CHAPTER IV.. Problems of Heat and Light.

PROBLEMS OF HEAT AND LIGHT.

A. What is the cause of heat?

B. How know you, that any thing is hot but yourself?

A. Because I perceive by sense it heats me.

B. It is no good argument, the thing heats me; therefore it is hot. But what alteration do you find in your body at any time by being hot?

A. I find my skin more extended in summer than in winter; and am sometimes fainter and weaker then ordinary, as if my spirits were exhaled; and I sweat.

B. Then that is it you would know the cause of. I have told you before that by the motion I suppose both in the sun, and in the earth, the air is dissipated, and consequently that there would be an infinite number of small empty places, but that the world being full, there comes from the next parts other air into the spaces they would else make empty. When therefore this motion of the sun is exercised upon the superficies of the earth, if there do not come out of the earth itself some corporal substance to supply that tearing of the air, we must return again to the admission of vacuum. If there do, then you see how by this motion fluid bodies are made to exhale out of the earth. The like happens to a man’s body or hand, which when he perceives, he says he is hot. And so of the earth when it sendeth forth water and earth together in plants, we say it does it by heat from the sun.

A. It is very probable, and no less probable, that the same action of the sun is that which from the sea and moist places of the earth, but especially from the sea, fetcheth up the water into the clouds. But there be many ways of heating besides the action of the sun or of fire. Two pieces of wood will take fire if in turning they be pressed together.

B. Here again you have a manifest laceration of the air by the reciprocal and contrary motions of the two pieces of wood, which necessarily causeth a coming forth of whatsoever is aereal or fluid within them, and (the motion pursued) a dissipation also of the other more solid parts into ashes.

A. How comes it to pass that a man is warmed even to sweating, almost with every extraordinary labour of his body?

B. It is easy to understand, how by that labour all that is liquid in his body is tossed up and down, and thereby part of it also cast forth.

A. There be some things that make a man hot without sweat or other evaporation, as caustics, nettles, and other things.

B. No doubt. But they touch the part they so heat, and cannot work that effect at any distance.

A. How does heat cause light, and that partially, in some bodies more, in some less, though the heat be equal?

B. Heat does not cause light at all. But in many bodies, the same cause, that is to say, the same motion, causeth both together; so that they are not to one another as cause and effect, but are concomitant effects sometimes of one and the same motion.

A. How?

B. You know the rubbing or hard pressing of the eye, or a stroke upon it, makes an apparition of light without and before it, which way soever you look. This can proceed from nothing else but from the restitution of the organ pressed or stricken, unto its former ordinary situation of parts. Does not the sun by his thrusting back the air upon your eyes press them? Or do not those bodies whereon the sun shines, though by reflection, do the same, though not so strongly? And do not the organs of sight, the eye, the heart, and brains, resist that pressure by an endeavour of restitution outwards? Why then should there not be without and before the eye, an apparition of light in this case as well as in the other?

A. I grant there must. But what is that which appears after the pressing of the eye? For there is nothing without that was not there before; or if there were, methinks another should see it better, or as well as he; or if in the dark, methinks it should enlighten the place.

B. It is a fancy, such as is the appearance of your face in a looking-glass; such as is a dream; such as is a ghost; such as is a spot before the eye that hath stared upon the sun or fire. For all these are of the regiment of fancy, without any body concealed under them, or behind them, by which they are produced.

A. And when you look towards the sun or moon, why is not that also which appears before your eyes at that time a fancy?

B. So it is. Though the sun itself be a real body, yet that bright circle of about a foot diameter cannot be the sun, unless there be two suns, a greater and a lesser. And because you may see that which you call the sun, both above you in the sky, and before you in the water, and two suns, by distorting your eye, in two places in the sky, one of them must needs be fancy. And if one, both. All sense is fancy, though the cause be always in a real body.

A. I see by this that those things which the learned call the accidents of bodies, are indeed nothing else but diversity of fancy, and are inherent in the sentient, and not in the objects, except motion and quantity. And I perceive by your doctrine you have been tampering with Leviathan . But how comes wood with a certain degree of heat to shine, and iron also with a greater degree; but no heat at all to be able to make water shine?

