119 



only an oblique parallelepiped cell being possible, for tin- 

 lonoclinic two kinds of cells, for the rhombic four, for the trigonal 

 mlv one rhombohedral cell, for the tetragonal system two kinds of 

 for the hexagonal only an equilateral trigonal prism, of which 

 six contiguous ones are shown in the figure, and for the cubic 

 it mi three kinds of cells. The cells of the rhombic, tetragonal 

 id cubic system which have a point in the centre of the parallelo- 

 )iped cells drawn in fig. JQJ, can be also chosen in such a way 

 it no point lies within the cell ; in the cubic system for instance, 

 le elementary cell would then have an octahedral form, with a 

 >int at each corner of the octahedron, etc. 

 7. In connection with this we shall at the same time draw 

 ittention to a fact which will appear of interest to us in future for 

 ic understanding of special groups of phenomena. It concerns 

 ic existence of so-called elements of pseudo-symmetry in such space- 

 ittices, a fact which finds its explanation in the special circum- 

 stance that there may exist a gradual passage of form from the one 

 cind of unit-cell to the other. 



Thus comparison of the elementary cells of both the tetragonal 

 :ells with the types a and b of the rhombic and the cubic system, 

 ;ill make it clear at once that a suitable change of the principal 

 limensions in one or two directions will make their form approach 

 closely as desired to that of a cubic cell. In the same way, if the 

 dihedral angle of the oblique monoclinic cell, differing from 90, 

 ipproaches very closely to this value, the cell becomes almost that 

 )f a rhombic space-lattice. 



When the principal ternary axis of the rhombohedral cell is suitably 

 lengthened or shortened, the polar dihedral angles can approach 

 to 90 as closely as possible, the rhombohedral cell being therefore 

 )nverted almost into a cube. Indeed, the rhombohedron is a dis- 

 torted cube, namely, if the latter be compressed or dilated in the 

 direction of one of its four trigonal symmetry-axes. 



If the prism-angle of a rhombic prismatic cell be almost 60 or 

 120, it approaches very closely to the equilateral triangular cell 

 of the hexagonal space-lattice, etc. 



In all such cases the lower symmetrical space-lattice exhibits 

 a greater or smaller approximation to a space-lattice of higher 

 symmetry. It is said to possses a limiting or pseudo-symmetry: the 

 space-lattice is called pseudo-cubic, pseudo-hexagonal, etc., to indicate 

 that, - - although having truly a lower degree of symmetry, - 



