Quantitative measurements on gases
were first made by the English chemist Robert Boyle (1627  1691). Boyle
used two instruments to measure pressure: the manometer, which measures differences
in pressure, and the barometer, which measures the total pressure of the
atmosphere.

The operation of a manometer, which is simply a bent piece of tubing,
preferably glass with one end closed. When the fluid level in both arms is
the same, the pressure of the sample of gas inside the closed end must equal
the pressure of the external atmosphere since the downward force on the two
columns of liquid is then equal. When the liquid levels are unequal, the
pressures must differ. The difference in pressure can be measured in units
of length of the vertical column of liquid. The mmHg, or its modern version
the torr, originated in this use of the manometer. Mercury is particularly
convenient for use in manometers (and barometers) because at room temperature
it has low vapor pressure, does not wet glass, and has a high density. Other
liquids such as linseed oil or water have also been used in manometers.

The barometer was invented by Torricelli, one of Galileo's students.
It is a device for measuring the total pressure of the atmosphere. A Torricellian
barometer can easily be constructed by taking a glass tube about a meter long,
sealing one end, filling the tube completely with mercury, placing your thumb
firmly over the open end, and carefully inverting the tube into an open dish
filled with mercury. The mercury will fall to a height independent of the
diameter of the tube and a vacuum will be created above it. The height of
the mercury column will be the height which the atmospheric pressure can
support. The standard atmospheric pressure, one atmosphere (atm), is 760
mmHg but the actual atmospheric pressure varies depending upon altitude and
local weather conditions. For this reason barometers can be used to help
predict the weather. A falling barometer indicates the arrival of a low pressure
air system, which often means stormy weather. A rising barometer indicates
the arrival of a high pressure air system, and that often means clear weather.

While mercury is again the most convenient liquid for use in barometers
it is by no means the only liquid which can be used. Preparation of a water
barometer, and many of the early barometers did use water, requires use of
a vacuum pump (or arms 13 meters long).

Units of Pressure

Units of pressure were originally all based on the length of
the column of liquid, usually mercury, supported in a manometer or barometer.
By far the most common of these units was the mmHg, however, the modern SI
unit of pressure is derived from the fundamental units of the SI. Pressure
is force per unit area, and force is the product of mass times acceleration,
so the SI unit of pressure is the kg m s^{2}/m^{2} or newton/m^{2},
which is called the pascal (Pa).

All of the older units of pressure have now been redefined in terms
of the pascal. One standard atmosphere, the pressure of the atmosphere at
sea level, is by definition exactly 101,325 Pa. The torr, named in honor of
Torricelli, is defined as 1/760 of a standard atmosphere or as 101,325/760
Pa. The mmHg can be considered identical to the torr. The term bar is used
for 100000 Pa, which is slightly belowone standard atmosphere.

Robert Boyle and his Law 
Boyle used the manometer and barometer to study the pressures and
volumes of different samples of different gases. The results of his studies
can be summarized in a simple statement which has come to be known as Boyle's
law:

At any constant temperature, the product of the pressure and the volume
of any size sample of any gas is a constant.

For a particular sample of any gas, Boyle's law can be shown graphically
as is done in the Figure below. It is more common to express it mathematically
as P_{1}V_{1} = P_{2}V_{2}.

The pressure and the volume vary inversely; as the
pressure increases the volume of the sample of gas must decrease.

Example: A sample of gas occupies a
volume of 47.3 cm^{3} at 20^{o}C when the pressure is 30
cm of mercury. If the pressure is increased to 75 cm of mercury, the
sample will occupy a volume of 47.3 cm^{3} (30 cmHg/75 cmHg) = 18.9
cm^{3}.
