There are two specific heats of gases- Specific Heat at Constant Pressure (Cp) and Specific Heat at Constant Volume (Cv). Heat supplied at constant pressure is used up in two ways: (a) for doing work in expansion and (b) for raising the temperature by 1° C, while at constant volume heat supplied is used only for rise of temperature. Therefore, Cp is always greater than Cv. They are related as: Cp−Cv = R/J. This relation is called Mayer’s Relation