silver iodide

substance used to seed clouds, substance used to induce rain

Silver iodide (AgI) is a chemical compound used in photography and as an antiseptic in medicine. Silver iodide is highly insoluble in water and has a crystalline structure similar to that of ice, allowing it to induce freezing (heterogeneous nucleation) in cloud seeding for the purpose of  rainmaking.
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Noun
1. an iodide that is used in photography, in seeding clouds to make rain, and in medicine
(hypernym) iodide