THE FORMATION OF ICH CRYSTALS 209 
touched freezes, while the streamer disappears; the 
behavior of this streamer which relays the freezing 
action is beautifully demonstrated by slow-motion pic- 
tures, one frame of which is shown in Fig. 3. This 
frame shows the instant when the droplet marked A 
becomes frozen by a streamer from the already frozen 
droplet B. 
There are innumerable variations in the shape of 
window hoars observable in nature, for example, spiral 
patterns, odd arabesque designs, snow-like forms, etc. 
The ordinary glass plate is always covered with an in- 
visible film of some organic substance. It is found that 
the combination of the effect of this mvisible film and 
the atomic nature of ice gives rise to the variation 
observed in the patterns. When the glass surface is 
chemically clean, crystallization takes place very slowly. 
Even when the crystallization is almost complete, the 
hoar crystal is very thin and greatly distorted. The 
effect of an adsorbed film is very well demonstrated by 
exposing the glass plate to alcohol vapor for a short 
time, so that the surface is covered with an invisible 
film of alcohol molecules. Alcohol has a strong affinity 
with water, and the growth of the ice crystal may be 
expected to suffer a marked deformation. The results 
of such an experiment are as might be expected. One 
example is shown in Fig. 4. 
The opposite effect can be observed on a glass plate 
covered with a thin, invisible film of paraffin wax. 
Since the water is repelled by the paraffin film, crystal- 
lization must be free from the effect of the surface. The 
glass plate is well cleaned and desiccated, and then ex- 
posed to paraffin vapor by being kept in a horizontal 
position 5 em above the surface of molten (not boiling) 
paraffin wax. Under favorable conditions hoar crystals 
very much like snow crystals can be obtained on the 
plate. The best example is seen in Fig. 5. The three 
stages of development of a hoar crystal thus obtaimed 
are shown. It is apparent that the window hoar crystal 
also develops hexagonal symmetry if the effect of the 
base surface is eliminated. 
Snow Crystals. The snow crystal is a solid product of 
precipitation formed in the atmosphere by sublimation 
of water vapor on minute solid nuclei. The symmetry of 
a snow crystal is due to its free development in a sus- 
pended state in air. The theory of crystal lattices can 
explain the symmetry of the angle between the faces, 
but it cannot touch upon the question of the symmetry 
of the macroscopic form of a crystal. The extraordinarily 
symmetrical pattern, sometimes observable in the 
hexagonal plane crystals of snow, is favored by the 
rotational motion around the vertical axis, while it is 
fallmg in a nearly horizontal position according to 
hydrodynamic theory. 
The formation of a snow crystal is best classified as 
taking place in two stages: (1) the formation of the germ 
or initial stage of the crystal, and (2) its subsequent 
growth into a snow crystal proper. The snow crystal 
proper is that which we observe on the ground, being 
several millimeters in dimension. The many varieties 
in the shape of a crystal are usually discussed with 
respect to the snow crystal proper, although some 
varieties are also observable among the germs. The 
latter are usually very tiny, being a few hundredths of 
a millimeter in dimension. The well-known experiments 
on cloud modification by I. Langmuir and V. J. Schaefer 
are those of transforming the supercooled cloud drop- 
lets into the germs. The nature of germs and the condi- 
tions for their formation will be found in the article by 
Schaefer? In this article the description is confined to 
the snow crystal proper, which henceforth is simply 
called the snow crystal. 
Dobrowolski’s book [5] is the most comprehensive 
study of snow crystals thus far published. The book by 
Bentley and Humphreys [8] is famous for the vast 
collection of over three thousand photomicrographs of 
snow crystals. The crystal appears quite different if 
the mode of illumination is changed. Transmitted light 
is usually used. Photography using transmitted light 
is advantageous in obtaining a clear picture of the 
boundary and internal structure of the crystal. How- 
ever, ordinary transmitted light does not show clearly 
the topography of the surface. An illumination by 
reflected light increases the beauty of the photograph, 
outlinmg the white image clearly against the dark 
background, but the delicate structure inside the crystal 
is not revealed. Recently M. Hanajima improved the 
technique of photomicrography to a remarkable extent 
by using oblique illumination. He succeeded in taking 
photomicrographs showing both the internal structure 
and the slight ruggedness of the crystal surface. The 
method is shown in Fig. 6. 
PHOTOGRAPHIC LENS 
OF LARGE APERTURE 
PLATE 
MICROSCOPE 
OBJECTIVE OF 
LOW MAGNIFICATION 
Fic. 6.—Method for taking photomicrographs of snow crystals 
(after M. Hanajima). 
HEAT RAY 
FILTER 
Snow crystals of various types photographed by this 
method of illumination are illustrated in Figs. 7-14. 
CLASSIFICATION OF SNOW CRYSTALS 
Principles of Classification of Snow Crystals. The 
famous astronomer Kepler was reputedly the first to 
point out the hexagonal symmetry of snow crystals. 
Descartes [4, p. 298] left the first scientific record of 
snow crystals. He made observations of snow crystals 
in 1635 at Amsterdam. Hooke, the discoverer of plant 
cells, gave the first sketches of snow and frost crystals 
observed through a microscope in his Mvcrographia. 
Hellmann [9, p. 37] in Berlin and Nordenskjéld [12] in 
Stockholm independently classified snow crystals into 
three kinds: planar, columnar, and a combination of the 
two. The basic idea of this system is retained in the 
general classification of snow crystals today. 
2. Consult “Snow and Its Relationship to Experimental 
Meteorology” by V. J. Schaefer, pp. 221-234 in this Com- 
pendium. 
