The nucleation and growth of particles can be independently determined by simultaneous measurement of two quantities having different functional dependencies upon particle number and size. Light scattering and optical absorption are the two quantities chosen to observe the precipitation kinetics of a gold colloid. When an exposed photosensitive glass is treated at high temperatures, gold precipitates on a fixed number of sites. On the other hand, increasing the concentration of reactants causes spontaneous gold precipitation in which growth sites form continuously throughout the precipitation process. This indicates that the photosensitive reaction forms nuclei in a supersaturated solution which cannot nucleate spontaneously. In agreement with this, the experiment shows that the number of growth sites increases with exposure while the growth rate is the same for all exposures. Finally, assuming that the particles are spheres, absolute values are derived for the initial change of particle size with time and for the number of nuclei as a function of exposure.