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. VII. Of Hard and Soft, and of the Atomes that flie in the Air. (Book 7)

A.

COncerning this Cover of the Earth, made up of an infinite number of parts of different natures, I had much ado to finde any tolerable method of enquiry. But I re∣solved at last to begin with the Questions con∣cerning Hard and Soft, and what kinde of Motion it is that makes them so. I know that in any pulsion of Air, the parts of it go innu∣merable and inexplicable ways; but I ask only if every point of it be moved.

B.

No. If you mean a Mathematical point, you know it is impossible. For nothing is movable but Body. But I suppose it divisible (as all other Bodies) into parts divisible. For no Substance can be divided into Nothings.

A.

Why may not that Substance within our Bodies, which are called Animal spirits, be a∣nother kind of Body, and more subtile than the common Air?

B.

I know not why, no more than you or any man else knows why it is not very Air, though purer perhaps than the common Air, as being strained through the blood into the Brain and

Nerves. But howsoever that be, there is no doubt, but the least parts of the common Air, (respectively to the whole) will easilier pierce (with equal Motion) the Body that resisteth them, than the least parts of water. For it is by Motion onely that any mutation is made in any thing; and all things standing as they did, will appear as they did. And that which changeth Soft into Hard, must be such as makes the parts not easily to be moved without being moved all together; which cannot be done but by some Motion compounded. And we call Hard, that whereof no part can be put out of order without disordering all the rest; which is not easily done.

A.

How Water and Air beaten into extreme small Bubbles is hardned into Ice, you have told me already, and I understand it. But how a soft Homogeneous Body, as Air, or Water, should be so hardned, I cannot ima∣gine.

B.

There is no hard Body that hath not also some degree of Gravity; and consequently, being loose, there must be some Efficient Cause, that is, some Motion, when it is severed from the Earth, to bring the same to it again. And seeing this compounded Motion gives to the Air and Water an Endeavour from the Earth, the Motion which must hinder it, must be in a way contrary to the compounded Mo∣tion of the Earth. For whatsoever, having

been asunder, comes together again, must come contrary ways, as those that follow one ano∣ther go the same way, though both move up∣on the same Line.

A.

What Experiment have you seen to this purpose?

B.

I have seen a drop of glass like that of the second Figure, newly taken out of the furnace, and hanging at the end of an Iron rod, and yet Fluid, and let fall into the water and hardned. The Club-end of it A A co∣ming first to the water, the tail B C following it. 'Tis proved before, that the motion that makes it is a compounded Motion, and gives an Endeavour outward to every part of it; and that the Motion which maketh Cold, is such as shaving the Body in every point of con∣tact, and turning it, gives them all an Endea∣vour inward. Such is this Motion made by the sinking of the hot and fluid glass into the water. 'Tis therefore manifest that the Mo∣tion which hardneth a Soft Body, must in every point of contact be in the contrary way to that which makes a hard body Soft. And further, that slender tail B C shall be made much more hard than common Glass. For towards the upper end, in C, you cannot easily break it, as small as it is. And when you have broken it, the whole Body will fall into dust, as it must do, seeing the bending is so difficult. For all the parts are bent with such force, that

upon the breaking at D, by their sudden resti∣tution to their liberty, they will break toge∣ther. And the cause why the tail B C, being so slender, becomes so hard, is, that all the En∣deavour in the great part A B, is propagated to the small part B C, in the same manner as the force of the Sun-beams is derived almost to a point by a Burning-glass. But the Cause why, when it is broken in D, it breaks also in so many other places, is, that the Endeavour in all the other parts (which is called the Spring) unbends it; from whence a Motion is caused the contrary way, and that Motion continued bends it more the other way and breaks it, as a Bow over-bent is broken into shivers by a sudden breaking of the string.

A.

I conceive now how a Body which ha∣ving been Hard and softned again, may be re∣hardned; but how a Fluid and meer Homo∣geneous Body, as Air or Water, may be so, I see not yet. For the hardning of water is making a hard Body of two Fluids, whereof one (which is the water) hath some tenacity; and so a man may make a Bladder hard with blowing into it.

B.

As for meer Air, which hath no Natural Motion of it self, but is moved onely by other Bodies of a greater consistence, I think it im∣possible to be hardned. For the parts of it so easily change places, that they can never be fixt by any Motion. No more I think can

Water, which though somewhat less Fluid, is with an insensible force very easily broken.

