Otto Von Guericke - Air Pressure and The Vacuum

Air Pressure and The Vacuum

Probably around 1650 von Guericke invented a vacuum pump consisting of a piston and an air gun cylinder with two-way flaps designed to pull air out of whatever vessel it was connected to, and used it to investigate the properties of the vacuum in many experiments. This pump is described in Chapters II and III of Book III of the Experimenta Nova and in the Mechanica Hydraulico-pneumatica (p. 445-6). Guericke demonstrated the force of air pressure with dramatic experiments.

With his experiments Guericke disproved the hypothesis of "horror vacui", that nature abhors a vacuum. Aristotle in e.g. Physics IV 6-9 had argued against the existence of the void and his views commanded near universal endorsement by philosophers and scientists up to the 17th century. Guericke showed that substances were not pulled by a vacuum, but were pushed by the pressure of the surrounding fluids.

All of von Guericke's work on the vacuum and air pressure is described in Book III of the Experimenta Nova (1672). As regards the more detailed chronology of his work we have, in addition to the Experimenta Nova's description of his demonstrations at Regensburg in 1654, the two accounts published by Fr.Schott in 1657 and 1663.

In Chapter 27 he alludes to what transpired at Regensburg in 1654. The first experiment he explicitly records as having been demonstrated was the crushing of a non-spherical vessel as the air was withdrawn from it. He did not use a vacuum pump directly on the vessel but allowed the air in it to expand into a previously evacuated receiver.

The second was an experiment in which a number of men proved able to pull an airtight piston only about half way up a cylindrical copper vessel. Von Guericke then attached his evacuated Receiver to the space below the piston and succeeded in drawing the piston back down again against the force of the men pulling it up. In a letter to Fr. Schott of June 1656, reproduced in Mechanica Hydraulico-pneumatica, von Guericke gives a short account of his experiences at Regensburg. Based on this, Schimank gives a list of ten experiments which he considers likely to have been carried out at Regensburg. In addition to the above two, these included the extraction of air using a vacuum pump, the extinction of a flame in a sealed vessel, the raising of water by suction, a demonstration that air has weight, and a demonstration of how fog and mist can be produced in a sealed vessel. The Mechanica Hydraulico-pneumatica also provides the earliest drawing of von Guericke's vacuum pump. This corresponds to the description in the opening chapters of Book III of the Experimenta Nova of the first version of his pump.

Stimulated by the interest taken in his work von Guericke was scientifically very active in the decade after 1654. In June 1656 we find him writing to Fr. Schott (Mechanica Hydraulico-pneumatica p. 444) "Since the time when I produced the exhibition for the said eminent Elector, I have a better and clearer grasp of all these matters and many other topics as well." The celebrated hemispheres experiment was, as noted in the biographical section above, carried out between July 1656 and August 1657. In Chapter IV of Book III he describes a new and much improved design of vacuum pump and attributes its invention to the need for a more easily transportable machine with which he could demonstrate his experiments to Frederick William who had expressed the desire to see them. The new pump is also described on p. 67 of the Technica Curiosa. The demonstration in the Elector's Library at Cölln an der Spree took place in November 1663 and was recorded by a tutor to the Elector's sons. (Schneider p. 113.) A number of experiments, such as the rather cruel testing of the effect of a vacuum on birds and fish (Experimenta Nova Book III Chapter XVI), are not described in the Technica Curiosa. Although the Experimenta Nova does contain correspondence from 1665, there is no reason to doubt von Guericke's assertion that the work was essentially finished by March 1663.

Throughout Books II and III he returns again and again to the theme of there being no abhorrence of a vacuum and that all the phenomena explained by this supposed principle are in fact attributable to the pressure of the atmosphere in conjunction with various incorporeal potencies which he held to be acting. Thus the Earth's "conservative potency" (virtus conservativa) provided the explanation for the fact that the Earth retains its atmosphere though travelling through space. In countering the objection of a Dr. Deusing that the weight of the atmosphere would simply crush the bodies of all living things, he shows explicit awareness of the key property of a fluid - that it exerts pressure equally across all planes. In Chapter XXX of Book III he writes: "Dr. Deusing ought to have borne in mind that the air does not just press on our heads but flows all around us. Just as it presses from above on the head, it likewise presses on the soles of the feet from below and simultaneously on all parts of the body from all directions."