Silver iodide particles have been found to serve as nuclei for the formation of ice crystals in super‐cooled water and in water vapor super‐saturated with respect to ice. It is believed that silver iodide serves as a very effective nucleus because it very closely resembles ice in crystal structure. Both dimensions of the unit cell of ice and silver iodide are the same to within approximately one percent. The maximum temperature at which the silver iodide particles serve as nuclei is approximately −4°C for particles one micron in diameter, and −8°C for particles 100 Angstrom units in diameter. A silver iodide smoke generator has been constructed which consumes 1 mg of silver iodide per second and produces 1013 effective nuclei per second.

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