Colloidal silica protects colloidal gold both against electrolyte and against spontaneous coagulation by increase of temperature. The stability of unprotected and, in particular, protected sols toward electrolyte increases with increasing temperature. Unprotected sols containing no added electrolyte coagulate spontaneously at 150 degrees to 250 degrees C., but protected sols are stable at 350 degrees C. The stability of protected and unprotected negative gold sols toward electrolyte is increased by the addition of NaOH, within a certain limit, but is decreased by the addition of HCl. The conditions of transportation and deposition of gold in acid and alkaline hydrothermal solutions are discussed.Silica exists as a colloid in water at 25 degrees and 100 degrees , and probably at 350 degrees , in dilutions considerably under the true solubilities reported by Hitchen and by Gruner. Their values may represent a colloidal peptization equilibrium that simulates a solubility curve.