to be known as Ostwald Ripening and is observed in many situations. An everyday example is the gritty texture that ice cream develops as it ages. On a geologic timescale, many minerals exhibit Ostwald Ripening as their crystal forms evolve as the mineral ages. Related to solubility and crystallization was Ostwald's finding that dissolution of a solid depends on the size of the crystal. When the crystals are small, typically less than a micron, the solubility of the solid in the solution phase is increased. Ostwald quantified this effect mathematically in a relationship that became known as the Ostwald-Freundlich equation. Ostwald first published his finding in 1900, and his mathematical equation was refined by German chemist Herbert Freundlich in 1909. This mathematical relationship also applies to the partial pressure of substance in the system. The Ostwald-Freundlich equation takes into account the surface tension of the particle in the system, in addition to curvature and temperature. The size dependence of solubility is sometimes utilized in the formulation of pharmaceuticals that have low solubility so as to enhance their uptake by the patient. The size dependence also has a role in Ostwald Ripening. 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. Atomic theory Ostwald introduced the word mole into the lexicon of chemistry around 1900. He defined one mole as the molecular weight of a substance in units of mass grams. The concept was linked to the ideal gas, according to Ostwald. Ironically, Ostwald's development of the mole concept was directly related to his philosophical opposition to atomic theory, against which he (along with Ernst Mach) was one of the last holdouts. He explained in a conversation with Arnold Sommerfeld that he was convinced by Jean Perrin's experiments on Brownian Motion. In 1906 Ostwald was elected a member of the International Committee on Atomic Weights. As a consequence of World War I, this membership ended