Pressure is energy density or force per unit area. Pressure pushes out on balloons and keeps them firm. A change in pressure sometimes makes your ears pop when you go up in a fast elevator.

The principal effect of increasing pressure on a substance, is a reduction in volume and a shortening of inter-atomic distances. These changes cause many continuous and discontinuous changes in the physical properties of the substance.

Simon Stevin (1548-1620) published books on liquid forces and pressures in 1605.

In 1650 Otto von Guericke (1602-1686) invented an air pump, and used it to create, and demonstrate, vacuums.

Evangelista Torricelli (1608-1647) invented the barometer and was the first to measure air pressure.

In 1661 Robert Boyle (1627-1691) determined that the volume of a gas varies inversely with its pressure when the temperature is held constant.

In 1787, Jacques Alexandre Charles (1746-1823) observed that the volume of a gas varies in proportion to its temperature when the pressure is held constant.

Blaise Pascal (1623-1662) rediscovered and popularized Stevin's work.

Daniel Bernoulli (1667-1748) explained air pressure.

Around 1865, James Clerk Maxwell (1831-1879) observed that the physical properties of aggregates of bodies would have to be expressed statistically.

Around 1875, Ludwig Boltzmann (1844-1906), showed that large-scale, physical phenomena, such as volume, pressure and entropy, could be explained by statistically examining the microscopic properties of a system.

About 1900, Johannes Diderik van der Waals (1837-1923) proposed modifying the "ideal gas law" to (pressure + a / volume) * (volume - b) = temperature * R, where a and b are parameters that vary from gas to gas, and "R" is a universal constant that equates mechanical energy to energy in the form of temperature.