DECREMENTS. 



73 



ments upon the edge of that form, except in some 

 particular cases which will be referred to in a future 

 section. 



Fig\ 107 shews the effect of a decrement by one 

 row of molecules on the edges of a cube ; producing 

 the planes of the rhombic dodecahedron. 



Fig. 108. 



Fig. 108 shews the manner in which an inter- 

 mediary decrement, taking place at the same time 

 upon the three adjacent angles of the cube, is con- 

 ccived to produce six planes on the solid angle. 



The causes which occasion decrements do not ap- 

 pear at present to be understood : crystals so minute 

 as to be seen only by the aid of a microscope, are 

 found variously modified ; hence the circumstance, 

 whatever it may be, which occasions the modifica- 

 tion, begins to operate very soon after the crystal has 

 been formed. 



Perhaps it may influence the arrangement of the 

 first few molecules which combine to produce the 

 crystal in its nascent state ; and as we find that crys- 

 tals during their increase in magnitude, sometimes 

 undergo a change of form, by the extinction of some 

 modifying planes, or the production of others, it is 

 evident that the cause which occasions a decrement, 

 may be suspended, or may be fii'st brought into ope- 

 ration, at any period during the increase of a crystal 

 in size. 



