(13) Furthermore let the temperature of each heat reservoir, at the instant when it comes into use, be equal to the simultaneous temperature of the body; then the cycle process becomes a reversible one and the inequality of the second law becomes an equality,
and substitution of above value for
gives
In this equation there occur only quantities referring to the state of the body itself and therefore it can be interpreted without any reference to the heat reservoirs. It contains the following proposition:
"If a homogeneous body by suitable treatment is allowed to pass through a series of continuous states of equilibrium and thus finally to come back to its initial condition, the summation of the differential,
