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 rec