64 COLOUR, FORM, AND MOVEMENT. 



crystals are not more obvious to the ordinary observer than 

 they are is that they are so frequently broken by mutual 

 pressure, etc. 



We have taken a piece of granite to exemplify the con- 

 dition of mineral matter, and in it we may see crystalline 

 structure. Not, certainly, in the simplest form in which 

 it might be seen ; still a close examination will reveal 

 the straight lines of innumerable crystals inextricably 

 mixed up. 



If our pupils wish to see a simpler case, it may be 

 possible to exhibit a large quartz crystal which has had 

 room to grow, and consequently displays its angles and 

 faces perfect. In this connection we may appeal for a 

 closer examination of the fern-like tracery of the frost 

 figures on the window pane, or an examination of the 

 snowflakes as they fall upon us out of doors. And above 

 all we must give effective point to the lesson by growing 

 crystals. 



What are the simpler characters of crystals which may 

 be pointed out to our pupils ? 



1. They are bounded by straight lines, not curves. 



2. They have flat surfaces, termed faces. 



3. In crystals of the same substance the angle between 

 corresponding faces is constant, whether the crystal be 

 large or small. 



4. There are different types, distinguished by the number, 

 length, and position of their " axes." 



Some Common Crystals and their Appearance. 



1. Snow. Snow crystals have six rays. They consist of 

 " solid rods or flat scales, each with six sides, others are 

 six-sided pyramids, the most common are six-pointed 

 stars." The six rays constitute three lateral axes, and 

 there is a very short vertical axis in the centre. They are 

 white because of the large amount of air enclosed between 

 their numerous points (Fig. 8). 



