This paper will summarize the experimental work at the University of Chicago on the problem of the onset of thermal instability in a layer of fluid heated from below. The purpose of this work has been to test certain theoretical predictions of the Rayleigh number at which instability sets in, and to determine the type of instability which appears at the critical point. The earlier experiments of this series were done at the hydrodynamics laboratory of the University of Chicago in connexion with a program of meteorological reseach[1, 2, 3, 4]. The current work is being done at the newly organized hydromagnetics laboratory of the Enrico Fermi Institute of Nuclear Studies. This laboratory utilizes the magnet of the old Chicago cyclotron, with pole pieces 92·7 cm in diameter and a gap of 22·1 cm. The magnet was reconstructed to allow the field strength to be varied from 0 to 13,000 gauss; the field is uniform to better than 1 % over the experimental area. The new laboratory is under the administrative supervision of Professors S. K. Allison and S. Chandrasekhar; the experiments are being done by Y. Nakagawa. The theoretical investigations are primarily the work of Chandrasekhar[5, 6, 7, 8, 9, 10] and it will be convenient to review some of his results before discussing the experiments.