Experimenting with Air

Abstract

Developments in hydrostatics in the second half of the seventeenth century were influenced by developments in pneumatics. Those developments were very much informed by novel experiments carried out in contrived situations, beginning with Torricelli’s famous experiment with what we now know as the mercury in glass barometer. That experiment was soon followed by others that exploited the space above the mercury in that device. It was shown that a partially inflated bladder expands when introduced into the space above the mercury and that a volume of air introduced into that space causes a much greater depression of the mercury than an equal volume of water. These latter experiments strongly suggested that air has a spontaneous capacity to expand that is a property distinct from weight. Once it is realized that air is distinguishable from solids by virtue of its capacity to expand then the question of how liquids differ from solids becomes an issue, since liquids, while sharing with solids and air the possession of weight, do not have a capacity to expand.