CRYSTALLOGRAPHY 



11 



ni; should the cry>tal Mop gro\\ ing after five sheets were laid 

 down it can he seen that t he cros> sect ion i.-> a ><|iiaiv, and t he solid 

 formed, a cube. If tor any reason growth is irregular and molecules 

 an- laid on fa.ter at one end. l-'ig. 10 b, the cross section is now no 

 longer a square, 'but a parallelogram ; the solid is no longer a cube, 

 but i- elongated in one direct ion. All angles are right angles and 

 cannot vary as long as the molecules are laid on in this order. We 

 cannot imagine t he angles varying from aright angle, any more 

 than it would be possible for a cube to possess angles not right angles. 

 In measuring and comparing angles between similar faces, those 

 faces are considered similar which cut the point-system in the same 

 direction or inclination. The six faces of the cube are, in regard 

 to the point-system, interchangeable. The configuration of the 

 molecules in the plane of each face is the same, therefore the physi- 

 cal properties of each face will be the same and the angles between 

 them will be the same. This is also true for the elongated cube, for 

 the addition of molecules on one side will distort the form, but 

 cannot possibly change the arrangement of those molecules al- 

 ready laid down, upon which the value of the interfacial angles 

 depends. In all similar faces the molecules are the same distance 

 apart in any given direction: they will lie parallel or equally in- 

 clined to the same lines of force; they will show the same luster, 

 polish, and hardness; they 

 are equally soluble and yield 

 the same corrosion figures; 

 they will expand with an 

 increase of temperature 

 equally along parallel di- 

 rections. The distribution 

 of the magnetic force and 

 electric charge will be alike ; 

 in fact, all physical proper- 

 ties of whatever description 

 will be exactly the same, 

 and must be considered in 

 the identification of similar 

 faces. 



The goniometer. The FIG. 11. The Penficld Card Goniometer, 

 exactness of the interfacial 



angle is so great that the accuracy of the angles of chemically pure 

 crystals far surpasses the capabilities of any instrument we may 



