MOLECULES IN A CRYSTAL. 487 



be ascribed to a crystal, for it harmonises both with the 

 geometrical and physical characters. 



The line joining any two particles is a line having a 

 whole series of particles strung upon it at equal intervals 

 along its whole length ; every parallel line of particles is 

 identical with it ; but along other lines the molecular interval 

 is in oeneral different. In such a structure we should ex- 

 pect the physical properties (elasticity, thermal expansion 

 and conductivity, etc.) to be the same along all parallel 

 lines, and different in general along lines which are not 

 parallel ; now these are precisely the characters exhibited 

 by a crystal. 



Again, the plane through any three particles is such that 

 it is filled with particles arranged like the knots upon a net, 

 and every parallel plane is exactly similar to it. If such a 

 structure is bounded by plane faces, we should expect them 

 to be just these planes, or (as I have previously called them) 

 "webs " of particles. It will be found that all these planes 

 are related to one another in such a way that, if we know 

 the directions of any four, we can predict with absolute 

 certainty the directions of all other planes which may be 

 constructed out of the lattice. Now this relation is pre- 

 cisely that which governs the mutual inclinations of a crystal's 

 faces, known to crystallographers as the Law of Rational 

 Indices. 



Or we may regard the structure in another way. It is 

 clear that the nature of the whole lattice is completely 

 defined by one of the cells or parallelepipedal spaces 

 enclosed by six adjacent particles. Thus, returning to the 

 hailstorm illustration : if the hailstones are descending in 

 vertical lines in a still air ; and if the distance between two 

 adjacent lines is equal to that between two consecutive 

 hailstones in the same line ; and if adjacent hailstones are 

 on the same horizontal level ; then the cell formed by six 

 adjacent particles may be a cube, and the whole lattice may 

 consist, as it were, of particles placed at the corners of 

 cubes packed in side by side so as to fill space ; a sort of 

 arrangement like that of a wire egg-rack. 



The cube being then one possible form, we may next 