B. That which shineth hath the same motion in its parts that I have all this while supposed in the sun and earth. In which motion there must needs be a competent degree of swiftness to move the sense, that is, to make it visible. All bodies that are not fluid will shine with heat, if the heat be very great. Iron will shine and gold will shine; but water will not, because the parts are carried away before they attain to that degree of swiftness which is requisite.

A. There are many fluid bodies whose parts evaporate, and yet they make a flame, as oil, and wine, and other strong drinks.

B. As for oil I never saw any inflamed by itself, how much soever heated, therefore I do not think they are the parts of the oil, but of the combustible body oiled that shine; but the parts of wine and strong drinks have partly a strong motion of themselves, and may be made to shine, but not with boiling, but by adding to them as they rise the flame of some other body.

A. How can it be known that the particles of wine have such a motion as you suppose?

B. Have you ever been so much distempered with drinking wine, as to think the windows and table move?

A. I confess, though you be not my confessor, I have; but very seldom; and I remember the window seemed to go and come in a kind of circling motion, such as you have described. But what of that?

B. Nothing, but that it was the wine that caused it; which having a good degree of that motion before, did, when it was heated in the veins, give that concussion, which you thought was in the window, to the veins themselves, and, by the continuation of the parts of man’s body, to the brain; and that was it which made the window seem to move.

A. What is flame? For I have often thought the flame that comes out of a small heap of straw to be more, before it hath done flaming, than a hundred times the straw itself.

B. It was but your fancy. If you take a stick in your hand by one end, the other end burning, and move it swiftly, the burning end, if the motion be circular, shall seem a circle; if straight, a straight line of fire, longer or shorter, according to the swiftness of the motion, or the space it moves in. You know the cause of that.

A. I think it is, because the impression of that visible object, which was made at the first instant of the motion, did last till it was ended. For then it will follow that it must be visible all the way, the impressions in all points of the time being equal.

B. The cause can be no other. The smallest spark of fire flying up seems a line drawn upward; and again by that swift circular motion which we have supposed for the cause of light, seems also broader than it is. And consequently the flame of every thing must needs seem much greater than it is.

A. What are those sparks that fly out of the fire?

B. They are small pieces of the wood or coals, or other fuel loosened and carried away with the air that cometh up with them. And being extinguished before their parts be quite dissipated into others, are so much soot, and black, and may be fired again.

A. A spark of fire may be stricken out of a cold stone. It is not therefore heat that makes this shining.

B. No it is the motion that makes both the heat and shining; and the stroke makes the motion. For every of those sparks, is a little parcel of the stone, which swiftly moved, imprinteth the same motion into the matter prepared, or fit to receive it.

A. How comes the light of the sun to burn almost any combustible matter by refraction through a convex glass, and by reflection from a concave?

B. The air moved by the sun presseth the convex glass in such manner as the action continued through it, proceedeth not in the same straight line by which it proceeded from the sun, but tendeth more toward the centre of the body it enters. Also when the action is continued through the convex body, it bendeth again the same way. By which means the whole action of the sun-beams are enclosed within a very small compass: in which place therefore there must be a very vehement motion; and consequently, if there be in that place combustible matter, such as is not very hard to kindle, the parts of it will be dissipated, and receive that motion which worketh on the eye as other fire does.

The same reason is to be given for burning by reflection. For there also the beams are collected into almost a point.

A. Why may not the sun-beams be such a body as we call fire, and pass through the pores of the glass so disposed as to carry them to a point, or very near?

B. Can there be a glass that is all pores? if there cannot, then cannot this effect be produced by the passing of fire through the pores. You have seen men light their tobacco at the sun with a burning glass, or with a ball of crystal, held which way they will indifferently. Which must be impossible, unless the ball were all pores. Again, neither you nor I can conceive any other fire than we have seen, nor than such as water will put out. But not only a solid globe of glass or crystal will serve for a burning-glass, but also a hollow one filled with water. How then does the fire from the sun pass through the glass of water without being put out before it come to the matter they would have it burn?

A. I know not. There comes nothing from the sun. If there did, there is come so much from it already, that at this day we had had no sun.

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