 The planets move around the earth in the ether (a tenuous kind of air), which belongs to the ‘mercury quaternion’: it includes watery bodies and mercury. Terrestrial fire is a weakened form of sidereal fire. It is related to oily substances and sulphur, and constitutes the ‘sulphur quaternion’. The two quaternions oppose each other: air/ether vs. fire/sidereal fire. Air and ether loose power when terrestrial and sidereal fires grow more energetic—Bacon's sulphur and mercury are not principles in the sense of Paracelsus, but simply natural substances. The Paracelsian principle of salt is excluded by Bacon and the substance, which plays a role only in the sublunary realm, is for him a compound of natural sulphur and mercury (Rees and Upton 1984, 25).