508 .-- ED.] In the minds of many persons, however, grave doubts persisted.
Solution appeared to be an essentially irreversible phenomenon.

It was therefore, in all strictness, impossible to calculate the entropy of a solution, and consequently to be certain of the value of the thermodynamic potential.

The objection would be serious even to-day, and, in calculations, what is called the paradox of Gibbs would be an obstacle.
We should not hesitate, however, to apply the Phase Law to solutions, and this law already gives us the key to a certain number of facts.

It puts in evidence, for example, the part played by the eutectic point-- that is to say, the point at which (to keep to the simple case in which we have to do with two bodies only, the solvent and the solute) the solution is in equilibrium at once with the two possible solids, the dissolved body and the solvent solidified.

The knowledge of this point explains the properties of refrigerating mixtures, and it is also one of the most useful for the theory of alloys.