230 
tals were grown in small-scale supercooled 
clouds, and on metal surfaces, but the simul- 
taneous growth of all the crystal forms on the 
same fibre has revealed the sharpness of the 
boundaries between one habit and another. For 
example, the transition between the plates and 
needles at —3°C, and that between hollow 
prisms and plates at —S8°C, oecurred within 
temperature intervals of less than one degree. 
Crystals having an almost identical variation 
of habit with temperature have also been grown 
from the vapor of heavy water but with the 
transition temperatures all shifted upwards by 
nearly 4°C, corresponding to the difference be- 
tween the melting points of H.O and D,O. 
These experiments, reported by Hallett and 
Mason [1958], appear conclusive in showing that 
very large variations of supersaturation do not 
change the basic crystal habit as between prism 
and plate-like growth although, of course, the 
growth rates are profoundly affected. On the 
other hand, the supersaturation appears to 
govern the development of various secondary 
features such as the needle-like extensions of 
hollow prisms, the growth of spikes and sectors 
at the corners of hexagonal plates, and the fern- 
like development of the star-shaped crystals, 
all of which occur only if the supersaturation 
exceeds values which correspond roughly to 
saturation relative to liquid water. 
The effect of suddenly changing the tempera- 
ture and supersaturation of the growth form 
of a particular erystal could be observed simply 
by raising or lowering the fiber in the chamber. 
Whenever a crystal was thus transferred to a 
new environment, the continued growth as- 
sumed a new habit characteristic of the new 
conditions. For example, when needles grown 
DISCUSSION 
at about —5°C were lowered in the chamber 
to where the temperature was between —12°C 
and —16°C, stars grew on their ends. In a 
similar manner it has been possible to produce 
combination forms of all the basic crystal 
types. Such radical changes in the crystal shape 
could not be produced by varying the super- 
saturation at constant temperature but, in some 
cases, were produced by only a degree or two 
change in temperature at constant supersatura- 
tion. 
The growth habit of ice crystals is not es- 
sentially altered by growing them in hydrogen 
or air at reduced pressure as recently reported 
by Japanese workers. 
The exact nature of the growth mechanism 
by which only a degree or two variation in tem- 
perature can completely change the crystal 
shape and which, furthermore, allows the habit 
to be reversed four times in a temperature 
range of only 25°C, is still something of a 
mystery. However, our current studies of the 
detailed evolution of the various crystal forms, 
in relation to their surface properties, are 
yielding some valuable clues. 
REFERENCES 
Bryant, G. W., J. Hatverr, anp B. J. Mason, The 
epitaxial growth of ice on single-crystalline sub- 
strates, Physics and Chem. of Solids, 1960 (in 
press). 
Haier, J., anp B. J. Mason, The influence of 
temperature and supersaturation on the habit of 
ice crystals grown from the vapour, Proc. R. 
Soc., A 247, 440-453, 1958. 
Mason, B. J., anp J. Maypanx, The ice-nucleating 
properties of some natural mineral dusts, Q. J. 
R. Met. Soc., 84, 235-241, 1958. 
Mason, B. J., np A. P. vAN DEN Hevuvet, The prop- 
erties and behavior of some artificial ice nuclei, 
Proc. Phys. Soc., 1959 (in press). 
Discussion 
Dr. C. L. Hosler—How did you measure the 
supersaturations ? 
Dr. B. J. Mason—This is a very important 
question. In a diffusion chamber one can not 
measure the supersaturation if one disturbs the 
air. So what I tried to do and what worked out 
to be almost too good to be true, was to insert 
two parallel sheets of ice, one at a temperature 
level 7, and the other at a temperature level 
T.. Knowing the temperature profile, one can 
actually caleulate what the supersaturation will 
be at any level in terms of the upper or lower 
temperature, and so on. One can check it be- 
cause one can arrange that the supersaturation 
at one particular level, according to theory, 
would be just 100 or 98% or whatever value 
one wants to take. Now put im a source of 
nuclei, say sodium chloride; let it settle down 
in this region and observe optically. For in- 
stance, if it started to grow where the humidity 
was 78% and that coincided with the theory, 
one would be quite happy about the theory. 
