Direct force measurements are reconciled with the well-known anomalous colloidal behavior of silica sols, To overcome the difficulty of surface-altering treatments the force between layers of nanometric colloidal silica particles adsorbed from solution is measured using a surface force apparatus, Coating substrates with nanometric particles gives a surface that is smooth only at the nanometer scale, but which is fully representative of a colloidal silica-water interface. Force measurements and colloidal stability can thus be compared, leading to direct evidence of chemical adhesion between silica particles.