318 ROLAND LIST 
Fig. 1—Thin section through center of hailstone 57.15 under polarized light; real length of the 
figure, 2.1 cm 
practically constant. If the fallmg particle, how- 
ever, should change its relative direction of fall, 
begin to rotate or be arrested in its initial rota- 
tion, then normally a new center of symmetry 
(Il) is formed and a second generation of unit 
crystals become oriented about this other point. 
Frequently different generations of this kind are 
interrupted by a so-called intermediary phase 
which consists of relatively much smaller crys- 
tals. 
A single hailstone may exhibit a third or even 
more centers of symmetry. Figure 1 gives a par- 
ticular example of a hailstone core having three 
centers of symmetry. Changes of symmetry, 
equivalent to changes of growing conditions, are 
happening over a whole intermediate surface of 
a growing hailstone and can be recognized later 
in the thin sections. This fact helps us to con- 
clude from the variations of symmetry to earlier 
shapes which the hailstone has passed through. 
In the case of hailstone 57.15 (Fig. 1 and 2), for 
instance, we can recognize a conical structure 
which turned into an ellipsoid (symmetry-center 
I). Continued growth led to a rotary ellipsoid 
(symmetry-center II) and afterwards reached 
the final shape of a triaxial ellipsoid compressed 
along the line of the smallest axis (svmmetry- 
center IIT). 
The causes leading to the formation of new 
crystallites are unknown. The assumption that 
there is some connection with the incorporation 
of additional freezing nuclei [List, 195S8ab] can- 
not be maintained, as these would have to be 
active at approximately 0°C. The whole mecha- 
nism follows, however, an apparently strict pat- 
tern, as can be seen from the structural pictures, 
and further research should throw hght on its 
causative principle. 
Bubble structures—Analogous conclusions may 
be drawn from the arrangement of air bubbles 
contained in the ice: bubbles arranged in lines 
indicate the direction of growth; an inner shell 
of bubbles indicates an earher form of the hail- 
stone. Zones of equal density may, with certain 
exceptions, be taken to indicate zones where the 
same growth conditions prevailed. Figure 2 shows 
the same section from hailstone 57.15 as Figure 
1 but seen this time by translucent light. From 
this picture it is clear that the intermediary 
phase I-II has resulted from a change in the 
conditions of growth, causing a shell of bubbles 
to form partially around the growth zone I. As 
