20 ROUND THE YEAR 



notice that the radius of the circle which bounds the 

 hexagon is exactly the length of one of the sides. It 

 is convenient to have ready a card cut to the figure of 

 a rhomb, with angles of 60 and 120. These can be 

 got from the hexagon. Lines drawn from the angles 

 to the centre meet at 60, and each angle of the 

 hexagon is 120. As a rule, half of one ray of the 

 crystal is enough to show the crystalline form, and it 

 is generally best to draw no more. 



So far we have neglected the thickness of the 

 crystals, and have tieated them as flat. But snow- 

 crystals are of three dimensions, and the third 

 dimension is often too large to be neglected. We 

 saw that the angles of the flat figure projected farther 

 from the centre than the rest, and generally gathered 

 more floating particles to themselves. It is the same 

 with the edges and solid angles of the crystal of 

 three dimensions. Suppose a great number of small 

 spheres to cohere into a crystalline form, which for 

 the sake of simplicity we will suppose to be cubical 

 On one of the flat faces each particle will be half 

 immersed and half exposed. The particles along an 

 edge will be one-quarter immersed and three-quarters 

 exposed The particle at a solid angle will be one- 

 eighth immersed and seven-eighths exposed. The 

 greater the exposure the greater the possibility of 

 attracting floating particles, and this helps us to 

 understand how edges grow faster than flat faces, 

 and solid angles faster than edges. 1 But exceptions 

 to the rule are not uncommon, especially in very 

 small crystals. 



1 Sollas, Nature, Dec. 29, 1892. 



