The existence of a vacuum, a space completely empty of matter, had been debated since at least the ancient Greek philosophers, and probably much longer. In 1643 the Italian physicist Evangelista Torricelli showed that for all practical purposes a vacuum was indeed possible.
Torricelli discovered the vacuum accidentally when he was conducting experiments that were designed to solve the problem of pumping water out of a deep well. He tried to scale down the problem using mercury instead of water because liquid mercury is much more dense than water and he hoped to be able to observe the same phenomenon at a lower height. He took a tube closed at one end and filled it with mercury. He stuck the open end in a bowl of mercury and slowly raised the closed end, where eventually a gap appeared about the mercury. The gap could not have been air because when he lowered the tube again the gap vanished immediately, quicker than air could have dissipated.
The discovery of the vacuum was eventually applied to advances in technology and its principle is used in heating and cooling systems, light bulbs, steam engines, cathode ray tubes, and more.