Page:The Works of Aristotle - Vol. 6 - Opuscula (1913).djvu/104

823$a$ sink, because their parts are practically solid. Oil and fat always float on the surface of water. We will now give the reason of this. We know that heat and moisture are present in these substances; and it is characteristic of moisture to cohere with particles of water, while it is characteristic of heat that it causes moisture to rise and makes its way towards the particles of air; and it is the habit of water to raise objects to its surface, and of air to rise upwards; and water does not rise above its surface, because the whole surface of the water is one and the same, and consequently the air rises with the oil above the water. Some stones too float on water, because rarity is present in them and is greater in quantity than the matter of which they are formed, and consequently the space occupied by air will be greater than that occupied by the earthy element. It is the nature of water to take up a position above the earth, and of air to rise above water; the material, therefore, which composes the stone, which is of the element of earth, sinks in the water, while the element of air enclosed in the stone rises above the water. Each element therefore attracts its like in a contrary direction to the element with which it is combined. If, then, one element is equal to the other, half the stone will be submerged and half will project above the surface; but if the air is present in greater quantity, the stone will float above the water. The weight of trees is made up in the same way. (These stones are due to a violent collision of waves, and are originally foam which forms a white oily substance; when the wave is dashed against the sand, the sand will collect the oily foam, and the dryness of the sea will dry it up together with the superfluous salt, and the particles of sand will collect, and thus in the long process of time stones will be formed.)