an air-free space above the free surface of a liquid in a vessel that is closed above the liquid.

In 1643 the Italian physicist V. Viviani found that if a sufficiently long glass tube, closed at one end, is filled with mercury and immersed at the open end in a container of mercury, the mercury in the tube descends, and a vacuum is formed in the tube above the surface of the mercury. E. Torricelli was the first to explain this phenomenon; hence the term “Torricellian vacuum.” He stated that the pressure of the atmosphere on the surface of the mercury in the container is counterbalanced by the weight of the mercury column. At sea level the height of the column is approximately 760 mm; if the tube is longer than 760 mm, a vacuum is formed above the surface of the mercury. Torricelli thus refuted the notion then dominant in physics that nature abhors a vacuum and therefore mercury fills a tube, water fills the suction line of a pumping device, and so on. In addition, he thus demonstrated the existence of atmospheric pressure.

Torricelli showed that the pressure of the atmosphere can be measured, and he invented the mercury barometer.