Torricelli's Tube

In 1643, Evangelista Torricelli proved air has weight using a tube of mercury. He filled a glass tube, sealed one end, flipped it upside down in a dish of mercury, and watched the column drop to a height of about 760 mm. Above the mercury, a vacuum formed — the first one created in a laboratory.

This was the first barometer, a device that measures atmospheric pressure by the height of a liquid column. The unit mmHg is still used in medicine today.