The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice (solid carbon dioxide). Liquid propane, which expands into a gas, has also been used. This can produce ice crystals at higher temperatures than silver iodide. The use of hygroscopic materials, such as table salt, is becoming more popular[citation needed] after promising research.[3] Cloud seeding to increase snowfall is done when temperatures within the clouds are between 19 and −4 °F (−7 and −20 °C).[4] Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation.