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Prof. W. J. Sollas. 

 F IG- 2 - FIG. 3. 



When, as until recently, sodium chloride was assigned a holo- 

 hedral symmetry, it would have been natural to suppose that the 

 arrangement of its molecules was that of fig. 3 ; but lately it has 

 been placed among hemihedral crystals with which potassium chlo- 

 ride has long been associated. The structure of these salts, and 

 judging by analogy of most of the alkaline haloids, is thus that 

 represented by fig. 2. 



It is of interest to observe that in the holohedral arrangement 

 similar atoms are brought into contact ; this is not the case with 

 hemihedral symmetry. Perhaps in this is to be found an explana- 

 tion of the fact that diatomic compounds of monads do not possess 

 holohedral symmetry. 



A plagiohedral asymmetry is revealed in the structure both of 

 common salt and potassium chloride by the action of solvents, which 

 produce etch-figures bounded by faces of a hexakis-octahedron. 

 The factors which determine the forms produced by solution are 

 unknown, but probably include the disposition of the chemical bonds 

 of the atoms in the molecule, and the relation of the water molecules 

 to the disposition of the atoms in the crystal. It is evident, how- 

 ever, both from models and figure that there is a skew in the distri- 

 bution of the chemical bonds of our molecules, and this may be 

 connected with the skew faces produced by solution. 



Planes can be drawn through the assemblage of fig. 2 parallel to 

 the faces of the cubelets without crossing any of the molecules; 

 these may be regarded as planes of cleavage. 



When a cube of rock salt is truncated at two opposite edges, 

 parallel to planes of a rhombic dodecahedron, and pressure is 

 exerted on the crystal in a direction normal to these faces, a per- 

 sistent compression results in the direction of the diagonal, and 

 accompanying this a doubly refracting stripe appears. With in- 

 creased pressure a clean fracture is produced, parallel to one of the 

 rhombic dodecahedral faces to which pressure is applied. 



All this is in complete consistence with the structure hypothetic- 

 ally assigned to the crystal. The effect of pressure, as I imagine it, 



