Decameron physiologicum, or, Ten dialogues of natural philosophy by Thomas Hobbes ... ; to which is added The proportion of a straight line to half the arc of a quadrant, by the same author.

CAP. III. Of Vacuum. (Book 3)

A.

'TIs hard to suppose, and harder to be∣lieve that the Infinite and Omnipo∣tent Creator of all things should make a work so vast as is the world we see, and leave a few little spaces with nothing at all in them; which put altogether in respect of the whole Creature, would be insensible.

B.

Why say you that? Do you think any Argument can be drawn from it to prove there is Vacuum?

A.

Why not? For in so great an Agitation of Natural Bodies, may not some small parts of them be cast out, and leave the places em∣pty from whence they were thrown?

B.

Because he that created them is not a Fancy, but the most real substance that is; who being Infinite, there can be no place em∣pty where he is, nor full where he is not.

A.

'Tis hard to answer this Argument, be∣cause I do not remember that there is any Ar∣gument for the maintenance of Vacuum in the writings of Divines: Therefore I will quit that Argument, and come to another. If you take a Glass Vial with a narrow neck, and ha∣ving

suckt it, dip it presently at the neck into a bason of water, you shall manifestly see the water rise into the Vial. Is not this a certain signe that you had suckt out some of the Air, and consequently that some part of the Vial was left empty?

B.

No: For when I am about to suck, and have Air in my mouth, contracting my Cheeks I drive the same against the Air in the Glass, and thereby against every part of the sides of the hard Glass. And this gives to the Air within an Endeavour outward, by which (if it be presently dipt into the water) it will pe∣netrate and enter into it. For Air if it be prest will enter into any Fluid, much more into wa∣ter. Therefore there shall rise into the Vial so much water as there was Air forced into the Bason.

A.

This I confess is possible, and not im∣probable.

B.

If sucking would make Vacuum, what would become of those women that are Nur∣ses? Should they not be in a very few days exhausted, were it not that either the Air which is in the Childs mouth penetrateth the Milk as it descends, and passeth through it, or the Breast is contracted?

A.

From what Experiment can you evi∣dently infer that there is no Vacuum?

B.

From many, and such as to almost all men are known and familiar. If two hard Bodies,

flat and smooth, be joyned together in a com∣mon Superficies parallel to the Horizontal Plain, you cannot without great force pull them asunder, if you apply your force perpen∣dicularly to the common Superficies: But if you place that common Superficies erect to the Horizon, they will fall asunder with their own weight. From whence I argue thus: Since their Contiguity, in what posture soever, is the same, and that they cannot be pull'd asun∣der by a perpendicular force without letting in the ambient Air in an instant, which is im∣possible; or almost in an instant, which is dif∣ficult: and on the other side, when the com∣mon Superficies is erect, the weight of the same hard Bodies are able to break the Contiguity, and let in the Air successively, it is manifest that the difficulty of Separation proceeds from this, that neither Air nor any other Body can be moved to any (how small soever) distance in an instant; but may easily be moved (the hardness at the sides once mastered) successive∣ly. So that the Cause of this difficulty of Se∣paration is this, that they cannot be parted ex∣cept the Air or other matter can enter and fill the space made by their diremption. And if they were infinitely hard, not at all. And hence also you may understand the Cause why any hard Body, when it is suddenly broken, is heard to crack; which is the swift Motion of the Air to fill the space between.

Another Experiment, and commonly known, is of a Barrel of Liquor, whose Tap-hole is very little, and the Bung so stopt as to admit no Air; for then the Liquor will not run: but if the Tap-hole be large it will, because the Air prest by a heavier Bodie will pierce through it into the Barrel. The like reason holds of a Gardeners Watering-pot, when the holes in the bottom are not too great. A third Experiment is this: Turn a thin Brass kettle the bottom upwards, and lay it flat upon the Water. It will sink till the wa∣ter rise within to a certain height, but no high∣er: Yet let the bottom be perforated, and the Kettle will be full and sink, and the Air rise again through the water without. But if a Bell were so laid on, it would be fill'd and sink, though it were not perforated, because the weight is greater than the weight of the same bulk of water.

A.

By these Experiments, without any more, I am convinced, that there is not actually in Nature any Vacuum; but I am not sure but that there may be made some little place empty, and this from two Experiments, one whereof is Tor∣ricellius his Experiment, which is this: Take a Cylinder of Glass, hollow throughout, but close at the end, in form of a Sack.

