Experiments in a cold chamber indicate that the crystallization of supercooled clouds can not be explained on the basis of the concept of sublimation nuclei. Materials that are not crystalline, and others which do not have a crystal form and dimensions resembling ice, exhibit the ability to produce ice crystals in a cloud. The effectiveness of some substances in crystallizing clouds appears to be related to the part they may play in altering the surface structure of small drops. All of the active materials provide polarizable ions or molecules which, on entering the surface of droplets, lower the surface free energy. This lowering of the energy may permit the small droplets to change to ice crystals with a decrease in the energy of the system, whereas, in the absence of the polarizable ions, the increase in surface energy upon freezing would exceed the decrease in internal energy and thus the change of state could not take place.