TETRAGONAL SYSTEM 



79 



of the isometric system. While outwardly these seven pri-m- 

 of the first on lei- are exactly alike, yet inwardly they all possess 

 their distinctive -ymmetry. The physical properties are distributed 

 on the f:iee of each prism in accord with, and they conform to, the 

 symmetry of the type. The seven prisms of the first order will 

 dilTer then in their symmetry. 



When a solvent is applied to a crystal face or a natural crystal- 

 line surface, as a cleavage surface, the crystal will not pass into solu- 

 tion equally or with the same speed in all directions. The rapidity 

 with which molecules pass into solution or are torn off from the crys- 

 talline network will depend upon the symmetry of the network. 

 The solvent's action will not act evenly all over the surface, but will 

 st art at points scattered over the crystal face ; and solution will begin 

 at each one of these points as a nucleus or center, the molecules 



1 i<i. 146. Photograph of Etch Figures on Halite, enlarged Five Diameters. 



going into solution one after the other, with a speed that varies 

 with the direction. If the solvent's action is stopped after a very 

 short time and the crystalline surface is examined, in many cases, if 

 Hie concentration and character of the solvent has been favorable 

 and it has not been allowed to act too long, the surface will be pe- 

 culiarly pitted. All of these pits or etch figures will be of the same 

 shape on all faces of the same crystal form. They are bounded by 

 straight or slightly curved lines and are of the nature of negative or 

 reentrant crystals, with their equivalent faces and axes arranged 

 parallel, as is shown in Fig. 146. These etch figures may be produced 

 by plunging a crystal into a solvent for a very short time, as they 

 are the result of the first action of the solvent on the crystal face. 



