1. The solubilities of silicic acid gels in water at 25.9° and 97° were studied. 2. It was shown that the silicic acid that passed into solution was present entirely in, the form of simple molecules and ions capable of reacting with molybdic acid. The equilibrium concentration was attained slowly at 25° (12.5·10−5 g/ml SiO2) and rapidly at 97° (41.5°10−5 g/ml SiO2). 3. A study was made of the behavior of silicic acid during the evaporation of its solutions and during the cooling of solutions saturated at 97°. 4. A study was made of the effect of prior heat treatment of a silicic acid gel on its solubility. It was shown that prior heating at 100, 200, 500, 800, or 900° has no effect on the solubility of silicic acid, but the rate of dissolution falls as the temperature of the previous heat treatment is raised. The solubility of silicic acid calcined at 1120° is very greatly reduced.