Crystals. 



1 25 





forms of crystals are not irregular or accidental, but definite, and based 

 on certain fixed laws; and he pointed out that whilst certain forms are 

 derivable from a given nucleus, there are others which cannot occur. 



Moreover he observed that when any change in a crystal took 

 place by its combination with other forms, all similar parts (angles, 

 edges and faces) were modified in the same way. Most important of 

 all, he shewed that these changes could be indicated by rational 

 co efficients. 



Thus Haiiy became the discoverer of two of the three great laws of 

 crystallography, namely, the law of symmetry, and the law of 



WHOLE NUMBERS. The Other, THE LAW OF CONSTANCY OF ANGLES, 



we have already mentioned. 



Let us consider for a moment Haiiy's two laws, taking first : 



THE LAW OF SYMMETRY. 



E. S, Dana enunciates this as follows: "The symmetry of 

 crystals is based upon the law that either : 



I. All parts of a crystal similar in position with reference to 



the axes are similar in planes or modification, or 

 it. Each half of the similar parts of a crystal, alternate or 

 symmetrical in position or relation to the other half, 

 may be alone similar in its planes or modifications. 

 The forms resulting according to the first method are termed 

 holohedral forms and those according to the second, hemihedral.''' 



An easy experimental way of studying the symmetry of crystals is 

 to cut one, or the model .of one, in two, and place the parts against 

 the surface of a mirror, which may or may not produce the exact ap- 

 pearance, of the original crystal. If it does produce the exact 

 appearance we have severed the crystal in a plane of symmetry. 

 By referring to Fig. 6 it will readily be seen that a cube, for in 

 stance, possesses nine such planes, indicated by the dotted lines. 



Fig. 6. 



In a sphere there would of course be an infinite number of these 

 planes. 



