558 



THE NATURE BOOK 



its borders ; it continues to draw the 

 invisible vapour from the air and builds 

 it up into a symmetrical structure. If 



.V 



/• \. //. ir>-;. 

 'ALL WITHOUT IS SILENT AND BEAUTIFUL." 



one part of the crystal has an ad\'antage 

 over another, growth may take place 

 more rapidly at such a place, but stiU 

 the original plan is followed, and no 

 departure is ever made from the charac- 

 teristic form it received at its birth. 

 It would take too long to demonstrate, 



but it can be proved that a hexagonal 

 plate, abstracting material from the sur- 

 rounding medium, wiU grow more rapidly 



at the angles than in 

 the middle of an 

 edge, and so in an 

 ice crystal there is 

 a tendency for spi- 

 cules to grow out- 

 wards from the six 

 corners. In this 

 way we arrive at the 

 star-shaped flake so 

 characteristic of 

 snow. On a stiU 

 day, during the first 

 few minutes of a 

 snowfaU, we c a n 

 often catch these 

 beautiful stars on 

 a coat sleeve or 

 other cold object, 

 and examine their 

 wondrous form s. 

 Although they never 

 vary from the typi- 

 cal six-rayed type, 

 the}' present an in- 

 finite variety. Some- 

 times the raj'S are 

 simple rods of ice, 

 at other times the 

 rods themselves act 

 as centres of gi^owth 

 and secondar\- rods 

 shoot out hke 

 branches from a 

 stem. Such out- 

 growths always 

 make an angle of 

 60° with the pri- 

 mary rod, and thus 

 preser\-e the hex- 

 agonal s}'mmetry. 

 From the secondary 

 rods other branches 

 may spring, and on 

 the original founda- 

 tion structures may 

 although subject to 

 the control of forces, possess character- 

 istics of their own. The ice stars are so 

 fragile that when they collide, the 

 minute rods are broken, and the larger 

 sncn\llakes are as a rule composed of 

 matted aggregates of fragmentary spicules. 



:.-::aw IIM. i,.U . 



be built up which, 



