State territory — Acquisition of territorial sovereignty — Cession — Treaties of Paris, 1814 — 1815 — Treaties between France and Baden, 1820 and 1840 — Boundaries between French and Baden Communes — Territory separated from Baden by Rhine boundary — Creation of French Commune of Rosenau — Subsequent agreement between French Communes of Rosenau and Kembs to transfer part of ceded territory to Kembs — Whether valid — The law of France.