SNOW-FLAKES 17 



great distances tends to impair their form. They are 

 apt to stick together and to form flakes, or to gather 

 moisture on one side more than on another. When 

 the air is quite still and very cold, a thin mist near 

 the ground sometimes turns to snow. This is the 

 " diamond dust snow " of the Arctic regions, and it is 

 believed to consist of very small and unusually 

 perfect crystals. 



We cannot follow by the eye, even when aided by 

 the microscope, all the details of the growth of a 

 snow-crystal. But we can draw or photograph, and 

 try to interpret what we have seen. 



The finest particles of liquid water, condensed from 

 water-vapour, attach themselves to solid bodies, as if 

 attracted by them. They seem to be easily attracted 

 through small distances by spicules of ice, and in 

 solidifying they commonly place themselves regularly 

 with respect to particles which have previously 

 crystallised. 



Suppose that we have to begin with a single 

 needle of ice, and that the moisture suffices to form 

 more needles. They will shoot out from the sides of 

 the first at angles of 60, forming a six-rayed star. 

 The angles are often filled up by the shooting across 

 of fresh needles, which make the same angle of 60 

 with the primary rays, and thus a regular hexagon is 

 formed. It is not uncommon to find among snow- 

 crystals very perfect and simple hexagons. If they 

 grow, they will probably send out rays from their 

 corners. Why from the corners rather than from 

 their flat sides ? Because the corners project farther 

 into the field, and are more exposed to the contact 



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