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. X. Of Transparence, Refraction; and of the Power of the Earth to produce living Creatures. (Book 10)

A.

THinking upon what you said yester∣day, it lookt like a generation of living Creatures. I saw the love between the Loadstone and the Iron in their mutual attra∣ction, their engendring in their close and con∣trary Motion; and their issue in the Iron, which being touch'd hath the same attractive vertue. Now seeing they have the same in∣ternal motion of parts with that of the Earth, why should not their substance be the same, or very near a kin?

B.

The most of them (if not all) that have written of this Subject, when they call the Loadstone a Terrella, seem to think as you do. But I, except I could find proof for it, will not affirm it. For the Earth attracteth all kind of Bodies but Air, and the Loadstone none but Iron. The Earth is a Star, and it were too bold to pronounce any sentence of its substance, especially of the Planets, that are so lapt up in their several Coats, as that they cannot work on our Eyes, or any Organ of our other Senses.

A.

I come therefore now to the business of the day. Seeing all Generation, Augmenta∣tion, and Alteration is local Motion, how can a Body not Transparent be made Transpa∣rent?

B.

I think it can never be done by the Art of Man. For as I said of Hard and Heavy Bodies in the Creation, so I think of Diapha∣nous, that the very same Individual Body which was not Transparent then, shall never be made Transparent by Humane Art.

A.

Do not you see that every day Men make Glass, and other Diaphanous Bodies not much inferior in beauty to the fairest Gems?

B.

It is one thing to make one Transparent of many by mixture, and another to make Transparent of not Transparent. Any very hard Stone, if it be beaten into small Sands,

such as is used for Hour-Glasses, every one of those Sands, if you look upon it with a Mi∣croscope, you will find to be Transparent; and the harder and whiter the Sone is, so much the more Transparent, as I have seen in the Stone of which are made Milstones, which Stone is here called Greet. And I doubt not but the Sands of white Marble must be more Transparent. But there are no Sands so Tran∣sparent that they have not a scurf upon them as hard, perhaps, as the Stone it self; which they whose profession it is to make Glass, have the Art to scour and wash away. And therefore I think it no great wonder to bring those Sands into one Lump, though I know not how they do it.

A.

I know they do it with Lie made with a Salt extracted from the Ashes of an Herb, of which Salt they make a strong Lie, and mingle it with the Sand, and then bake it.

B.

Like enough. But still it is a Com∣pound of two Transparent Bodies, whereof one is the Natural Stone, the other is the Morter. This therefore doth not prove, that one and the same Body, of not Transparent can be made Transparent.

A.

Since they can make one Transparent Body of many, why do they not of a great many small sparks of natural Diamant com∣pound one great one? It would bear the charges of all the Materials, and beside, enrich them.

B.

'Tis probable it would. But it may be they know no Salt that howsoever prepared, which (with how great a Fire soever) can make them melt. And, it may be the true Chrystal of the Mountain, which is found in great pieces in the Alps, is but a compound of many small ones, and made by the Earths Annual Motion. For it is a very swift Mo∣tion. Suppose now that within a very small Cavern of those Rocks whose smallest Atomes are Chrystal, and the Cavity fill'd with Air; and consider what a tumult would be made by the swift reciprocation of that Air; whether it would not in time separate those Atomes from the Rock, and jumbling them together make them rub off their scruf from one ano∣ther, and by little and little to touch one ano∣ther in polish'd plains, and consequently stick together, till in length of time they become one lump of clean Chrystal.

A.

I believe that the least parts of created substances lay mingled together at first, till it pleased God to separate all dissimilar natures, and congregate the similar, to which this Annual Motion is proper. But they say that Chrystal is found in the open Air hanging like Icicles upon the Rocks. Which (if true) defeats this supposition of a narrow Cavern. And therefore I must have some further ex∣perience of it before I make it my opinion. But howsoever, it still holds true that Dia∣phanous

Bodies of all sorts, in their least parts, were made by God in the beginning of the World. But it may be true, notwithstand∣ing those Icicles. For the force of the Air that could break off those Diaphanous Atomes in a Cavern, can do the same in the open Air. And I know that a less force of Air can break some Bodies into small pieces, not much less hard than Chystal, by corrupting them.

B.

That which you now have said is some∣what. But I deny not the possibility, but only doubt of the Operation. You may there∣fore pass to some other Question.

A.

Well, I will ask you then a Question about Refraction. I know already that for the Cause of Refraction (when the Light falleth through a thinner Medium upon a thicker) you assign the resistance of the thicker Body; but you do not mean there by Rarum and Densum, two Bodies whereof in equal spaces one has more substance in it than the other.

B.

No. For equal spaces contain equal Bodies. But I mean by Densum any Body which more resisteth the Motion of the Air, and by Rarum that which resisteth less.

A.

But you have not declared in what that resistance consisteth.

B.

I suppose it proceedeth from the Hard∣ness.

A.

But from thence it will follow, that all Transparent Bodies that equally refract are

equally Hard. Which I think is not true, be∣cause the Refraction of Glass is not greater (at least in comparison of their Hardnesses) than that of Water.

B.

I confess it. Therefore I think we must take in Gravity to a Share in the production of this Refraction. For I never considered Refraction but in Glass; because my business then was only to find the Causes of the Phaeno∣mena of Telescopes and Microscopes. Let therefore A B (in Fig. 7.) be a hard, and consequently, a heavy Body. And from above (as from the Sun) let C A be the line of Incidence, and produced to D. And draw A E perpendicular to A B. It is manifest that the Hardness in A B shall turn the stream of the Light inwards toward A E, suppose in the line A e. It is also evident that the Endea∣vour in B, which is (being heavy) down∣ward, shall turn the stream again inward, to∣wards A E as in A b. Thus it is in Refracti∣on from the Sun downwards. In like manner, if the light come from below, as from a Candle in the point D, the line of Incidence will be D A, and produced will pass to C. And the resistance of the Hardness in A will turn the stream A C inward, suppose into B l, and make C l equal to D e. For passing into a thinner Medium, it will depart from the per∣pendicular in an Angle equal to the Angle D A e, by which it came nearer to it in A e.

