VACUUM, an empty space. Strictly speaking, for a space to be a vacuum it must contain no traces of gas of any kind, but even with the most refined modern means of exhaustion there will be a few million million gas molecules in a glass bulb five inches or so in diameter which has been rendered as empty as possible. This represents, however, a pressure only a ten thou sand-millionth of the atmospheric pressure. It is customary to speak of a space in which the pressure has been reduced to a thousandth of an atmosphere or less as a vacuum, and to specify the pressure, while a space in which the pressure has been reduced to a lesser extent, however little below atmospheric pressure, is sometimes referred to as a partial vacuum. There is, however, no fixed rule in these matters, the term vacuum being in general ap plied to a space in which a low gaseous pressure prevails, and whether a given pressure is considered to be low or not generally depends upon the class of phenomenon or experiment under con sideration.
The study of gases at very low pressures has assumed enormous importance in the last fifty years both from the point of view of pure science and from the point of view of the electrical industry. The study of the discharge of electricity in evacuated tubes which became so marked a feature of physical research at the end of last century, and has remained so since, led to the discovery of the electron and of X-rays, and to the host of important discoveries in atomic physics which is associated with the Cavendish Labora tory at Cambridge, and with such names as Lenard and Wien on the continent. On the other hand the manufacture of electric lamps of all kinds, of X-ray tubes and of thermionic valves has necessitated a profound study in the great industrial laboratories not only of the means of producing vacua and measuring low gaseous pressure, but also of the behaviour and influence of the gases occluded on metal and glass surfaces, and of many properties of the low pressure discharges. It would, perhaps, not be too much to say that the physics of the vacuum dominates the modern laboratory.
Guericke's Experiments.—The first experiments on vacua were made by Otto von Guericke (1602-1686) whose original plan was to fill a vessel with water, and then pump the water out with a water pump, so leaving an exhausted space. His earliest
attempts were certainly carried out before 1652 ; Gerland suggests as early as 5632 to 1638. He filled a cask with water, and pumped out the water by means of a brass fire-squirt, adapted for the purpose. Naturally enough the cask proved not to be air-tight, and the air was heard to rush in through the pores and crevices. He then tried the same experiment with a large copper sphere in place of the cask, but after the pumping had proceeded for some time, the copper sphere collapsed "with a great noise, to the terror of all," a fact which, later, Guericke correctly attributed to the pressure of the air. These early experiments are described in Guericke's famous book De Vacuo Spatio (of which the full title is Experimenta Nova [ut vocantur] Magdeburgica de Vacuo Spatio), which was not, however, published until 1672. After these abortive attempts Guericke constructed several true air pumps, consisting simply of cylinder and piston with two valves, the whole pump being immersed in water to make the joints as far as possible airtight. These pumps are not described by Guericke, but an account of one of them was given, with Guericke's approval, by Caspar Schott in 1657, in his Mechanica Hydraulico-Pneumatica.