One of the properties of a gas is its pressure. The **pressure, P, **is the force
exerted by the gas divided by the area on which the force is exerted. Pressure =
force/area Because of gravity our atmosphere exerts a downward force and therefore a
pressure on Earth's Surface. The SI unit of force is kg · m/s^{2} and is called
the *newton* (N). The SI unit of pressure is N/m^{2}. It is given the name **pascal
**(Pa) after Blaise Pascal, a French scientist. The pascal is a very small unit of
pressure and so pressures are usually given in kilopascals, (kPa). Chemists have
traditionally used two other units of pressure, based on the mercury barometer. A barometer is *an instrument for measuring the pressure of the
atmosphere. *Such a barometer can be made from a glass tube about one meter in length
that is closed at one end. The tube is completely filled with mercury and inverted into a
container that has additional mercury. At sea level the mercury in the tube falls to a
height of about 760 mm above the level of mercury in the container. The height of the
mercury column is a measure of the pressure of the atmosphere. This height is used to
define a non-SI unit of pressure, which is **mmHg, or millimeters of mercury,** also
called the **torr** (after Evangelista Torricelli, the inventor in 1643 of the mercury
barometer.) The **atmosphere (atm) **is a related non-SI unit of pressure equal to
exactly 760 mmHg. A summary of the relationship among the units of pressure is:

1 atm = 760 mmHg = 760 torr = 1.01325 x 10^{5} Pa = 101.325 kPa = 14.7 lb/in^{2}