CHAPTER III.. Problems of Vacuum.

PROBLEMS OF VACUUM.

A. What convincing argument is there to prove, that in all the world there is no empty place?

B. Many; but I will name but one; and that is, the difficulty of separating two bodies hard and flat laid one upon another. I say the difficulty, not the impossibility. It is possible, without introducing vacuum, to pull asunder any two bodies, how hard and flat soever they be, if the force used be greater than the resistance of the hardness. And in case there be any greater difficulty to part them, besides what proceeds from their hardness, than there is to pull them further asunder when they are parted, that difficulty is argument enough to prove there is no vacuum.

A. These assertions need demonstration. And first, how does the difficulty of separation argue the plenitude of all the rest of the world?

B. If two flat polished marbles lie one upon another, you see they are hardly separated in all points at one and the same instant; and yet the weight of either of them is enough to make them slide off one from the other. Is not the cause of this, that the air succeeds the marble that so slides, and fills up the place it leaves?

A. Yes, certainly. What then?

B. But when you pull the whole superficies asunder, not without great difficulty, what is the cause of that difficulty?

A. I think, as most men do, that the air cannot fill up the space between in an instant; for the parting is in an instant.

B. Suppose there be vacuum in that air into which the marble you pull off is to succeed, shall there be no vacuum in the air that was round about the two marbles when they touched? Why cannot that vacuum come into the place between? Air cannot succeed in an instant, because a body, and consequently cannot be moved through the least space in an instant. But emptiness is not a body, nor is moved, but is made by the act itself of separation. There is therefore, if you admit vacuum, no necessity at all for the air to fill the space left in an instant. And therefore, with what ease the marble coming off presseth out the vacuum of the air behind it, with the same ease will the marbles be pulled asunder. Seeing then, if there were vacuum, there would be no difficulty of separation, it follows, because there is difficulty of separation, that there is no vacuum.

A. Well, now, supposing the world full, how do you prove it possible to pull those marbles asunder?

B. Take a piece of soft wax; do not you think the one half touches the other half as close as the smoothest marbles? Yet you can pull them asunder. But how? Still as you pull, the wax grows continually more and more slender; there being a perpetual parting or discession of the outermost part of the wax one from another, which the air presently fills; and so there is a continual lessening of the wax, till it be no bigger than a hair, and at last separation. If you can do the same to a pillar of marble, till the outside give way, the effect will be the same, but much quicker, after it once begins to break in the superficies; because the force that can master the first resistance of the hardness, will quickly dispatch the rest.

A. It seems so by the brittleness of some hard bodies. But I shall afterward put some questions to you, touching the nature of hardness. But now to return to our subject. What reason can you render (without supposing vacuum) of the effects produced in the engine they use at Gresham college?

B. That engine produceth the same effects that a strong wind would produce in a narrow room.

A. How comes the wind in? You know the engine is a hollow round pipe of brass, into which is thrust a cylinder of wood covered with leather, and fitted to the cylinder so exactly as no air can possibly pass between the leather and the brass?

B. I know it; and that they may thrust it up, there is a hole left in the cylinder to let the air out before it, which they can stop when they please. There is also in the bottom of the cylinder a passage into a hollow globe of glass, which passage they can also open and shut at pleasure. And at the top of that globe there is a wide mouth to put in what they please to try conclusions on, and that also to be opened and shut as shall be needful. It is of the nature of a pop-gun which children use, but great, costly, and more ingenious. They thrust forward and pull back the wooden cylinder (because it requires much strength) with an iron screw. What is there in all this to prove the possibility of vacuum.

A. When this wooden cylinder covered with leather, fit and close, is thrust home to the bottom, and the holes in the hollow cylinder of brass close stopped, how can it be drawn back, as with the screw they draw it, but that the space it leaves must needs be empty: for it is impossible that any air can pass into the place to fill it?

B. Truly I think it close enough to keep out straw and feathers, but not to keep out air, nor yet matter. For suppose they were not so exactly close but that there were round about a difference for a small hair to lie between; then will the pulling back of the cylinder of wood force so much air in, as in retiring it forces back, and that without any sensible difficulty. And the air will so much more swiftly enter as the passage is left more narrow. Or if they touch, and the contact be in some points and not in all, the air will enter as before, in case the force be augmented accordingly. Lastly, though they touch exactly, if either the leather yield, or the brass, which it may do, to the force of a strong screw, the air will again enter. Do you think it possible to make two superficies so exquisitely touch in all points as you suppose, or leather so hard as not to yield to the force of a screw? The body of leather will give passage both to air and water, as you will confess when you ride in rainy and windy weather. You may therefore be assured that in drawing out their wooden leather cylinder, they force in as much air as will fill the place it leaves, and that with as much swiftness as is answerable to the strength that drives it in. The effect therefore of their pumping is nothing else but a vehement wind, a very vehement wind, coming in on all sides of the cylinder at once into the hollow of the brass pipe, and into the hollow of the glass globe joined to it.