B.

How long?

A.

As long as you will, so it be more than 29 inches.

B.

And how broad?

A.

As broad as you will, so it be broad e∣nough to pour into it Quicksilver. And fill it with Quicksilver, and stop up the Entrance with your finger, so as to unstop it again at your pleasure. Then set down a Bason, or (if you will) a Sea of Quicksilver, and in∣verting the Cylinder full as it is, dip the end into the Quicksilver, and remove your finger, that the Cylinder may empt it self. Do you conceive me? For there is so many passing by, that I cannot paint it.

B.

Yes, I conceive you well enough. What follows?

A.

The Quicksilver will descend in the Cylinder, not till it be level with that in the Bason according to the nature of heavie Flu∣ids, but stay and stand above it at the height of 29 inches or very neer it, the bottom being now uppermost that no Air can get in.

B.

What do you infer from this?

A.

That all the Cavitie above 29 inches is fill'd with Vacuum.

B.

'Tis very strange that I, from this same Experiment, should infer (and I think evi∣dently) that it is fill'd with Air. I pray, tell me, when you had inverted the Cylinder, full as it was, and stopt with your finger, dipt into the Bason, if you had then removed your fin∣ger, whether you think the Quicksilver would not all have fallen out?

A.

No sure. The Air would have been prest upward through the Quicksilver it self: For a man with his hand can easily thrust a Bladder of Air to the bottom of a Bason of Quicksilver.

B.

It is therefore manifest that Quicksilver can press the Air through the same Quicksil∣ver.

A.

'Tis manifest; and also it self rise into the Air.

B.

What cause then can there be, why it should stand still at 29 inches above the level of the Bason, rather than any place else?

A.

'Tis not hard to assigne the cause of that. For so much Quicksilver as was above the 29 inches, will raise the first level of that in the bason, as much as if you had pour'd it on; and thereby bring it to an Aequilibrium. So that I see plainly now, that there is no ne∣cessity of Vacuum from this Experiment. For I considered onely that naturally Quicksilver cannot ascend in Air, nor Air descend in Quicksilver, though by force it may.

B.

Nor do I think that Torricellius or any other Vacuist thought of it more than you. But what is the second Experiment?

A.

There is a Sphere of Glass, which they call a Recipient, of the Capacity of three or four Gallons. And there is inserted into it the end of a hollow Cylinder of Brass above a foot long; so that the whole is one Vessel,

and the bore of the Cylinder three inches Dia∣meter. Into which is thrust by force a solid Cylinder of Wood, covered with leather so just, as it may in every point exactly touch the Concave Superficies of the Brass. There is also to let out the Air which the wooden Cy∣linder as it enters (called the Sucker) drives before it, a Flap to keep out the External Air while they are pulling the Sucker. Besides, at the top of the Recipient there is a hole to put into it any thing for Experiment. The Sucker being now forc'd up into the Cylinder, what do you think must follow?

B.

I think it will require as much strength to pull it back, as it did to force it in.

A.

That is not it I ask, but what would hap∣pen to the Recipient.

B.

I think so much Air as would fill the place the Sucker leaves, would descend into it out of the Recipient; and also that just so much from the External Air would enter into the Recipient, between the Brass and the Wood, at first very swiftly, but, as the place increased, more leasurely.

A.

Why may not so much Air rather de∣scend into the place forsaken, and leave as much Vacuum as that comes to, in the Reci∣pient? For otherwise no Air will be pumpt out; nor can that wooden Pestle be called a Sucker.

B.

That's it I say. There is no Air either pumpt or sucked out.

A.

How can the Air pass between the Lea∣ther and the Brass, or between the Leather and the Wood being so exactly contiguous, or through the Leather it self?

B.

I conceive no such exact contiguity, nor such fastness of the Leather: For I never yet had any that in a storm would keep out either Air or Water.

A.

But how then could there be made in the Recipient such strange alteration both on animate and inanimate Bodies?

B.

I will tell you how: The Air descends out of the Recipient, because the Air which the Sucker removeth from behinde it self as it is pulling out, has no place to retire into without. And therefore is driven into the Engine between the wood of the Sucker and the brass of the Cylinder, and causes as much Air to come into the place forsaken by the re∣tiring Sucker; which causeth by oft repetition of the force, a violent circulation of the Air within the Recipient, which is able quickly to kill any thing that lives by respiration, and make all the alterations that have appeared in the Engine.