In the preceding chapter we drew attention to the fact that any spatially extended mass is under the sway of both gravity and levity. We then saw that with the transition of matter from the solid via the liquid to the gaseous state, not only does the specific gravity of the substance decrease, but at the same time an increase takes place of what we might call 'specific levity'. In the gaseous state, therefore, we find gravity-bound matter becoming so far levity-bound that it assumes the property of actively expanding in space.