A. I see the reason already of one of their wonders, which is, that the cylinder they pump with, if it be left to itself, after it is pulled back, will swiftly go up again. You will say the air comes out again with the same violence by reflection, and I believe it.

B. This is argument enough that the place was not empty. For what can fetch or drive up the sucker, as they call it, if the place within were empty? For that there is any weight in the air to do it, I have already demonstrated to be impossible. Besides, you know, when they have sucked out, as they think, all the air from the glass globe, they can nevertheless both see through it what is done, and hear a sound from within when there is any made; which, if there were no other, but there are many other, is argument enough that the place is still full of air.

A. What say you to the swelling of a bladder even to bursting, if it be a little blown when it is put into the receiver, for so they call the globe of glass?

B. The streams of air that from every side meeting together, and turning in an infinite number of small points, do pierce the bladder in innumerable places with great violence at once, like so many invisible small wimbles, especially if the bladder be a little blown before it be put in, that it may make a little resistance. And when the air has once pierced it, it is easy to conceive, that it must afterward by the same violent motion be extended till it break. If before it break you let in fresh air upon it, the violence of the motion will thereby be tempered, and the bladder be less extended; for that also they have observed. Can you imagine how a bladder should be extended and broken by being too full of emptiness?

A. How come living creatures to be killed in this receiver, in so little a time as three or four minutes of an hour?

B. If they suck into their lungs so violent a wind thus made, you must needs think it will presently stop the passage of their blood; and that is death; though they may recover if taken out before they be too cold. And so likewise will it put out fire; but the coals taken out whilst they are hot will revive again. It is an ordinary thing in many coal-pits, whereof I have seen the experience, that a wind proceeding from the sides of the pit every way, will extinguish any fire let down into it, and kill the workmen, unless they be quickly taken out.

A. If you put a vessel of water into the receiver, and then suck out the air, the water will boil; what say you to that?

B. It is like enough it will dance in so great a bustling of the air; but I never heard it would be hot. Nor can I imagine how vacuum should make anything dance. I hope you are by this time satisfied, that no experiment made with the engine at Gresham College, is sufficient to prove that there is, or that there may be vacuum.

A. The world you know is finite, and consequently, all that infinite space without it is empty. Why may not some of that vacuum be brought in, and mingled with the air here?

B. I know nothing in matters without the world.

A. What say you to Torricellio’s experiment in quicksilver, which is this: there is a basin at A filled with quicksilver, suppose to B, and C D a hollow glass pipe filled with the same, which if you stop with your finger at B, and so set it upright, and then if you take away your finger, the quicksilver will fall from C downwards but not to the bottom, for it will stop by the way, suppose at D. Is it not therefore necessary that that space between C and D be left empty? Or will you say the quicksilver does not exactly touch the sides of the glass pipe?

B. I will say neither. If a man thrust down into a vessel of quicksilver a blown bladder, will not that bladder come up to the top?

A. Yes, certainly, or a bladder of iron, or anything else but gold.

B. You see then that air can pierce quicksilver.

A. Yes, with so much force as the weight of quicksilver comes to.

B. When the quicksilver is fallen to D, there is so much the more in the basin, and that takes up the place which so much air took up before. Whither can this air go if all the world without that glass pipe B C were full? There must needs be the same or as much air come into that space, which only is empty, between C and D: by what force? By the weight of the quicksilver between D and B. Which quicksilver weigheth now upward, or else it could never have raised that part higher, which was at first in the basin. So you see the weight of quicksilver can press the air through quicksilver up into the pipe, till it come to an equality of force as in D, where the weight of the quicksilver is equal to the force which is required in air to go through it.

A. If a man suck a phial that has nothing in it but air, and presently dip the mouth of it into water, the water will ascend into the phial. Is not that an argument that part of the air had been sucked out, and part of the room within the phial left empty?

B. No. If there were empty space in the world, why should not there be also some empty space in the phial before it was sucked? And then why does not the water rise to fill that? When a man sucks the phial he draws nothing out, neither into his belly, nor into his lungs, nor into his mouth; only he sets the air within the glass into a circular motion, giving it at once an endeavour to go forth by the sucking, and an endeavour to go back by not receiving it into his mouth; and so with a great deal of labour glues his lips to the neck of the phial. Then taking it off, and dipping the neck of the phial into the water before the circulation ceases, the air, with the endeavour it hath now gotten, pierces the water and goes out: and so much air as goes out, so much matter comes up into the room of it.

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