History of Vacuum

In 1664 the Italian Evangelista Torricelli created the first experiment with a vacuum by setting a glass tube filled with mercury upside down in a basin of mercury. The mercury in the tube fell about 76cm and air could not enter the space it left in the tube. This became known as the Torricellian vacuum. An ideal vacuum contains absolutely nothing, but this does not actually exist in reality. Even in the depths of outer space where matter is spread the thinnest there are still gases and particles. Modern technology canachieve a vacuum of 10-12 Pa, but this leaves 105 gas particles in a 1 litervessel. Ideal vacuum therefore remains purely theoretical.

There is no vessel in the real world that is totally proof against gas leakage.To maintain the vacuum in an enclosed space, you must continuously remove the gas particles. In other words, you must constantly pump gas out of that enclosed space.