CHANGE OF CRYSTALLINE FORM 151 



tage by the number of planes of symmetry corresponding, 

 viz % 9, 7, 5, 3, i, o, and dividing the sum of these products 

 by TOO. Like all statistical relations this has the disad- 

 vantage that the application to particular cases often fails. 



2. Influence of Changes in the Molecule on the 

 Crystalline Form. 



In the preceding, attention was paid to the direct relation 

 between symmetry or the want of it in the molecule and 

 the crystal. A second line of research is that of the 

 influence which changes in the molecule exert on corre- 

 sponding changes of form. Let us take first the smallest 

 changes which show themselves in the orientation or 

 relative position in the molecule, as polymorphism, while 

 the molecule as a whole remains the same. Here we have 

 clearly a point of departure in judging of the relations 

 between molecular configuration and crystalline structure, 

 since the same configuration is associated with two different 

 figures. The observations collected on this subject are, 

 however, so far of a purely empirical character, and amount 

 to no more than the important remark that, when the 

 crystalline form of a polymorphic substance changes, there 

 is often a striking equality in the angular and axial 

 relations to be noted J . Let us consider examples. 



Potassium nitrate, potassium sulphate, and calcium 

 carbonate, which can all take hexagonal form, i. e. with 

 the angle 60, show in their possible rhombic forms prism 

 angles not very different from 60 actually 60 36', 59 36', 

 and 63 44' in the three cases. 



Basic copper nitrate, Cu.^(OH) 3 N0 3 , which is found in 

 rhombic form with the axial ratio 



a : b : c = 0-9217 : i : 1-1562, 

 is converted into a monoclinic form with almost the same 



ratio, 



a : b : c = 0-919 : i : 1-1402, 



1 Arzruni's collection in Graham-Otto, 1893, 74. 



