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. VI. Of the Causes and Effects of Heat and Cold. (Book 6)

A.

'TIs a fine day, and pleasant walking through the Fields, but that the Sun is a little too hot.

B.

How know you that the Sun is hot?

A.

I feel it.

B.

That is to say, you know that your self, but not that the Sun is hot. But when you finde your self hot, what Body do you feel?

A.

None.

B.

How then can you infer your heat from the Sense of Feeling? Your walking may have made you hot: Is Motion therefore hot? No. You are to consider the Concomitants of your heat; as, that you are more faint, or more ruddy, or that you sweat, or feel some Endea∣vour of Moisture or Spirits tending outward; and when you have found the Causes of those Accidents, you have found the Causes of Heat, which in a living Creature, and specially in a Man, is many times the Motion of the parts within him, such as happen in sickness, anger, and other passions of the minde; which are not in the Sun nor in Fire.

A.

That which I desire now to know, is what Motions and of what Bodies without me are the Efficient Causes of my Heat.

B.

I shewed you yesterday in discoursing of Rain, how by this compounded Motion of the Suns Body, the Air was every way at once thrust off West and East; so that where it was contiguous, the small parts of the water were forced to rise, for the avoiding of Vacuum. Think then that your hand were in the place of water so exposed to the Sun. Must not the Sun work upon it as it did upon the Water? Though it break not the skin, yet it will give to the inner Fluids and looser parts of your hand, an Endeavour to get forth, which will extend the skin, and in some climates fetch up the bloud, and in time make the skin black. The Fire also will do the same to them that often sit with their naked skins too neer it. Nay, one may sit so neer (without touching it) as it shall blister or break the skin, and fetch up both spirits and bloud mixt into a pu∣trid oyly matter sooner, than in a Furnace Oyl can be extracted out of a Plant.

A.

But if the Water be above the Fire in a Kettle, what then will it do? Shall the par∣ticles of water go toward the Fire, as it did toward the Sun?

B.

No. For it cannot. But the Motion of the parts of the Kettle which are caused by the Fire, shall dissipate the Water into Vapour till it be all cast out.

A.

What is that you call Fire? Is it a hard or Fluid Body?

B.

It is not any other Body but that of the shining coal; which coal, though extinguish∣ed with Water, is still the same Body. So al∣so in a very hot Furnace, the hollow spaces between the shining coals, though they burn that you put into them, are no other Body than Air moved.

A.

Is it not Flame?

B.

No. For flame is nothing but a multi∣tude of Sparks, and Sparks are but the A∣tomes of the Fewel dissipated by the incredi∣ble swift Motion of the Movent, which makes every Spark to seem a hundred times greater than it is, as appears by this; That when a man swings in the Air a small stick fired at one end (though the Motion cannot be very swift) yet the Fire will appear to the eye to be a long, streight, or crooked Line. There∣fore a great many sparks together flying up∣ward, must needs appear unto the sight as one continued Flame. Nor are the sparks striken out of a Flint any thing else but small particles of the stone, which by their swift Motion are made to shine. But that Fire is not a substance of it self, is evident enough by this, that the Sun-beams passing through a Globe of Water will burn as other fire does. Which beams, if they were indeed Fire, would be quenched in the passage.

A.

This is so evident, that I wonder so wise men as Aristotle and his followers, for so long a time could hold it for an Element, and one of the primary parts of the Universe. But the Natural heat of a man or other living Creature, whence proceedeth it? Is there any thing within their Bodies that hath this com∣pounded Motion?

B.

At the breaking up of a Deer I have seen it plainly in his Bowels as long as they were warm. And it is called the Peristaltique Motion, and in the Heart of a Beast newly ta∣ken out of his Body; and this Motion is called Systole and Diastole. But they are both of them this compounded Motion, whereof the former causeth the food to Winde up and down through the guts, and the later makes the Circulation of the Blood.

A.

What kind of Motion is the Cause of Cold? Methinks it should be contrary to that which causeth Heat.

B.

