Air is the most subtle element, and is moist and warm: heavier, indeed, than fire: but lighter than earth and water: it is the cause of respiration and voice: it is colourless, that is, it has no colour by nature: it is clear and transparent, for it is capable of receiving light: it ministers to three of our senses, for it is by its aid that we see, hear and smell: it has the power likewise of receiving heat and cold, dryness and moisture, and its movements in space are up, down, within, without, to the right and to the left, and the cyclical movement.