_B._ When the quicksilver is fallen to D, there is so much the more in the basin, and that takes up the place which so much air took up before. Whither can this air go if all the world without that glass pipe B C were full? There must needs be the same or as much air come into that space, which only is empty, between C and D: by what force? By the weight of the quicksilver between D and B. Which quicksilver weigheth now upward, or else it could never have raised that part higher, which was at first in the basin. So you see the weight of quicksilver can press the air through quicksilver up into the pipe, till it come to an equality of force as in D, where the weight of the quicksilver is equal to the force which is required in air to go through it. _A._ If a man suck a phial that has nothing in it but air, and presently dip the mouth of it into water, the water will ascend into the phial. Is not that an argument that part of the air had been sucked out, and part of the room within the phial left empty? _B._ No. If there were empty space in the world, why should not there be also some empty space in the phial before it was sucked? And then why does not the water rise to fill that? When a man sucks the phial he draws nothing out, neither into his belly, nor into his lungs, nor into his mouth; only he sets the air within the glass into a circular motion, giving it at once an endeavour to go forth by the sucking, and an endeavour to go back by not receiving it into his mouth; and so with a great deal of labour glues his lips to the neck of the phial. Then taking it off, and dipping the neck of the phial into the water before the circulation ceases, the air, with the endeavour it hath now gotten, pierces the water and goes out: and so much air as goes out, so much matter comes up into the room of it. ========== CHAPTER IV. 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.