So it is in some respect. For seeing the Motion that begets Heat, tendeth to the sepa∣ration of the parts of the Body whereon it acteth, it stands with reason, that the Motion which maketh Cold, should be such as sets them closer together. But contrary Motions are (to speak properly) when upon two ends of a Line two Bodies move towards each other, the Effect whereof is to make them meet. But each of them (as to this Question) is the same.

A.

Do you think (as many Philosophers have held and now hold) that Cold is no∣thing but a privation of Heat?

B.

No. Have you never heard the Fable of the Satyre that dwelling with a Husband∣man, and seeing him blow his fingers to warm them, and his Pottage to cool it, was so scan∣dalized, that he ran from him, saying he would no longer dwell with one that could blow both Hot and Cold with one breath? Yet the Cause is evident enough. For the Air which had gotten a Calefactive power from his vital parts, was from his mouth and throat gently diffused on his fingers, and retained still that power. But to cool his Pottage he streigh∣tened the passage at his lips, which extingui∣shed the Calefactive Motion.

A.

Do you think Wind the general Cause of Cold? If that were true, in the greatest Winds we should have the greatest Frosts.

B.

I mean not any of those uncertain Winds which, I said, were made by the Clouds, but such as a Body moved in the Air makes to and against it self. For it is all one Motion of the Air, whether it be carried against the Body, or the Body against it. Such a Wind as is constant (if no other be stirring) from East to West made by the Earth turning dayly up∣on its own Centre. Which is so swift, as (ex∣cept it be kept off by some hill) to kill a man, as by Experience hath been found by

those who have passed over great Mountains, and specially over the Andies which are op∣posed to the East. And such is the Wind which the Earth maketh in the Air by her An∣nual Motion, which is so swift, as that (by the Calculation of Astronomers) to go Sixty miles in a minute of an hour. And therefore this must be the Motion which makes it so cold a∣bout the Poles of the Ecliptique.

A.

Does not the Earth make the Wind as great in one part of the Ecliptique as in ano∣ther?

B.

Yes. But when the Sun is in Cancer, it tempers the Cold, and still less and less, but least of all in the Winter-Solstice, where his beams are most oblique to the Superficies of the Earth.

A.

I thought the greatest Cold had been about the Poles of the Aequator.

B.

And so did I once. But the reason com∣monly given for it is so improbable, that I do not think so now. For the Cause they render of it, is onely, that the Motion of the Earth is swiftest in the Aequinoctial, and slowest about the Poles; and consequently (since Motion is the Cause of Heat, and Cold is but (as 'twas thought) a want of the same) they in∣ferr'd that the greatest Cold must be about the Poles of the Aequinoctial. Wherein they miscounted. For not every Motion causeth Heat, but this agitation onely, which we call

compounded Motion; though some have al∣leadged Experience for that opinion; as that a Bullet out of a Gun will with its own swift∣ness melt. Which I never shall believe.

A.

'Tis a common thing with many Philo∣sophers to maintain their Fancies with any rash report, and sometimes with a Lye. But how is it possible that so soft a Substance as water should be turned into so hard a Sub∣stance as Ice?

B.

When the Air shaves the Globe of the Earth with such swiftness, as that of Sixty miles in a minute of an hour, it cannot (where it meets with still water) but beat it up into small and undistinguishable bubbles, and in∣volve it self in them as in so many bladders or skins of Water. And Ice is nothing else but the smallest imaginable parts of Air and Water mixt; which is made hard by this compoun∣ded Motion, that keeps the parts so close toge∣ther, as not to be separated in one place with∣out disordering the Motion of them all. For when a Body will not easily yield to the im∣pression of an external Movent in one place without yielding in all, we call it Hard; And when it does, we say 'tis Soft.

A.

Why is not Ice as well made in a moved as in a still water? Are there not great Seas of Ice in the Northern parts of the Earth?

B.

