822a
A plant has three powers, the first derived from the element of earth, the second from that of water, the third from that of fire. From the earth the plant derives its solidity, from water the unity, and from fire the concretion of its solidity. We see much the same thing in vessels of pottery, which contain three elements—clay, which is, as it were, the material of pottery; secondly, water, which binds the pottery together; and, thirdly, fire, which draws its parts together, until it completes the process of manufacture. The appearance, then, of complete unity is due to the fire; because rarity is present in pottery according to the composition of its parts, and, when the fire heats them, the moist matter is solidified,[1] and the parts of the clay will cohere together. Dryness will thus take the place of moisture, owing to the predominance of the fire and the process of concoction[2] which takes place in all animals, plants, and metals. For concoction takes place where moisture and heat are present, when the struggle between them is allowed to run its course; and this is what will take place in the concoction of stone and metals.[3] It is not so in animals and plants; for their parts are not closely compacted, and so there is an escape of moisture from them. But in metals there is no such escape of moisture or sweating, because their parts have no rarity, and therefore they can give up nothing except parts of themselves to correspond to certain superfluities which are given off by animals and plants. This escape of moisture can only take place where rarity is present; and so where there is no rarity, nothing at all can be given off. Therefore that which cannot be increased is solid, because that which might increase lacks space in which to dilate and grow; and therefore stones, salt, and earth are always the same,
