"Experiment in which Torricelli, its inventor, thinks that mercury, and any other liquid's, ability to hold itself up in the void, may come from the natural external pressure of air."
Saggi di naturali esperienze fatte nell'Accademia del Cimento, Florence 1666

The possibility of the existence of the vacuum was firmly denied by Aristotle in his Physics, which held sway, almost uncontended until the mid seventeenth century. Aristotle's plenist physics gave rise to the development and general acceptance, in the Middle Ages, of the theory of nature's abhorrence of the void. The theory of repugnance to the void, which would make nature prevent the production of a void at any cost, was used to explain various phenomena, such as the workings of the pump and the syphon: the upwards movement of water in this device was in fact interpreted as an action performed by nature to prevent a void after suction.

The definitive challenge to the theory of horror vacui - which had already been discussed, without conclusive results, by many authors - is one of the great merits of a disciple of Galileo, Evangelista Torricelli. In a very famous, and extraordinarily simple experiment, performed in Florence in the Spring of 1644, Torricelli showed not only that nature did not 'abhor the void', but also that it was all too simple to bring it into being.

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