Cooling and Heating Curves for Water Solutions of Uranyl Nitrate between 0° and -70° C.—A thermo-junction was placed in 0.5 cc. of a solution of known concentration and galvanometer readings were taken every 10 seconds during the cooling. At about -35° a rapid evolution of heat took place accompanied by expansion sufficient to break the glass tube if the solution filled more than the rounded bottom. On heating the solution a corresponding absorption of heat was observed at -20°. From the observed temperature changes and the specific heat the amount of heat evolved or absorbed was computed. The results of 67 experiments in which the concentration was varied from 10 to 55 per cent. were plotted and show a sharp maximum at 48.0 per cent. for the evolved heat and at 47.5 per cent. for the absorbed heat.