Our purpose was, by a systematic study of the solubilities concerned, to ascertain whether any hydrates of orthophosphoric acid, other than a semi-hydrate, 2H3PO4,H2O (Joly's hydrate), exist. The solubilities of phosphoric acid and Joly's hydrate at various temperatures had not previously been measured. Crystalline, anhydrous phosphoric acid and Joly's hydrate were prepared in pure form. The solid was stirred in a thermostat with water until equilibrium was reached at each temperature, and the concentration of the solution was determined. The diagram (fig. 1) shows the results. AB is the curve of solubilities of Joly's hydrate from −16° to 29·35°, its melting point. BC contains the melting points of Joly's hydrate, depressed by phosphoric acid.