Yes, and perhaps also in the Southern parts. But I cannot imagine how Ice can be

made in such agitation as is always in the open Sea made by the Tides and by the Winds. But how it may be made at the Shoar, it is not hard to imagine. For in a River or Current, though swift, the water that adhereth to the banks is quiet, and easily by the Motion of the Air driven into small insensible bubbles; and so may the water that adhereth to those bub∣bles, and so forwards till it come into a stream that breaks it, and then it is no wonder though the fragments be driven into the open Sea, and freeze together into greater lumps. But when in the open Sea, or at the Shoar, the Tide or a great Wave shall arise, this young and tender Ice will presently be washt away. And there∣fore I think it evident, that as in the Thames the Ice is first made at the banks where the Tide is weak or none, and broken by the stream comes down to London, and part goes to the Sea floating till it dissolve, and part (being too great to pass the Bridge) stoppeth there and sustains that which follows, till the River be quite frozen over: So also the Ice in the Northern Seas begins first at the banks of the Continent and Islands which are scituated in that Climate, and then broken off, are car∣ried up and down, and one against another, till they become great Bodies.

A.

But what if there be Islands, and narrow Inlets of the Sea, or Rivers also about the Pole of the Aequinoctial?

B.

If there be, 'tis very likely the Sea may also there be covered all over with Ice. But for the truth of this, we must stay for some fur∣ther discovery.

A.

When the Ice is once made and hard, what dissolves it?

B.

The Principal Cause of it, is the weight of the water it self; but not without some abate∣ment in the Stream of the Air that hardned it; as when the Sun-beams are less oblique to the Earth, or some contrary Wind resisteth the stream of the Air. For when the impediment is removed, then the nature of the water on∣ly worketh, and (being a heavie Body) down∣ward.

A.

I forgot to ask you, Why two pieces of Wood rub'd swiftly one against another, will at length set on fire.

B.

Not onely at length, but quickly, if the Wood be dry. And the Cause is evident, viz. the compounded Motion which dissipates the external small parts of the Wood. And then the inner parts must of necessity (to preserve the Plenitude of the Universe) come after; first the most Fluid, and then those also of greater consistence, which are first erected, and (the Motion continued) made to flie swiftly out; whereby the Air driven to the Eye of the beholder, maketh that fancie which is cal∣led Light.

A.

Yes. I remember you told me before,

that upon any strong pressure of the Eye, the resistance from within would appear a Light. But to return to the enquiry of Heat and Cold, there be two things that beyond all o∣ther put me into admiration. One is the swift∣ness of kindling in Gunpowder. The other is the freezing of Water in a Vessel (though not far from the fire) set about with other water with Ice and Snow in it. When Paper or Flax is flaming, the flame creeps gently on; and if a house full of Paper were to be burnt with putting a Candle to it, it will be long in burning; whereas a spark of fire would set on flame a mountain of Gunpowder in almost an instant.

B.

Know you not Gunpowder is made of the powder of Charcole, Brimstone, and Salt∣peter? Whereof the first will kindle with a spark, the second flame as soon as toucht with fire; and the third blows it, as being com∣posed of many Orbs of Salt fill'd with Air, and as it dissolveth in the flame, furiously blowing increaseth it. And as for making Ice by the fire side. It is manifest that whilst the Snow is dissolving in the external vessel, the Air must in the like manner break forth, and shave the Superficies of the inner vessel, and work through the water till it be frozen.

A.

I could easily assent to this, if I could conceive how the Air that shaves (as you say) the outside of the Vessel, could work through

it. I conceive well enough a pail of water with Ice or Snow dissolving in it, and how it causeth Wind. But how that Wind should communicate it self through the vessel of wood or metal, so as to make it shave the Su∣perficies of the water which is within it, I do not so well understand.

B.

I do not say the inner Superficies of the vessel shaves the water within it. But 'tis manifest that the Wind made in the Pail of water by the melting Snow or Ice presseth the sides of the Vessel that standeth in it; and that the pressure worketh clean through, how hard soever the Vessel be; and that again worketh on the water within, by restitution of its parts, and so hardeneth the water by degrees.

A.

I understand you now. The Ice in the Pail by its dissolution transfers its Hardness to the water within.

