Robert Boyle showed that air “has a Spring that enables it to sustain or resist a pressure” and also it has “an active Spring . . . as when it distends a flaccid or breaks a full-blown Bladder in our exhausted receiver” (Boyle 1999, 6.41-42).1 In this respect, he distinguished between air and other fluids, since liquids such as water are “not sensibly compressible by an ordinary force” (ibid., 5.264). He explained the air’s tendency to resist and to expand by (...) hypothesizing the Air near the Earth to be such a heap of little Bodies, lying one upon another, as may be resembled to a Fleece of Wooll. For this (to omit other likenesses betwixt them) consists of many slender and flexible hairs; each of which, may indeed, like a little Spring, be easily bent or rouled up; but will also, like a Spring, be still endeavouring to stretch it self out again (Boyle 1999, 1.165). (shrink)