Any particle upon which an ice crystal may grow by the process of sublimation. A. Wegener (1911) was the first to suggest that the atmosphere might contain nuclei upon which sublimation (deposition) of water vapor directly to ice occurs in a manner similar to condensation upon condensation nuclei. In experiments where various nuclei are added to supercooled clouds, it is generally difficult to determine whether these nuclei function as deposition nuclei or as freezing nuclei. It has never been demonstrated that natural deposition nuclei (as distinguished from freezing nuclei) exist in the atmosphere. In the laboratory, cases of artificial preparation of true sublimation nuclei have been reported that activate ice crystals without reaching anywhere close to water saturation; lack of such nuclei in the atmosphere results from the predominance of mixed nuclei.