A.

It is the opinion of many learned men, that Ice (in long time) will be turned into Christal; and they alleadge Experience for it. For they say that Christal is found hanging on high Rocks in the Alps like Isicles on the Eaves of a house, and why may not that have former∣ly been Ice, and in many years have lost the power of being reduced?

B.

If that were so, it would still be Ice, though also Christal: Which cannot be, be∣cause Christal is heavier than Water, and therefore much heavier than Ice.

A.

Is there then no transubstantiation of Bodies but by mixture?

B.

Mixture is no transubstantiation.

A.

Have you never seen a Stone that seemed to have been formerly Wood, and some like Shells, and some like Serpents, and others like other things?

B.

Yes. I have seen such things, and par∣ticularly I saw at Rome in a Stone-cutters work-house a Billet of Wood, as I thought it, partly covered with bark, and partly with the grain bare, as long as a mans Arm, and as thick as the Calf of a mans Leg; which handling I found extreme heavie, and saw a small part of it which was polished, and had a very fine Gloss, and thought it a substance between Stone and Metal, but neerest to Stone. I

have seen also a kind of Slate painted natural∣ly with Forest-work. And I have seen in the hands of a Chymist of my acquaintance at Paris, a broken Glass, part of a Retort, in which had been the Rozin of Turpentine, wherein though there were left no Rozin, yet there appeared in the piece of Glass many Trees; and Plants in the ground about them, such as grow in Woods; and better designed than they could be done by any Painter; and continued so for a long time. These be great wonders of Nature, but I will not undertake to shew their causes. But yet this is most cer∣tain, that nothing can make a hard Body of a Soft, but by some Motion of its parts. For the parts of the Hardest Body in the world can be no closer together than to touch; and so close are the parts of Air and Water, and con∣sequently they should be equally Hard, if their smallest parts had not different Natural Motions. Therefore if you ask me the Causes of these Effects, I answer, They are different Motions. But if you expect from me how and by what Motions, I shall fail you. For there is no kind of Substance in the World now, that was not at the first Creation, when the Creator gave to all things what Natural and special Motion he thought good. And as he made some Bodies wondrous great, so he made others wondrous little. For all his works are wondrous. Man can but guess, nor

guess further, than he hath knowledge of the variety of Motion. I am therefore of opinion, that whatsoever perfectly Homogeneous is Hard, consisteth of the smallest parts (or, as some call them, Atomes) that were made Hard in the beginning, and consequently by an Eter∣nal Cause; and that the hardness of the whole Body is caused onely by the contact of the parts by pressure.

A.

What Motion is it that maketh a hard Body to melt?

B.

The same compounded Motion that heats, namely, that of Fire, if it be strong e∣nough. For all Motion compounded is an Endeavour to dissipate (as I have said before) the parts of the Body to be moved by it. If therefore hardness consist onely in the pressing Contact of the least parts, this Motion will make the same parts slide off from one ano∣ther, and the whole to take such a figure as the weight of the parts shall dispose them to, as in Lead, Iron, Gold, and other things mel∣ted with Heat. But if the small parts have such figures as they cannot exactly touch, but must leave spaces between them filled with Air or other Fluids, then this Motion of the fire, will dissipate those parts some one way, some another, the Hard part still hard; as in the burning of Wood or Stone into Ashes or Lime. For this Motion is that which maketh Fermentation, scattering dissimilar parts, and congregating similar.

A.

Why do some hard Bodies resist breaking more one way than another?

B.

The Bodies that do so, are for the most part Wood, and receive that quality from their generation. For the heat of the Sun in the Spring-time draweth up the moisture at the Root, and together with it the small parts of the Earth, and twisteth it into a small twig by its Motion upwards to some length, but to very little other dimensions, and so leaves it to dry till the Spring following; and then does the same to that, and to every small part round about it; so that upward the strength is doubled, and the next year trebled, &c. And these are called the grain of the Wood, and but touch one another, like sticks with little or no binding, and therefore can hardly be broken across the grain, but easily all-along it. Also some other hard Bodies have this quality of being more fragile one way than a∣nother, as we see in quarrels of a Glass-win∣dow, that are aptest many times to break in some crooked Line. The cause of this may be, that when the glass, hot from the Furnace, is poured out upon a Plain, any small stones in or under it will break the stream of it into di∣vers lines, and not onely weaken it, but also cause it falsly to represent the Object you look on through it.

