Collaborating with German chemist Raphael E. Liesegang, Ostwald recognized that substances can crystallize in a periodic fashion wherein the crystallization behavior follows a spatial or temporal pattern. In certain circumstances, the result of this periodic crystallization behavior is easily visually observed, for example, in various geologic formations. Liesegang had previously investigated this phenomenon in specific laboratory experiments, showing his results to Ostwald. Ostwald then developed a mathematical model for the phenomenon that served to explain the observations and realized how widespread is the periodic crystallization behavior. These observations came to be known as Liesegang rings.