What does a Pressure of Such-and-Such Really Mean?
We always hear about the barometric pressure – or the level of a vacuum – in terms of ‘mm or mercury’ or ‘inches’. 1 atmosphere (at sea level under some unidentified ideal conditions) is also said to be 14.7 pounds per square inch. Why? The earth is covered with a vast ocean of air. Despite common experiences, even air has mass and mass implies weight. We know it has volume or else your automobile would have a real problem with flat tires. Most of the volume (the contribution from the volume of the the protons, neutrons, and electrons in the atoms are negligible but not precisely zero) results from the constant motion of the molecules (in air or other gas) bouncing against each-another due to their thermal motion. This also keeps the air in a gaseous state. At really low temperatures, the motion is reduced resulting in liquid and solid phases of even air. At exactly absolute zero (0 °K or -459 °F or -273 °C) all motion ceases. However, even then most of the volume of the frozen air is
