eight inches deep, and then exhausted the air, and continued pumping until all that could possibly be got both out of the receiver, and out of the two fluids, was extricated: then, by a particular contrivance from the top of the receiver, he mixed the fluids together; upon which they produced such a prodigious quantity of air as instantly blew up the receiver, although it was pressed down by the atmosphere with upwards of 400 pound weight.