Fig. 2—An epitaxial deposit of ice crystals growing only at the steps of a hexagonal growth spiral on 
cadmium iodide 
threshold temperature when introduced into a 
supercooled water cloud in either a diffusion- or 
mixing-cloud chamber. They also cause highly 
purified bulk water to freeze at these same tem- 
peratures. Ammonium fluoride, cadmium iodide, 
and iodine, being soluble in water, are inactive 
in a water-saturated atmosphere but produce ice 
crystals in an environment maintained between 
water and ice saturation, at the temperatures 
indicated. In effect, small particles of these sub- 
stances act as sublimation nuclei, but on enter- 
ing a water-droplet cloud, they go quicky into 
solution and lose their ice-nucleating ability. In 
addition to the substances shown in Table 1, 
whose nucleating ability increases steadily as 
the temperature is lowered beyond the threshold 
value, we find a number of metallic oxides (for 
example, oxides of copper, cadmium, manganese, 
and tin) are slightly active at temperatures be- 
tween —6 and —10°C but show no appreciable 
increase in nucleating ability at lower tempera- 
tures; again, these appear to act as sublima- 
tion rather than freezing nuclei. 
Although there is a tendency for the more 
effective nucleators to be hexagonally symmetri- 
cal crystals in which the atomic arrangement is 
reasonably similar to that of ice, Table 1 shows 
that there are a number of exceptions; but, for 
all the substances which are active above —15°C, 
it is possible to find a low-index crystal face in 
which the atomic spacings will differ from those 
in either the basal or prism faces of ice by only 
a few per cent. However, there is not, in general, 
a high correlation between the threshold nuclea- 
tion temperature and the degree of misfit between 
the ice and the nucleus structures, indicating that 
nucleating ability is only partly determined by 
simple geometrical factors. 
Full details of our work on artificial ice nuclei 
are given in a paper by Mason and van den 
Heuvel [1959]. 
Detailed study of the epitaxial growth of ice 
crystals—In an attempt to investigate the nu- 
cleating mechanism in more detail we have 
studied the growth of ice on well-defined faces 
of single crystals of various nucleating agents 
under carefully determined conditions of tem- 
perature and supersaturation. Oriented deposits 
of ice crystals have been observed on hexagonal 
crystals of silver iodide, lead iodide, cupric sul- 
