SNOW-FLAKES 



their place of intersection. The fourth axis is usually 

 so short that the crystal has hardly an appreciable 

 thickness ; but it may be long. Some snow-crystals 

 take the form of double stars separated by a 

 prismatic rod longer than the diameter of the stars. 

 Here the prismatic rod lies along the fourth axis, 

 which is often called the principal axis. 



If we mark points on the three lateral axes, 

 equidistant from their intersection, and join these one 

 to another by straight 

 lines, we shall get a 

 regular hexagon, whose 

 angles are, of course, 



8 -f = 120 (Fig. 4.) 



Hexagonal plates often 

 enter into snow-crystals, 

 forming the centre, or 

 tipping the rays ; some 

 crystals are nothing but 

 hexagonal plates. 



Solid water in all its 



forms is essentially similar to the snow-crystal. During 

 a thaw six-sided prisms sometimes stand out from 

 the surface of the melting ice. The spicules which 

 shoot across the surface of freezing water make 

 angles of 60. Hoar-frost, when studied with a lens, 

 is seen to be built up of six-sided crystals. 



If we take any two adjacent triangles in Fig. 4 and 

 join them, they will form a four-sided rhomb. Three 

 such rhombs make up the hexagon. Rhombohedrons 

 are often seen to form the side branches of a principal 



FIG. 4. A regular hexagon in a circle, 

 with three axes crossing at angles 

 of 60. 



