On Vacuum

Nature Abhors a Vacuum

4. Galileo Galilei: Experiments Demonstrating that Nature Abhors a Vacuum

Translated by Henry Crew and Alfonso de Salvio²⁶

Annotated by Edward Grant

[59]

Salv. A truly ingenious device!²⁷ I feel, however, that for a complete explanation other considerations might well enter; yet I must not now digress upon this particular topic since you are waiting to hear what I think about the breaking strength of other materials which, unlike ropes and most woods, do not show a filamentous structure. The coherence of these bodies is, in my estimation, produced by other causes which may be grouped under two heads. One is that much-talked-of repugnance which nature exhibits towards a vacuum; but this horror of a vacuum not being sufficient, it is necessary to introduce another cause in the form of a gluey or viscous substance which binds firmly together the component parts of the body.²⁸

First I shall speak of the vacuum, demonstrating by definite experiment the quality and quantity of its force [virtù]. If you take two highly polished and smooth plates of marble, metal, or glass and place them face to face, one will slide over the other with the greatest ease, showing conclusively that there is nothing of a viscous nature between them. But when you attempt to separate them and keep them at a constant distance apart, you find the plates exhibit such a repugnance to separation that the upper one will carry the lower one with it and keep it lifted indefinitely, even when the latter is big and heavy.²⁹

This experiment shows the aversion of nature for empty space, even during the brief moment required for the outside air to rush in and fill up the region between the two plates. It is also observed that if two plates are not thoroughly polished, their contact is imperfect so that when you attempt to separate them slowly the only resistance offered is that of weight; if however, the pull be sudden, then the lower plate rises, but quickly falls back, having followed the upper plate only for that very short interval of time required for the expansion of the small amount of air remaining between the plates, in consequence of their not fitting, and for the entrance of the surrounding air. This resistance which is exhibited between the two plates is doubtless likewise present between the parts of a solid, and enters, at least in part, as a concomitant cause of their coherence.

[60]

Sagr. Allow me to interrupt you for a moment, please; for I want to speak of something which just occurs to me, namely, when I see how the lower plate follows the upper one and how rapidly it is lifted, I feel sure that, contrary to the opinion of many philosophers, including perhaps even Aristotle himself, motion in a vacuum is not instantaneous. If this were so the two plates mentioned above would separate without any resistance whatever, seeing that the same instant of time would suffice for their separation and for the surrounding medium to rush in and fill the vacuum between them. The fact that the lower plate follows the upper one allows us to infer, not only that motion in a vacuum is not instantaneous,³⁰ but also that, between the two plates, a vacuum really exists, at least for a very short time, sufficient to allow the surrounding medium to rush in and fill the vacuum; for if there were no vacuum there would be no need of any motion in the medium. One must admit then that a vacuum is sometimes produced by violent motion [violenza] or contrary to the laws of nature, (although in my opinion nothing occurs contrary to nature except the impossible, and that never occurs).

^26. Reprinted with the kind permission of the Macmillan Company from the First Day of the Dialogues Concerning Two New Sciences by Galileo Galilei, translated from the Italian and Latin into English by Henry Crew and Alfonso de Salvio with an Introduction by Antonio Favaro (New York: Macmillan, 1914), pp. 11–12. As we shall see here and in another brief selection (Selection 56.3) Galileo did accept the existence of interstitial vacua but nevertheless maintained the traditional viewpoint that nature abhorred a separate vacuum and sought as quickly as possible to occupy it with matter. Not until Pascal explained the role of atmospheric pressure was this long-held opinion emphatically denied (for Pascal’s exposition, see next selection).

^27. A reference to a device for descending from a window at a controlled rate, described by Sagredo, one of the interlocutors in the dialogue.

^28. Later, Galileo identifies this other cause as interstitial vacua, which he suggests function as a binding or attractive force (forza del vacuo) holding together the particles of a solid body (see Selection 56.3).

^29. The opinions of Salviati and Sagredo represent Galileo’s viewpoint on this subject. Admitting a momentary vacuum between the plates (see the last two paragraphs) represents a radically different response than was given in a typically medieval exposition by Roger Bacon to explain much the same phenomena.

After demonstrating the impossibility of void by rational argument, Bacon offers a series of supporting illustrations and experiments, the first of which is basically that presented by Galileo. "Let two plane tables be taken and joined together so that nothing lies between them. If one were raised above the other, the air would enter between the tables, first reaching the extreme parts, and then the central point. But in this raised position, and before the air reaches the central point, a void will exist there." In order to deny the formation of a void, Bacon says that one might suppose that when the tables are parted, air reaches all parts of the tables instantaneously. Finding this repugnant, he proposes another solution. "I reply to this by saying that the two round tables could not be raised if one were above the other unless there should be an inclination of some part. Hence it would be necessary that some part be inclined before it could be raised, for otherwise a void would be produced. This applies to all of nature. . . . It is evident, then, that anyone who wishes to raise [anything] must incline one part beforehand. . . . For this reason a vacuum should not be assumed." (My translation is from Bacon’s Questions on the Eight Books of the Physics of Aristotle, Book IV ("Whether a vacuum can be assumed below the heavens"), in Opera hactenus inedita Rogeri Baconi, Fasc. XIII: Questiones supra libros octo Physicorum Aristotelis, edited by F. M. Delorme, O.F.M., with the aid of Robert Steele [Oxford: Clarendon Press, 1935], pp. 225–226.)

^30. Some scholastics argued that motion in a hypothetical vacuum would not be instantaneous (Selection 47, n. 6, and Selection 55.1). Of great interest here is the fact that Galileo holds that an actual vacuum would exist momentarily before nature rushed to fill it.