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 things 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.