SNOW CRYSTAL NUCLEI 



Fig. 4. A nucleus and the electron-diffraction pattern. 



the center nucleus. The particles had been 

 in the branch of the snow crystal. The 

 particle of 4 yu diameter in Fig. 3c is the 

 nucleus of this snow crystal. The nucleus 

 consists of two kinds of materials which are 

 divided into two parts, upper and lower, as 

 shown in Fig. 3c. The shape and electron 

 diffraction pattern of the lower part of the 

 nucleus is similar to an attapulgite, and the 

 upper part is similar to kaolinite. They are 

 a kind of clay mineral which is of wide dis- 

 tribution. Fig. 4a shows the center nucleus of 

 a dendritic snow crystal. The nucleus is a 

 thin hexagonal crystal. The shape is similar 

 to kaolinite, whose largest extent of the nu- 

 cleus is 4 M- Many minute substances are 

 seen around the nucleus; some of them are 

 found to be soluble in water. The electron 

 diffraction pattern of the nucleus in Fig. 4b 

 shows an hexagonal cross grating which is 

 expected to be obtained when the direction 

 of the incident beam is normal to the base 

 plane of kaolinite, a clay mineral. 



The size of the condensation nuclei lies 

 between 0.01 and 0.5 m, and it seems that 

 there are two kinds of this nuclei. Fig. 5 

 shows this point clearly. A frequency curve 

 the size of condensation nuclei has been made 



0.1 0. 2 0.3 0. 4 0.5 



Diameter of condensation nuclei /» 



Fig. 5. Frequency distribution of condensation 

 nuclei. 



from 1200 data obtained from four electron 

 micrographs. One sees clearly two kinds of 

 condensation nuclei, the most frequent diam- 

 eter of the smaller kind being 0.05 n, as 

 compared with 0.15 /x for the larger kind. 

 The concentration of condensation nuclei is 

 larger in thick-rimmed crystals than in thin 

 crystals, and also influences the concentra- 

 tion of air pollution in the atmosphere in 

 which the snow crystal grows and through 

 which it passes. 



The Center Nucleus Substance. A snow 

 crystal is formed on a nucleus by sublimation 



257 