So also the resistance of the Gravity in the point A shall turn the stream of the Light into the line A i, and make the Angle l A i equal to the Angle e A b. And thus you see in what manner, though not in what proportion Hard∣ness and Gravity conjoyn their resistance in the Causing of Refraction.

A.

But you proved yesterday, that a hea∣vy Body does not Gravitate upon a Body equally heavy. Now this A B has upper parts, and lower parts; and if the upper parts do not Gravitate upon the lower parts, how can there be any Endeavour at all downward to contribute to the Refraction?

B.

I told you yesterday, that when a heavy Body was set upon another Body heavier or harder than it self, the Endeavour of it down∣ward was diverted another way, but not that it was extinguished. But in this case, where it lyeth upon Air, the first endeavour of the lowest part worketh downward. For neither Motion nor Body can be utterly extinguished by a less than an Omnipotent power. All Bodies as long as they are Bodies, are in Mo∣tion one way or other, though the farther it be communicated, so much the less.

A.

But since you hold that Motion is pro∣pagated through all Bodies, how hard or heavy soever they be, I see no Cause but that all Bodies should be Transparent.

B.

There are divers Causes that take away

Transparency. First, if the Body be not per∣fectly Homogeneous, that is to say, if the smallest parts of it be not all precisely of the same nature, or do not so touch one another as to leave no Vacuum within it; or though they touch, if they be not as hard in the con∣tact as in any other line. For then the Re∣fractions will be so changed both in their di∣rection, and in their strength, as that no Light shall come through it to the Eye; as in Wood and ordinary Stone and Metal. Secondly, The Gravity and hardness may be so great, as to make the Angle refracted so great, as the second Refraction shall not direct the beam of light to the Eye; as if the Angle of Re∣fraction were D A E, the Refracted line would be perpendicular to A B, and never come to the line A D, in which is the Eye.

A.

To know how much of the Refraction is due to the Hardness, and how much to the Gravity, I believe it is impossible, though the Quantity of the whole be easily measured in a Diaphanous Body given. And both you and Mr. Warner have demonstrated, that as the Sine of the Angle Refracted in one Inclina∣tion is to the Sine of the Angle Refracted in another Inclination, so is the Sine of one In∣clination to the Sine of the Angle of the other Inclination. Which Demonstrations are both published by Mersennus in the end of the first Volume of his Cogitata Physico-Mathematica.

But since there be many Bodies, through which though there pass Light enough, yet no Object appears through them to the Eye, what is the reason of that?

B.

You mean Paper. For Paper-Windows will enlighten a Room, and yet not show the Image of an Object without the Room. But 'tis because there are in Paper abundance of pores, through which the Air passing moveth the Air within; by the Reflections whereof any thing within may be seen. And in the same Paper there are again as many parts not Tran∣sparent, through which the Air cannot pass, but must be reflected first to all parts of the Object, and from them again to the Paper; and at the Paper either reflected again or transmitted, according as it falls upon Pores or not Pores; so that the Light from the Object can never come together at the Eye.

A.

There belongs yet to this Subject the Causes of the diversity of Colours. But I am so well satisfied with that which you have written of it in the 24th Chapter of your Book de Corpore, that I need not trouble you further in it. And now I have but one Que∣stion more to ask you, which I thought upon last night. I have read in an antient Historian that Living Creatures after a great deluge were produced by the Earth, which being then very soft, there were bred in it (it may be by the rapid Motion of the Sun) many Blisters,

which in time breaking, brought forth (like so many Eggs) all manner of living Crea∣tures great and small, which since it is grown hard it cannot do. What think you of it?

B.

It is true that the Earth produced the first living Creatures of all sorts but Man. For God said, (Gen. 1. vers. 24.) Let the Earth produce every living Creature, Cattle, and creeping thing, &c. But then again (ver. 25.) it is said that God made the Beast of the Earth, &c. So that it is evident that God gave unto the Earth that vertue. Which vertue must needs consist in Motion, because all Generation is Motion. But Man, though the same day, was made afterward.

A.

Why hath not the Earth the same vertue now? Is not the Sun the same it was? Or is there no Earth now soft enough?

B.

Yes. And it may be the Earth may yet produce some very small Living Creatures: And perhaps Male and Female. For the smallest Creatures which we take notice of, do en∣gender, though they do not all by conjun∣ction; therefore if the Earth produce living Creatures at this day, God did not absolutely rest from all his Works on the seventh day, but (as it is Cap. 2. ver. 2.) he rested from all the work he had made. And therefore it is no harm to think that God worketh still, and when and where and what he pleaseth. Be∣side, 'tis very hard to believe, that to produce

Male and Female, and all that belongs thereto, as also the several and curious Organs of Sense and Memory, could be the work of any thing that had not understanding. From whence, I think we may conclude, that whatsoever was made after the Creation, was a new Creature made by God no therwise than the first Crea∣tures were, excepting only Man.

A.

They are then in an Errour that think there are no more different kinds of Animals in the World now, than there were in the Ark of Noah.

B.

Yes doubtless. For they have no Text of Scripture from which it can be proved.

A.

The Questions of Nature which I could yet propound are innumerable. And since I cannot go through them, I must give over somewhere, and why not here? For I have troubled you enough, though I hope you will forgive me.

B.

So God forgive us both as we do one another. But forget not to take with you the Demonstration of a straight Line equal to an Arc of a Circle.

FINIS.