Note 26 — Chapter VI · THE STUDY OF NATURE. FUNDAMENTAL POSITIONS.

The relative density of the elements also determines their place in the universe. Fire and air are light; water and earth are heavy. Fire and air move away from the centre of the universe;26 water and earth are drawn towards it;27 and thus, from above to below—or, what is the same thing, from without to within—the four layers of fire, air, water, and earth are formed.28 The fire on the circumference goes by the name of Ether.29 Its most remote portion was called by Zeno Heaven;30 and it differs from earthly fire not only by its greater purity,31 but also because the motion of earthly fire is in a straight line, whereas the motion of the Ether is circular.32 Because of this difference of motion, Aristotle supposed a radical difference to exist between these two kinds of fire, but the Stoics did not feel it necessary to admit such a difference.33 They could always maintain that, when beyond the limits of its proper locality, fire tried to return to it as quickly as possible, whereas within those limits it moved in the form of a circle.