The Summary of Physics Restored

121Those who have believed that exhalations raised from the terrestrial region into the air are therefore earthy and converted into the element of fire because they catch fire there are greatly mistaken on both points. They are not of an earthy nature, but rather an airy one. For our air, moist through contact with water, acquires oiliness while lying long inactive in earth’s dry bosom, and through association with earth tempers moisture with dryness. When heat presses it to exhale through earth’s pores and fissures—or when an abundance of matter multiplies its strength—it bursts out of its shaken and broken prison with noise and crashing. Hence many earthquakes and chasms are experienced, often with devastation. Having gained its freedom, that exhalation flies into the region of light bodies and there, more fully digested into sulphurous matter by wandering motion and heat, catches fire. This matter is therefore not truly earthy, since it is averse to heaviness and coldness. Because the conjunction of heat, dryness, and moisture has made it oily and combustible, it should be regarded as fuel for accidental fire rather than as Nature’s elemental fire. It is a kind of spurious generation, which has not deserved to be numbered among the elements or designated by that name. Aristotle therefore rightly calls such ignitions imperfect mixtures. The same judgment applies to the smoke of burnt things: because smoke is oily, it readily takes flame, which is nothing other than smoke set alight.