Colloidal silica of 30, 20, 10, and 5% concentrations by weight have been used to produce small-angle scattering. The resulting scattering showed plateaus which varied in angular position with concentration in the very small angle portion and then showed an approximation to the single particle scattering in the larger small angle region. This scattering from individual scattering centers followed that predicted by Debye. Average distances of separation of randomly spaced particles as determined from the particle-to-particle scattering plateaus checked those calculated from the concentrations. In addition, scattering from various sized particles ranging from diameters of 100 to 500 000 A were measured. Two theories, a refraction and reflection or ray theory and a diffraction or wave theory, have been applied to the analysis of this scattering. A comparison of the particle sizes as obtained using these two theories with the particle size as obtained by means other than x-ray scattering shows that for the larger particles, geometrical optics can be used satisfactorily; while for the small particles the diffraction theory gives the correct scattering relation. Particle size measurements were made on both wetted (colloidal) and dried samples.