On the Structure, the Gravity, the Equilibrium and the Elastic Force of Air

Abstract

Above it was shown more than enough that water is in its proper position and remains together by its gravity. Moreover, we showed that no positive lightness is found in combined bodies, at least in those which are commonly called predominantly aerial. Particularly, air is heavy and indivisible. Aristotle recognized this when he said: all elements have gravity except fire and all have lightness except earth. From this he infers: consequently, it is necessary that earth and the bodies which have earth above all have gravity everywhere, water everywhere except in earth, air except in water and earth. In their area indeed all have gravity except fire, even air itself. The sign of this is that an inflated goatskin pulls more than an empty one. This can also be demonstrated in the same way by the arguments which we presented in the preceding chapter. We considered there pieces of wood, glass phials and bubbles rising through water. We demonstrated that they are impelled upwards not by a force of lightness but by extrusion by the ambient fluid. If instead of wood or a phial, air is set at the bottom of quicksilver or water, oil or spirit of wine, the same happens. We will show that air does not rise spontaneously moved by a force of lightness but by the compression of the heavier fluid environment which impels it violently upwards. Although this matter appears to be completely settled, it will be useful, however, to confirm it from the equilibrium of air with other fluids.