_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.