The change of entropy is, properly speaking, the most characteristic fact of a thermal change.
It is important, however, to remark that if we can thus easily define and measure the difference of entropy between two states of the same body, the value found depends on the state arbitrarily chosen as the zero point of entropy; but this is not a very serious difficulty, and is analogous to that which occurs in the evaluation of other physical magnitudes--temperature, potential, etc.
A graver difficulty proceeds from its not being possible to define a difference, or an equality, of entropy between two bodies chemically different.

We are unable, in fact, to pass by any means, reversible or not, from one to the other, so long as the transmutation of matter is regarded as impossible; but it is well understood that it is nevertheless possible to compare the variations of entropy to which these two bodies are both of them individually subject.
Neither must we conceal from ourselves that the definition supposes, for a given body, the possibility of passing from one state to another by a reversible transformation.

Reversibility is an ideal and extreme case which cannot be realized, but which can be approximately attained in many circumstances.

So with gases and with perfectly elastic bodies, we effect sensibly reversible transformations, and changes of physical state are practically reversible.

The discoveries of Sainte-Claire Deville have brought many chemical phenomena into a similar category, and reactions such as solution, which used to be formerly the type of an irreversible phenomenon, may now often be effected by sensibly reversible means.