1.   So, at the thermal equilibrium, the pressure and  volume are inversely proportional to each other, i.e.
                                                   ---  Boyle's Law.
     2.   Similarly, by simple experiments we can observe that at constant pressure, volume of the gas is directly
           proportional to the temperature and at constant volume, pressure of the gas is directly proportional to the
           temperature.
                                                ---   Charles Law.

We can summarize the above two observations to the following relations,
                                              ................................... ( 2 )        

          If we take two different kinds of gases at same pressure, volume and temperature, then the quantity of the gas should be the same. The quantity of the gases can be better represented by the number of molecules. So, it is clear that, the constant in equn(2) is related to the number of molecules. So, equn(2) can be written as

                                                 ................................... ( 3 )
where joule/K  is Boltzmann constant.