In physics horror vacui, or plenism ("fullness", from Latin plēnum, English "plenty", cognate via Proto-Indo-European to "full"), is a theory first proposed by Aristotle in the Fourth book of Physics (or Physica)[1] that nature abhors a vacuum, and therefore empty space would always be trying to suck in gas or liquids to avoid being empty. The theory was widely accepted for a long time, but discarded in the 17th century as fluid mechanics advanced – see history of fluid mechanics for context.
The theory was supported and restated by Galileo Galilei in the early 17th century as resintenza del vacuo. Galilei was surprised by the fact that water could not raise above certain level in an aspiration tube in his suction pump, leading him to conclude that there is a limit to the phenomenon.[2] The theory was then rejected by later scientists. Galileo's pupil Evangelista Torricelli, in his investigations of the barometer (see barometer: history) and discovery of atmospheric pressure stated in 1644 that the level of mercury in a closed tube was dependent on the pressure of surrounding air – the mercury was pushed up by the atmospheric pressure, not "sucked" up by nature abhorring a vacuum. In 1647 Blaise Pascal proved this notion in his famous vide dans le vide (“emptiness in emptiness”) experiment (see Leere in der Leere at German Wikipedia for elaboration), and Pascal also demonstrated that Torricelli's explanation of the suction pump and siphon were correct (see siphon: history). The Magdeburg Hemispheres used by Otto von Guericke in 1650 were seen by some as proof that Aristotle's theory was not correct (see more on this topic in history of thermodynamics). For a scholarly discussion Leviathan and the Air-Pump, by Shapin and Schaffer 1985, is particularly instructive in the 17th century debate between Thomas Hobbes, supporting the plenum, and Robert Boyle's experimental demonstration of the vacuum.