Vacuum means literally absolutely empty space; but the perfect realization of this has hitherto been found to be experimentally impossible, although a point can be reached when the amount of gaseous matter left behind in a vessel is so small that it is almost unmeasurable, and is so attenuated that it has lost many of the properties usually ascribed to matter. By inverting a barometer tube carefully filled with mercury a space -- known as the Torricellian vacuum -- is left above a column about 30 inches high of mercury. This space, however, is not a perfect vacuum: it contains mercury vapour. Such a method of obtaining a vacuum is, however, not of great practical utility, and different sorts of air-pumps (q.v.) have been invented, the Sprengel being the one most commonly employed. The extensive use of incandescent electric lamps has caused vacua to be of considerable commercial importance, and has tended to cheapen and simplify the apparatus employed for their production. A vacuum is generally described as one of a certain fraction of an inch or of a millimetre: thus the vacuum of 1/4500 inch obtained by Andrews when carbonic acid gas was removed from the receiver of his air-pump was simply one in which the pressure of the remaining gas was only sufficient to support a column of mercury 1/4500 inch high. Since the initial pressure would have been about 30 inches it is seen that only about 1/135000 of the original gas remained behind.