When one has made use of materials which in part belong to that person and in part to another, in order to form a thing of a new description without having destroyed any of the materials, but in such a way that they cannot be separated without inconvenience, the thing formed is common to both proprietors in proportion, as respects the one, of the materials belonging to that person, and as respects the other, of the materials belonging to that person and the price of that person’s workmanship.