MINERALOGY 



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observed, but in which the physical properties are directional, 

 the crystalline force may be considered as being very feeble. The 

 crystalline force will vary not only with the substance, but will 

 differ with the direction in the same substance. When the mole- 

 cules pass to the solid state, from a condition in which they are free 

 to move, and become fixed under the influences of the crystalline 

 force, it is reasonable to suppose that in the direction in which the 

 crystalline force has the strongest attraction the molecules will be 

 packed closer together. In the direction in which the force is 

 f ^ feeble they will be farther apart. The 



forces influencing and surrounding each 

 molecule are exactly the same as the 

 forces influencing and surrounding 

 every other molecule ; there is no sin- 

 gular molecule. The distance from 

 any molecule to its nearest neighbor in 

 the same direction will be exactly 

 equal in all. In Fig. 3, a, b, c, d are 

 in complete crystalline position, while 

 e must be revolved 90, and f must be 

 both revolved and translated to reach a crystalline position in 

 regard to a, b, c, and d. 



The whole will form a regular molecular network, or point- 

 system, in which each molecule holds an exact position, just as 

 each individual man in a marching regiment must be in his 

 exact position, holding a fixed relation to those surrounding 

 him; and further, if our attention is directed to the complete 

 formation, it will be seen that as a consequence of the orderly 

 position of each man, the whole is bounded by straight lines. If it 

 were possible to place one regiment on top of another, the straight 

 lines would then become planes, and the solid thus formed would 

 be bounded by plane faces ; in a similar way crystals are bounded 

 by plane faces. 



The smooth plane faces bounding a crystal are its most striking 

 external character. It must always be remembered that they are 

 only reflections of the internal orderly arrangement of the mole- 

 cules. If the crystalline molecules are identical with, or a small 

 multiple of, the chemical molecule, then the crystalline units may 

 be considered infinitely small, as regards any power we may pos- 

 sess to distinguish them when packed together, forming crystal 

 faces. All artificially polished surfaces fall far short of the smooth- 



