A vessel which is technically empty still contains an enormous number of molecules.

Molecules of gas constitute a considerable hindrance in many physical instruments. Radio tubes, \(X\)-ray tubes, accelerators of elementary particles-all these instruments require a vacuum (derived from the Latin vacuus meaning “empty”), i.e. space free of gas molecules. There should also be a vacuum in an ordinary electric lamp. If air enters a lamp, it will oxidize and immediately burn out.

In the best vacuum instruments, vacuum of the order of \(10^{-8}\) mm Hg is produced.¹ A completely negligible pressure, it would seem: the level of mercury in a manometer would move by a hundred-millionth of a millimetre if the pressure changed by such an amount.

However, there are still several hundred million molecules in 1 cm\(^3\) at this meagre pressure.

It is interesting to compare the void of interstellar space with such a vacuum—there one finds an average of one elementary particle of matter in several cubic centimetres.

Special pumps are employed in order to obtain vacuum. An ordinary pump removing gas by means of the motion of a piston can create a vacuum of at best 0.01 mm Hg. A good or, as one says, high vacuum can be obtained with the aid of a so-called diffusion (mercury or oil) pump in which gas molecules are caught up in a stream of mercury or oil vapor.

Mercury pumps, bearing the name of their inventor, Langmuir, start working only after a preliminary exhaustion to a pressure of about 0.1 mm Hg; such a preliminary rarefaction is called a forevacuum.

This is the way it works. A small glass container is connected to a vessel with mercury, an evacuated space and a forepump, The mercury is heated and the forepump carries away its vapour. The mercury vapour captures molecules of the gas along the way and brings them to the forepump. The mercury vapour condenses (cooling by means of running water is provided for), and the liquid trickles down into the vessel from which the mercury began its journey.

A vacuum obtained under laboratory conditions, as we have just said, is still far from empty in the absolute sense of the word. A vacuum is greatly rarefied gas. The properties of such a gas may differ essentially from those of an ordinary gas.

The motion of the molecules “forming a vacuum” changes its character when the mean free path of a molecule becomes greater than the dimensions of the vessel containing the gas. The molecules then rarely collide with each other and travel in straight zigzags striking against first one and then another wall of the vessel. We shall speak in detail about the motion of molecules in the second book. It is known to the reader that mean free path of a molecule in air at atmospheric pressure is equal to \(5\times 10^{-6}\) cm. If we increase it by a factor of \(10^7\), it will be 50 cm, i.e. will be noticeably greater than an average sized vessel. Since the mean free path is inversely proportional to the density, and hence also to the pressure, the pressure must be \(10^{-7}\) of atmospheric pressure, or approximately \(10^{-4}\) mm Hg.

Even interplanetary space is not entirely empty. But the density of the matter in it is about \(5\times 10^{-24}~\mathrm{g/cm^3}\). The main component of interplanetary matter is atomic hydrogen. At the present time, it is considered that cosmic space contains several hydrogen atoms per 1 cm\(^3\). If a hydrogen molecule were enlarged to the size of a pea and placed in Moscow, its nearest “cosmic neighbour” would prove to be in Tula.