metry '). However this may be, it seems to us that a model of the 
crystal cannot be satisfactory, unless the idea of moleeule and the 
lower symmetry find expression in it. 
Also the occurrence of internal equilibria must be revealed by 
the erystal model, which is impossible with the prevalent conception. 
If we consider e.g. a mixed erystal of two molecule kinds, one of 
which is a polymer of the other, it must be possible that definite 
atoms are alternately bound or not bound to others. Of course it 
is possibile that in the formation of double molecules the distances 
are little modified, if at all, so that this chemical process is not 
expressed in the Röntgenogram, but then we should at least come 
to the conviction that the Röntgenogram does not teach us anything 
about the chemical forces which interest us most, so that in other 
words we cannot make out whether two neighbouring atoms are 
chemically bound or whether they are not. 
Thus it is e.g. possible that by means of the Röntgenogram no 
difference is found between a mixed crystal and a chemical binding, 
when they possess the same symmetry in solid state, though chemi- 
cally there exists a very great and exceedingly important difference 
between them. 
We thought it incumbent upon us to make these remarks, because 
from physical side the problem of the atom arrangement in the 
solid substance seems to be considered as all but solved, though the 
given solution is entirely incompatible with the most essential element, 
viz. with the chemical properties. 
5. The chemical requirements, therefore, include that the valency 
is expressed, while there are indications for NaCl and KCl that the 
symmetry is lower than has been assumed in Brage’s model. 
Accordingly Brace’s model for these chlorides must be subjected 
to a considerable modification; every atom lying on an axis of 
symmetry would have to be multivalent on the assumption of chemical 
bindings, unless the binding lies on the axis itself. To this is added 
that the valency would depend on the circumstance whether the 
atom is situated on a 2, 3, or 4 fold axis, or in the centre. 
When designing a new model for NaCl we have further been 
led by the assumption that the distance between the chemically 
bound atoms will not be greater than that between, not bound atoms, 
and the chemical force, therefore, never acts in a diagonal direction. 
Further by the assumption that the chemical binding, undoubtedly, 
is one of the factors that determine the class of symmetry of the 
1) If NaCl should be holoedrical, then it will hold in each case for KCI. 
