726 Sir J. J. Thomson on the Application of- the 



strained homogeneously in any way without fracture, still 

 remain an assemblage of cells which would enclose an atom 

 in each cell and have the right number of electrons per cell- 

 Thus the cubes might be distorted into parallelepipeda and 

 the cubical symmetry replaced by symmetries represented by 

 the triclinic, monoclinic, rhombic, or tetragonal systems in 

 crystallography. Such a distortion of the cells might be 

 expected to occur when there are many different kinds of 

 atoms in the system. It may be impossible to arrange these 

 so as to give complete cubical symmetry so that bodies with 

 complicated constitutions would tend to crystallize in the 

 more irregular systems. 



The view we are discussing is supported by a comparison 

 of the way in which elements of different valencies crystallize 

 with the results indicated by the preceding table. The state- 

 ments as to the forms in which the different elements 

 crystallize are taken from Groth's Cliemische Krystallo- 

 graplde, vol. i. 



Monovalent Elements. 



Very little seems to be known about the crystallization of 

 the alkali metals ; copper, silver, gold, which are monovalent,, 

 all crystallize in the regular system. 



Divalent Elements. 

 Beryllium, magnesium, zinc, and cadmium all crystallize 

 in the hexagonal system ; an observation by Moissan sug- 

 gests that calcium does so also; nothing is known as to the 

 crystallization of barium and strontium. 



Trivalent Elements. 

 Very little seems settled as to the crystallization of these 

 elements, except that aluminium crystallizes in the regular 

 system. 



Quadrivalent Elements. 

 Carbon, silicon, and lead crystallize in the regular system,. 

 tin probably in a tetragonal one. 



Pentavalent Elements. 

 Phosphorus crystallizes in the regular system ; arsenic in 

 two forms — one regular, the other trigonal. 



Sexavalent Elements. 

 Sulphur crystallizes in several forms, monoclinic and 

 rhombic ; selenium is monoclinic or trigonal ; tellurium 

 trigonal. 



Septavalent Elements. 

 Iodine crystallizes in a rhombic system. 



