GUIDE TO THE MIXERALOGIC COLLECTIONS 



11 



a and c are not interchangeable. The same statement holds 

 good for fig. 19 and 20. Cardboard models 6 and 7 will help to 

 make this clear. Fig. 16, 18 and 20 show that if one crystallo- 

 graphic axis is also an axis of trigonal, tetragonal or hexagonal 

 symmetry the other crystallographic axes will be at right angles 

 to it and interchangeable. 



Crystal form 



When every face of a crystal cuts the axes to which it is 

 referred at the same relative distances from the center or inter- 



Fig. 22 Fig. 23 Fig. 24 



section of the axes, the crystal is said to be composed of a single 

 crystal form. Two such crystals are shown in fig. 22 and 23 

 from which it will be noticed that all the faces of each crystal 

 form are similar. Compare models 1, 2, 3, 4 and 5, all of which 

 are crystal forms. Crystals may be composed of a single crys- 

 tal form or of combinations of two or more 

 forms. Such a combination is shown in fig. 24 

 which is made up of the two forms shown in fig. 

 22 and 23. 



Law of simple mathematical ratio 



If the faces of a crystal are extended to inter- 

 sect the axes it will be found that these points of 

 intersection lie at the ratio distance a, b, and c i c 

 characteristic of the substance, or at distances 

 winch are simple multiples or fractions of these ratio dis- 

 tances. Should a plane be parallel to one or two of the axes 

 its intercepts, or in other words the relative distances from the 

 center at which it cuts these axes, are infinity. Assuming the 



