SNOW CRYSTAL NUCLEI 



When the central part of a snow crystal is 

 examined under an ordinary microscope, it 

 is usually seen as a tiny ice crystal of a stellar 

 crystal with an hexagonal or small stellar 

 pattern having a diameter of about 30 n, in 

 the case of a hexagonal plate type. These 

 central patterns are called the central por- 

 tion of the snow crystal in this article. Since 

 the mean diameter of the central portion 

 was 20 M or 30 ix, this central portion could 

 be arranged in a void of the mesh. In the 

 case of sno^^'falls not accompanied by strong 

 wind, snow crj^stals fell into the igloo 

 through a window opened in the ceiling of 

 the snow cavern. They were received on a 

 clean glass plate. Then a snow crystal was 

 picked up and put on a mesh which was 

 covered with collodion film. It was important 

 to observe the mode of sublimation of snow 

 crystals to see whether the supposed nucleus 

 could be expected to remain on the collodion 

 film without displacement. Successive stages 

 of sublimation of specimen crystals were 

 taken through a microscope with a magnifi- 

 cation factor of about 60. A snow crystal be- 

 gins to evaporate by sublimation from the 

 tips of the crystal, and the thicker central 

 portion of the crystal remains to the last 

 stage without marked displacement, as 

 shown in Fig. 1. It takes about 10 minutes 

 for a small crystal to vanish by sublimation. 

 The snow crystal, thus arranged on the col- 

 lodion film, was kept in a desiccator in the 

 igloo. In this condition the snow sublimated, 

 leaving the nucleus on the collodion film. It 

 was concluded that the nucleus remains 

 safely on the collodion film of the mesh. The 



central portion which vanished in the last 

 stage of the optical micrograph could be ob- 

 served through an electron microscope, and 

 the center nucleus pattern could be taken in 

 an electron micrograph. 



The nuclei (9, 16, 21) of cloud and fog are 

 prepared in a similar method. They are col- 

 lected on a collodion film of the mesh by the 

 use of an impactor (5). Millipore (2, 10) 

 filters also are used to identify atmospheric 

 particles of micron size. 



Replicas of Snow Crystals. Replicas of 

 snow crystals for electron microscopy are 

 made by using 0.2-0.01 per cent solution of 

 Formvar (18) dissolved in ethylene dichlo- 

 ride. Snow crystals are received on a piece 

 of black velvet, then a suitable crystal is 

 picked up and put on a mesh. The snow 

 crystal is kept in a cold chamber at — 20°C 

 and is gradually evaporated by sublimation 

 from the crystal surface. At the midpoint of 

 the evaporation, the crystal surface is coated 

 with 0.1 per cent 'Tormvar" solution. The 

 replica film is not peeled off from the mesh, 

 but it is used in situ. The shadowing is done 

 in a vacuum vessel by evaporating chromium 

 to the top surface of the replica with a suit- 

 able angle. 



Fig. 2 shows a center nucleus and etched 

 surface (12, 14) of a natural snow crystal. 

 The shape of the snow crystal is an hexag- 

 onal plate with dendritic branches. The nu- 

 cleus is 3.5 M in the largest extension. The 

 rounded pattern around the nucleus is the 

 image of the center of the crystal. The diam- 

 eter of this rounded pattern is 8 /z. Numerous 



jmrM 



Fig. 1. Sublimation of a snow crystal on a mesh 



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