_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. ========== CHAPTER V. 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?