CHAPTER V.. Problems of Hard and Soft.

PROBLEMS OF HARD AND SOFT.

A. What call you hard, and what soft?

B. That body whereof no one part is easily put out of its place, without removing the whole, is that which I and all men call hard; and the contrary soft. So that they are but degrees one of another.

A. What is the cause that makes one body harder than another, or, seeing you say they are but degrees of one another, what makes one body softer than another, and the same body sometimes harder, sometimes softer?

B. The same motion which we have supposed from the beginning for the cause of so many other effects. Which motion not being upon the centre of the part moved, but the part itself going in another circle to and again, it is not necessary that the motion be perfectly circular. For it is not circulation, but the reciprocation, I mean the to and again, that does cast off, and lacerate the air, and consequently produce the fore-mentioned effects.

For the cause therefore of hardness, I suppose the reciprocation of motion in those things which are hard, to be very swift, and in very small circles.

A. This is somewhat hard to believe. I would you could supply it with some visible experience.

B. When you see, for example, a cross-bow bent, do you think the parts of it stir?

A. No. I am sure they do not.

B. How are you sure? You have no argument for it, but that you do not see the motion. When I see you sitting still, must I believe there is no motion in your parts within, when there are so many arguments to convince me there is.

A. What argument have you to convince me that there is motion in a cross-bow when it stands bent?

B. If you cut the string, or any way set the bow at liberty, it will have then a very visible motion. What can be the cause of that?

A. Why the setting of the bow at liberty.

B. If the bow had been crooked before it was bent, and the string tied to both ends, and then cut asunder, the bow would not have stirred. Where lies the difference?

A. The bow bent has a spring; unbent it has none, how crooked soever.

B. What mean you by spring?

A. An endeavour of restitution to its former posture.

B. I understand spring as well as I do endeavour.

A. I mean a principle or beginning of motion in a contrary way to that of the force which bent it.

B. But the beginning of motion is also motion, how insensible soever it be. And you know that nothing can give a beginning of motion to itself. What is it therefore that gives the bow (which you say you are sure was at rest when it stood bent) its first endeavour to return to its former posture?

A. It was he that bent it.

B. That cannot be. For he gave it an endeavour to come forward, and the bow endeavours to go backward.

A. Well, grant that endeavour be motion, and motion in the bow unbent, how do you derive from thence, that being set at liberty it must return to its former posture?

B. Thus there being within the bow a swift (though invisible) motion of all the parts, and consequently of the whole; the bending causeth that motion, which was along the bow (that was beaten out when it was hot into that length) to operate across the length in every part of it, and the more by how much it is more bent; and consequently endeavours to unbend it all the while it stands bent. And therefore when the force which kept it bent is removed, it must of necessity return to the posture it had before.

A. But has that endeavour no effect at all before the impediment be removed? For if endeavour be motion, and every motion have some effect more or less, methinks this endeavour should in time produce something.

B. So it does. For in time (in a long time) the course of this internal motion will lie along the bow, not according to the former, but to the new acquired posture. And then it well be as uneasy to return it to its former posture, as it was before to bend it.

A. That is true. For bows long bent lose their appetite to restitution, long custom becoming nature. But from this internal reciprocation of the parts, how do you infer the hardness of the whole body?

B. If you apply force to any single part of such a body, you must needs disorder the motion of the next parts to it before it yield, and there disordered, the motion of the next again must also be disordered; and consequently no one part can yield without force sufficient to disorder all: but then the whole body must also yield. Now when a body is of such a nature as no single part can be removed without removing the whole, men say that body is hard.

A. Why does the fire melt divers hard bodies, and yet not all?

B. The hardest bodies are those wherein the motion of the parts are the most swift, and yet in the least circles. Wherefore if the fire, the motion of whose parts are swift, and in greater circles, be made so swift, as to be strong enough to master the motion of the parts of the hard body, it will make those parts to move in a greater compass, and thereby weaken their resistance, that is to say, soften them, which is a degree of liquefaction. And when the motion is so weakened, as that the parts lose their coherence by the force of their own weight, then we count the body melted.

A. Why are the hardest things the most brittle, insomuch that what force soever is enough to bend them, is enough also to break them?

B. In bending a hard body, as (for example) a rod of iron, you do not enlarge the space of the internal motion of the parts of iron, as the fire does; but you master and interrupt the motion, and that chiefly in one place. In which place the motion that makes the iron hard being once overcome, the prosecution of that bending must needs suddenly master the motions of the parts next unto it, being almost mastered before.

