14)6 TERMINOLOGY. . 13G. 



which geometrically must enter into it, as really belonging 

 to them, although nature should not as yet have produced 

 them as simple forms. 



The System of Crystallisation derived from the rhombo- 

 hedron is termed the Rhombohcdral System. The system 

 whose fundamental form is the isosceles four-sided pyramid, 

 is called the quadrato-pyramidal, or, in shorter terms, the 

 Pyramidal System ; that from the scalene four-sided pyra- 

 mid is the rhombeo-pyramidal system, which, on account of 

 the great variety of prisms which it contains (. 91. 95. 98.), 

 receives the denomination of the Prismatic System ; and 

 that from the hexahedron is called the Tessular System, not 

 the hexahedral one, in order to intimate, that experience 

 has not as yet given any reason for assuming another sys- 

 tem of tessulur forms, although geometrically we may con- 

 ceive a system of forms of several axes, which stands in the 

 same relation to the regular dodecahedron of Geometry, in 

 which the tessular system is to the hexahedron. The other 

 systems, comprising the Hemiprismatic (. 153.) and the 

 Tetartoprismatic (. 154.) forms, have not as yet been pro- 

 vided with particular denominations. 



. 136. SERIES OF CRYSTALLISATION. 



The fundamental form being supposed to possess 

 determined dimensions, the assemblage of derived 

 forms becomes a Series of Crystallisation. 



The system of crystallisation is an idea of very great ex- 

 tent, but it is liable to certain restrictions in particular de- 

 terminate cases. These restrictions consist in ascertaining 

 or fixing the dimensions of the fundamental forms in the 

 different systems : for the relations of the derived forms 

 among each other and to the fundamental one are general, 

 and must remain unchanged in all series of crystallisation, 

 which belong to the same system. If, therefore, the funda- 

 mental form is supposed to possess determined dimensions, 

 the derivation will yield a System of Crystallisation, which. 



