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crystal. We have taken these circumstances into account in the 
following model. There is no atom in the centre of the figure, 
because when it were present, it would be an atom of a valeney 
of six. For the reason mentioned above the places have been left 
vacant on the 2 and 3 fold axes. The four-fold-axis has been 
perfectly covered with molecules in our model, except in the centre. 
Just as in Brace’s model the planes (111) are alternately exclusively 
covered with Na resp. Cl-atoms. The planes (100) and (110) all 
contain both Na and Cl-atoms. In every section there occur vacant 
places, the number of which will relatively decrease as the crystal 
gets larger. The condition for interference will, however, get more 
complicated here than in Brage’s model, because parallel planes 
are not perfectly equally covered. Testing by observation is rendered 
less simple in consequence of this. It is, however, clear that among 
other things the explanation for the difference between the interference 
images of NaCl and KCl also perfectly applies to our model. 
To construct this model one can start from the inner cube, indicated 
by fig. 1, the side of which is the double parameter of the lattice. 
There are only homonymous atoms in the centres of the side planes 
of this cube; these atoms are chemically bound with the atoms lying 
in the centres of the planes of the second cube (fig. 2), the side 
of which is four times the parameter of the lattice. The other net- 
points of the second cube are all as much as possible covered with 
atoms. The four atoms that lie nearest about the central atom in 
each plane, are bound with atoms of the third cube, fig. 3. Each 
plane of the third cube contains 5 atoms, which are bound with 
the following one. We can now imagine, that the crystal is built 
up of two kinds of crystal molecules of the size of fig. 2 or fig. 3 
or of still greater dimensions. These crystal molecules are derived 
of the innereube indicated by fig. 1, with Na resp. Cl atoms in the 
centres of the side planes. 
Anorg. Chem. Lab. of the University. 
Amsterdam, July 1916. 
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