The Summary of Physics Restored

75Those who attribute the greatest moistness to air because it is difficult to contain within a boundary of its own but easy to contain within another's are greatly mistaken. This is proper to thin and fluid bodies, not to moist ones, and therefore applies more to fire and heaven, whose natures are rarer, than to water or air. For rare bodies, because they flow of themselves in every direction, cannot remain within their own boundary and therefore need another's. Only dense and firm bodies are confined by their own outline and surface. Subtle bodies cannot do this, because through their thinness they flow and pour themselves out; the rarer they are, the more they flow out and the less they hold together. It follows that air is a rarer body, but not a moister one.