A. I have seen a small piece of glass, the figure whereof is this, A A B C. Which piece of glass if you bend toward the top, as in C, the whole body will shatter asunder into a million of pieces, and be like to so much dust. I would fain see you give a probable reason of that.

B. I have seen the experiment. The making of the glass is thus: they dip an iron rod into the molten glass that stands in a vessel within the furnace. Upon which iron rod taken out, there will hang a drop of molten but tough metal of the figure you have described, which they let fall into the water. So that the main drop comes first to the water, and after it the tail, which though straight whilst it hung on the end of the rod, yet by falling into the water becomes crooked. Now you know the making of it, you may consider what must be the consequence of it. Because the main drop A comes first to the water, it is therefore first quenched, and consequently the motion of the parts of that drop, which by the fire were made to be moved in a larger compass, is by the water made to shrink into lesser circles towards the other end B, but with the same or not much less swiftness.

A. Why so?

B. If you take any long piece of iron, glass, or other uniform and continued body; and having heated one end thereof, you hold the other end in your hand, and so quench it suddenly, though before you held it easily enough, yet now it will burn your fingers.

A. It will so.

B. You see then how the motion of the parts from A toward C is made more violent and in less compass by quenching the other parts first. Besides, the whole motion that was in all the parts of the main drop A, is now united in the small end B C. And this I take to be the cause why that small part B C is so exceeding stiff. Seeing also this motion in every small part of the glass, is not only circular, but proceeds also all along the glass from A to B, the whole motion compounded will be such as the motion of spinning any soft matter into thread, and will dispose the whole body of the glass in threads, which in other hard bodies are called the grain. Therefore if you bend this body (for example) in C (which to do will require more force than a man would think that has not tried) those threads of glass must needs be all bent at the same time, and stand so, till by the breaking of the glass at C, they be all at once set at liberty; and then all at once being suddenly unbent, like so many brittle and overbent bows, their strings breaking, be shivered in pieces.

A. It is like enough to be so. And if nature have betrayed herself in any thing, I think it is in this, and in that other experience of the crossbow; which strongly and evidently demonstrates the internal reciprocation of the motion, which you suppose to be in the internal parts of every hard body. And I have observed somewhat in looking-glasses which much confirms that there is some such motion in the internal parts of glass, as you have supposed for the cause of hardness. For let the glass be A B, and let the object at C be a candle, and the eye at D. Now by divers reflections and refractions in the two superficies of the glass, if the lines of vision be very oblique, you shall see many images of the candle, as E, F, G, in such order and position as is here described. But if you remove your eye to C, and the candle to D, they will appear in a situation manifestly different from this. Which you will yet more plainly perceive if the looking-glass be coloured, as I have observed in red and blue glasses; and could never conceive any probable cause of it, till now you tell me of this secret motion of the parts across the grain of the glass, acquired by cooling it this or that way.

B. There be very many kinds of hard bodies, metals, stones, and other kinds, in the bowels of the earth, that have been there ever since the beginning of the world; and I believe also many different sorts of juices that may be made hard. But for one general cause of hardness it can be no other than such an internal motion of parts as I have already described, whatsoever may be the cause of the several concomitant qualities of their hardness in particular.

A. We see water hardened every frosty day. It is likely therefore you may give a probable cause of ice. What is the cause of freezing of the ocean towards the poles of the earth?

B. You know the sun being always between the tropics, and (as we have supposed) always casting off the air; and the earth likewise casting it off from itself, there must needs on both sides be a great stream of air towards the poles, shaving the superficies of the earth and sea, in the northern and southern climates. This shaving of the earth and sea by the stream of air must needs contract and make to shrink those little circles of the internal parts of earth and water, and consequently harden them, first at the superficies, into a thin skin, which is the first ice; and afterwards the same motion continuing, and the first ice co-operating, the ice becomes thicker. And this I conceive to be the cause of the freezing of the ocean.

A. If that be the cause, I need not ask how a bottle of water is made to freeze in warm weather with snow, or ice mingled with salt. For when the bottle is in the midst of it, the wind that goeth out both of the salt and of the ice as they dissolve, must needs shave the superficies of the bottle, and the bottle work accordingly on the water without it, and so give it first a thin skin, and at last thicken it into a solid piece of ice. But how comes it to pass that water does not use to freeze in a deep pit?

B. A deep pit is a very thick bottle, and such as the air cannot come at but only at the top, or where the earth is very loose and spungy.

A. Why will not wine freeze as well as water?

B. So it will when the frost is great enough. But the internal motion of the parts of wine and other heating liquors is in greater circles and stronger than the motion of the parts of water; and therefore less easily to be frozen, especially quite through, because those parts that have the strongest motion retire to the centre of the vessel.

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