_B._ I know not why, no more than you or any man else knows why it is not very air, though purer perhaps than the common air, as being strained through the blood into the brain and nerves. But howsoever that be, there is no doubt, but the least parts of the common air, respectively to the whole, will easilier pierce, with equal motion, the body that resisteth them, than the least parts of water. For it is by motion only that any mutation is made in any thing; and all things standing as they did, will appear as they did. And that which changeth soft into hard, must be such as makes the parts not easily to be moved without being moved all together; which cannot be done but by some motion compounded. And we call hard, that whereof no part can be put out of order without disordering all the rest; which is not easily done. _A._ How water and air beaten into extreme small bubbles is hardened into ice, you have told me already, and I understand it. But how a soft homogeneous body, as air or water, should be so hardened, I cannot imagine. _B._ There is no hard body that hath not also some degree of gravity; and consequently, being loose, there must be some efficient cause, that is, some motion, when it is severed from the earth, to bring the same to it again. And seeing this compounded motion gives to the air and water an endeavour from the earth, the motion which must hinder it, must be in a way contrary to the compounded motion of the earth. For whatsoever, having been asunder, comes together again, must come contrary ways, as those that follow one another go the same way, though both move upon the same line. _A._ What experiment have you seen to this purpose?