A.

What is the Cause why a Bow of Wood or Steel, or other very hard Body, being bent,

but not broken, will recover its former de∣gree of straightness?

B.

I have told you already, how the smal∣lest parts of a hard Body have every one (by the generation of hardness) a Circular, or other compounded Motion; such Motion is that of the smallest parts of the Bow. Which Circles in the bending you press into narrower figures, as a Circle into an Ellipsis, and an El∣lipsis into a narrower but longer Ellipsis with violence; which turns their Natural Motion against the outward parts of the Bow so bent, and is an Endeavour to stretch the Bow into its former posture. Therefore if the impedi∣ment be removed, the Bow must needs reco∣ver its former Figure.

A.

'Tis manifest; and the cause can be no other but that, except the Bow have Sense.

B.

And though the Bow had Sense, and Appetite to boot, the Cause will be still the same.

A.

Do you think Air and Water to be pure and Homogeneous Bodies?

B.

Yes, and many Bodies both Hard and Heavie to be so too, and many liquors also besides water.

A.

Why then do men say they finde one Air healthy, another infectious?

B.

Not because the nature of the Air varies, but because there are in the Air drawn, or rather, beaten up by the Sun, many little

Bodies, whereof some have such Motion as is healthful, others such as is hurtful to the life of man. For the Sun (as you see in the ge∣neration of Plants) can fetch up Earth as well as Water; and from the driest ground a∣ny kind of Body that lieth loose, so it be small enough, rather than admit any Emptiness. By some of these small Bodies it is that we live; which being taken in with our breath, pass into our blood, and cause it (by their compounded Motion) to circulate through the Veins and Arteries; which the blood of it self (being a heavie Body) without it cannot do. What kind of substance these Atomes are, I cannot tell. Some suppose them to be Nitre. As for those infectious creatures in the Air, whereof so many die of the Plague, I have heard that Monsieur Des Cartes, a very ingenious man, was of opinion, that they were little Flies. But what grounds he had for it, I know not, though there be many Experi∣ments that invite me to believe it. For first, we know that the Air is never universally in∣fected over a whole Country, but onely in or neer to some populous Town. And therefore the cause must also be partly ascribed to the multitude thronged together, and constrained to carry their Excrements into the fields round about and neer to their habitation. Which in time fermenting breed Worms, which com∣monly in a month or little more, naturally be∣come

Flies; and though engendred at one Town, may flie to another. Secondly, in the beginning of a Plague, those that dwell in the Suburbs, that is to say, neerest to this corru∣ption, are the poorest of the people, that are nourished for the most part with the Roots and Herbs which grow in that corrupted dirt; so that the same filth makes both the blood of poor people, and the substance of the Fly. And 'tis said by Aristotle, that every thing is nouri∣shed by the matter whereof it is generated. Thirdly, when a Town is infected, the Gentle∣men, and those that live on wholsomest food, scarce one of Five hundred die of the Plague. It seems therefore, whatsoever creatures they be that invade us from the Air, they can dis∣cern their proper nourishment, and do not en∣ter into the mouth and nostrils with the breath of every man alike, as they would do if they were inanimate. Fourthly, a man may carry the infection with him a great way into the Country in his Clothes, and infect a Village. Shall another man there draw the Infection from the Clothes onely by his breath? Or from the Hangings of a Chamber wherein a man hath died? It is impossible. Therefore what∣soever killing thing is in the Clothes or Hang∣ings, it must rise and go into his mouth or no∣strils before it can do him hurt. It must there∣fore be a Fly, whereof great numbers get in∣to the blood, and there feeding and breeding

Worms, obstruct the Circulation of the blood, and kill the man.

A.

I would we knew the palate of those lit∣tle Animals; we might perhaps finde some medicine to fright them from mingling with our breath. But what is that which kills men that lie asleep too neer a Charcole-fire? Is it another kind of Fly? Or is Charcole veni∣mous?

B.

It is neither Fly nor venim, but the Ef∣fect of a flameless glowing fire, which dissi∣pates those Atomes that maintain the circula∣tion of the blood; so that for want of it, by degrees they faint, and being asleep cannot remove, but in short time, there sleeping die, as is evident by this, that being brought into the open Air (without other help) they re∣cover.

A.

'Tis very likely. The next thing I would be informed of, is the nature of Gravi∣ty. But for that, if you please, we will take another day.