The general law of expansion or compression of a perfect gas is pVn = Constant. It gives the relationship between pressure and volume of a given quantity of gas. The value of n depends upon the nature of gas., and condition under which the changes take place. The value of n may be between zero and infinity. But the following values of n are important from the subject point of view.

1. when n = 0. This means pV0 = constant, i.e.p = constant. In other words, for the expansion or constant of a perfect gas at constant pressure, n = 0.

2. when n = 1 ; then pv = constant, i.e the expansion or compression is isothermal or hyperbolic.

3. when n lies between 1 and n, the expension or compression is polytropic, i.e. pVn = Constant.

4. when n = & the expension or compression is adiabatic

5. when n = infinity the expansion or compression is at constant volume, i.e. v - Constant.