SNOW-FLAKES 21 



Hoar-frost often gives us good examples of the 

 tendency of crystals to grow out from their points 

 and edges. When the air is still, very cold, and laden 

 with moisture, a white crystalline growth forms on 

 trees, bushes and grass. If we examine it with a 



FIG. 7. Hollow hexagonal "hopper-crystals" of ice, from Surtshellir Cavern, 

 Iceland. From Grossmann and Lomas, Nature, Oct. 18, 1894. 



lens we shall see many needles, growing at the 

 point. We shall also find hexagonal " hopper- 

 crystals," that is hollow pyramids, attached by the apex 

 and with the cavity turned towards the sky. These 

 are due to more rapid growth of the edges of the 

 crystal. Air-currents and other disturbing causes 



