Robert von Helmholtz (1886) "Untersuchungen über Dämpfe und Nebel, besonders über solche von Lösungen" (Investigations of vapors and mists, especially of such things from solutions), Annalen der Physik, 263 (4): 508–543. On pages 523–525, Robert von Helmholtz converts Kelvin's equation to the form that appears here (which is actually the Ostwald–Freundlich equation).