Hamilton Perkins Cady (May 2, 1874 – May 26, 1943) was an American chemist who in 1907 in collaboration with David McFarland discovered that helium could be extracted from natural gas.
After attending Carleton College, Northfield, Minnesota, he enrolled at the University of Kansas, where he graduated in 1897 with an A.B., and in 1903 with a Ph.D. During his senior year he carried out the initial experiments on the electrical conductance of solutions of salts in liquid ammonia, opening up a new and fruitful field of research.
This constituted the first step in the development of the ammonia system of compounds, a concept which, owing to the later contributions of Edward C. Franklin and Charles A. Kraus, became an outstanding feature of American chemical achievement.
Following the completion of his undergraduate work he was granted a scholarship and then a fellowship at Cornell University where he spent the years 1897–1899 in research under the direction of Wilder D. Bancroft.
In 1899, Cady returned to the University of Kansas as assistant professor of chemistry and resumed his work with liquid ammonia in collaboration with Franklin, carrying out a thesis consisting of a study of concentration cells in this solvent.