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Ice was grown in a cold chamber mounted on a Vickers Projection Microscope and so constructed that the temperature and humidity could be independently controlled. Observations were made (magnification 280×to 560×) of the growth of ice crystals on a freshly-cleaved mica substrate. No growth occurred until saturation with respect to water was reached; it was concluded that nucleation of ice on mica is a two-stage process above −40C. Upon subsequent regrowth of ice, certain places were observed which acted as sublimation nuclei. These results are qualitatively discussed in terms of present nucleation theory.

A cold box was constructed to study ice crystal growth on single crystals of lead iodide. When the temperature of the PbI2 substrate was slowly lowered past the temperature of the ice surface, crystal growth was observed first at the edges of the crystal and then at certain preferred sites on the PbI2 surface. If water drops appeared on the crystals, there was a reduction in the number of active, nucleating sites. This was especially noticed at the edges of the crystal. It was noted that the threshold temperature for ice crystal growth, as determined by visual observation, was −6C. From the temperature-vapor pressure curve, it appears that PbI2 acts as a freezing nucleus from −6C to about −12C to −14C and a sublimation nucleus at temperatures lower-than −12C to −14C.

Clean, single crystals of silver iodide have been found to serve as a very poor nucleating agent for ice. The controlled introduction of impurities on the crystal surface by means of photolytic decomposition has been shown to greatly increase its nucleating ability.