Any change involving decrease of entropy is impossible.
The entropy cannot diminish.
Since the condition of heat-isolation is impracticable, the condition of maximum entropy cannot, as a rule, be directly applied, and it is necessary to find a more convenient method of expression.
In practice, however, there is always some frictional dissipation, accompanied by an increase of entropy and by a fall of pressure.
Changes of entropy must be calculated in terms of quantities of heat, and must be interpreted in a similar manner.
The entropy tends to a maximum, and the state is one of stable equilibrium when the value of the entropy is the maximum value consistent with the conditions of the problem.
The total entropy of the system is found by multiplying the entropy per unit mass of the substance in each state by the mass existing in that state, and adding the products so obtained.