Vacuum
Collier's New Encyclopedia (1921)
in physics, a portion of space void of matter. The possibility of the existence of a perfect vacuum has been a favorite subject of discussion among metaphysicians; and that the negative view was popular is indicated by the historical phrase "the vacuum which nature abhors." Descartes asserted that if the contents of a hollow vessel were taken out without anything entering to fill its place the sides of the vessel would be in contact. This assertion, however, is based on the dogma that the only essential property of matter was its extension; that, in other words, matter was indistinguishable from space. Such inquiries, however, can lead to nothing definite. The experimental philtical impossibility of obtaining fect vacuum. The so-called Torricellian vacuum, which exists above the mercury column of a barometer, is really filled with mercury vapor probably mixed with a small percentage of air. The very action by means of which the air pump produces its vacuum shows that there can never be an absolute void-a little air always remaining behind. Andrews, following up a suggestion of Davy, obtained remarkable vacua by first pumping in carbonic acid so as to expel as far as possible the air, and then after ordinary exhaustion leaving the carbonic acid to be taken up by moistened caustic potash which has been previously placed in the receiver. In this way Andrews removed all but 550, of the gas originally present, whereas by ordinary air pump action only of the gas can be removed. Frankland, Gassiott, Crookes, Dewar, and others, have devised various improvements. Dewar takes advantage of the property of the gases to condense on the surface of solids in a remarkably ingenious way. A piece of carbon, placed in the vacuum vessel, is kept heated while the vessel is being exhausted. After the exhaustion is completed as far as possible, the carbon is allowed to cool so as to permit the small quantity of gas present to condense in its pores. A gentle heating of the carbon at once releases a quantity of the gas, so that this method is invaluable in the study of electric discharges through rarefied gases, since the rarefaction can be so easily altered at will.
What Has Changed in the Last Century
Collier's in 1921 was right that a perfect void cannot be pumped out, and Dewar's trick of chilling charcoal to soak up the last traces survives as the modern cryosorption pump. But the numbers have moved enormously: with diffusion, turbomolecular and ion pumps, laboratories now reach extreme high vacuum around 10^-11 pascals, leaving only a few hundred molecules per cubic centimetre where the 1921 air pump left billions of billions. The deeper change is conceptual. Since the 1930s quantum field theory has held that even a space swept clean of all matter is not empty but seethes with zero-point fluctuations, a claim made concrete by Hendrik Casimir's 1948 prediction that two close metal plates would be pushed together by the vacuum itself, measured in the laboratory in the 1990s, and by the discovery in 1998 that some form of vacuum energy appears to be accelerating the expansion of the universe.
Written for this edition, 2026.