B.

You are merry. But supposing, as I do, that the Ice in the Pail is more than the water in the Vessel, you will finde no absurdity in the Argument. Besides, the Experiment, you know, is common.

A.

I confess it is probable. The Greeks have the word 〈 in non-Latin alphabet 〉〈 in non-Latin alphabet 〉 (whence the Latins have their word Frigus) to signifie the curling of water by the Wind; and use the same also for Horrour, which is the passion of one that co∣meth suddenly into a cold Air, or is put into

a sudden affright, whereby he shrinks, and his hair stands upright. Which manifestly shews that the Motion which causeth Cold, is that which pressing the Superficies of a Body, sets the parts of it closer together. But to pro∣ceed in my Quaeries. Monsieur Des Cartes, who (you know) hath written somewhere, that the noise we hear in Thunder, proceeds from brea∣king of the Ice in the Clouds; What think you of it? Can a Cloud be turned into Ice?

B.

Why not? A Cloud is but Water in the Air.

A.

But how? For he has not told us that.

B.

You know that 'tis onely in Summer, and in hot weather, that it Thunders; or if in Winter, it is taken for a Prodigie. You know also, that of Clouds, some are higher, some lower, and many in number, as you cannot but have oftentimes observed, with spaces be∣tween them. Therefore, as in all Currents of water, the Water is there swiftest where it is streightned with Islands, so must the Current of Air made by the Annual Motion be swiftest there, where it is checkt with many Clouds through which it must (as it were) be strai∣ned, and leave behinde it many small particles of earth, always in it, and in hot weather more than ordinary.

A.

This I understand, and that it may cause Ice. But when the Ice is made, how is it broken? And why falls it not down in shi∣vers?

B.

The particles are inclosed in small Ca∣verns of the Ice; and their Natural Motion being the same which we have ascribed to the Globe of the Earth, requires a sufficient space to move in. But when it is imprisoned in a less room that that, then a great part of the Ice breaks: And this is the Thunder-clap. The Murmur following is from the settling of the Air. The Lightening is the fancie made by the recoiling of the Air against the Eye. The fall is in Rain, not in Shivers; because the prisons which they break are extreme narrow, and the shivers being small, are dissolved by the Heat. But in less Heat they would fall in Drops of Hail, that is to say, half frozen by the shaving of the Air as they fall, and be in a very little time (much less than Snow or Ice) dissolved.

A.

Will not that Lightning burn?

B.

No. But it hath often kill'd men with Cold. But this extraordinary swiftness of Lightning consisteth not in the Expansion of the Air, but in a straight and direct stream from where it breaks forth; which is in many places successively, according to the Motion of the Cloud.

A.

Experience tells us that. I have now done with my Problems concerning the great Bodies of the world, the Stars, and Element of Air in which they are moved, and am therein satisfied, and the rather, because you have an∣swered

me by the Supposition of one onely Motion, and commonly known, and the same with that of Copernicus, whose Opinion is re∣ceived by all the Learned; and because you have not used any of these empty terms, Sym∣pathy, Antipathy, Antiperistasis, &c. for a na∣tural Cause, as the old Philosophers have done to save their credit. For though they were many of them wise men, as Plato, Aristotle, Seneca, and others, and have written excel∣lently of Morals and Politiques, yet there is very little Natural Philosophy to be gathered out of their Writings.

B.

Their Ethiques and Politiques are plea∣sant reading, but I finde not any argument in their discourses of Justice or Vertue drawn from the supreme Authority, on whose Laws all Justice, Vertue, and good Politiques de∣pend.

A.

Concerning this Cover, or (as some have called it) the Scurf or Scab of the Terre∣strial Star, I will begin with you to morrow. For it is a large Subject, containing Animals, Vegetables, Metals, Stones, and many other kinds of Bodies, the knowledge whereof is desired by most men, and of the greatest and most general profit.

B.

And this is it, in which I shall give you the least satisfaction; so great is the variety of Motion, and so concealed from humane senses.