NUCLEATION AND GROWTH OF ICE CRYSTALS 229 
phide, cadmium iodide, and brucite, and also on 
freshly-cleaved muscovite mica, mercuric 1odide, 
iodine and calcite. 
This study has revealed the great influence 
of the surface structure and topography of the 
host erystal. The ice crystals show a marked 
tendency to form at special sites on the surface, 
particularly at the edges of growth or cleavage 
steps. This is illustrated in Figures 1 and 2. Crys- 
tals will appear at these preferred locations un- 
der ice-supersaturations of order ten per cent 
but much higher supersaturations exceeding per- 
haps 100%, are required for nucleation on the 
very flat, perfect areas of the substrate surface. 
Some very striking colored effeets, which re- 
veal a good deal about the detailed mechanism 
of ice-crystal growth, have been observed with 
ice crystals growing on a blue erystal of natural 
cupric sulphide (covellite). Figure 3 shows the 
crystals viewed in reflected white light. Being 
only a few thousand angstroms high, the hexag- 
onal plates show interference colors which give 
a measure of their thickness. Inspection of four 
parts of Figure 3 taken at about 30 sec intervals, 
reveals that some crystals grow considerably 
in diameter with no discernible change of thick- 
ness. This suggests that molecules arriving on 
the upper surface of the crystal are not assimi- 
lated but migrate over this surface and are 
built in at the edges. 
The crystals generally thicken after meeting 
a cleavage step on the substrate or when they 
contact a neighboring crystal. This is shown 
very well by the line of five crystals which 
rapidly change color (thickness) after contact, 
with colored growth fronts spreading across 
their surfaces. These ‘accidents’ probably set up 
dislocations in the crystals from which growth 
fronts can emanate. 
There is a marked tendency for the ice 
crystals to cluster along cleavage steps on the 
substrate. A erystal setting astride a step may 
be of different thickness on either side as indi- 
cated by the two-tone effects of certain crystals 
in Figure 3. 
In order to investigate the nucleating prop- 
erties of these single crystalline surfaces in 
more detail, careful measurements have been 
made, at different temperatures, of the minimum 
vapor supersaturations required to produce 
oriented deposits of ice crystals. The results 
for silver iodide are as follows. At temperatures 
above —4°C only water droplets were deposited. 
As the temperature was lowered from —4°C to 
—12°C, increasing numbers of ice crystals formed 
on selected sites provided that the air surpassed 
saturation relative to liquid water. At tempera- 
tures below —12°C, however, crystals appeared 
when the air was sub-saturated relative to water 
but supersaturated relative to ice by at least 
12%. The observations suggest that between 
—4 and —12°C the initial deposit may have 
been liquid water, perhaps in droplets too small 
to be seen before they froze, while at tempera- 
tures below —12°C erystals may appear by 
sublimation direct from the vapor phase. 
Very similar results have also been obtained 
for lead iodide, cupric sulphide, and cadmium 
iodide, with shghtly different critical tempera- 
tures and supersaturations in each case, and 
also for an aerosol of silver iodide introduced 
into a diffusion cloud chamber in which the 
supersaturation could be accurately determined. 
These cloud-chamber experiments show that, 
even at temperatures above —12°C, it is not 
necessary for a silver iodide particle to enter 
a supercooled droplet in order to produce an 
ice crystal; it can act by adsorbing a film of 
liquid water. Full details of the work deseribed 
in this section appear in a paper by Bryan’, 
Hallett, and Mason [1960]. 
The growth forms of snow crystals—Ore of 
our most fascinating problems, and one of great 
importance to the crystal physicist, concerns 
the remarkable variety of shapes exhibited by 
natural snow crystals. In order to discover the 
factors which influence the crystal form, and in 
the hope of discovering the exact nature of the 
controlling mechanism, we are growing artificial 
snow crystals under very carefully controlled 
conditions. 
The crystals are grown on a thin fiber running 
vertically through the center of a diffusion cloud 
chamber in which the vertical gradients of 
temperature and supersaturation can be ac- 
curately controlled and measured. The results 
of many experiments covering a temperature 
range of 0 to —50°C and supersaturations vary- 
ing from a few per cent (in the presence of a 
water-droplet cloud) to about 300% (in very 
clean, droplet-free air) consistently show that 
the crystal habit varies along the length of the 
fiber in the following manner: 
0 to —3°C Thin hexagonal plates 
—3 to —5°C Needles 
—5 to —8C Hollow prisms 
—8 to —12°C Hexagonal plates 
2 tor —16r© 
—16 to —25°C 
—25 to —50°C 
Dendritic crystals 
Plates 
Hollow prisms 
This scheme is very similar to that which we 
obtained in earlier experiments in which crys- 
