ISOMORPHISM 153 



On passage from the active compounds to the racemate, 

 in which the enantiomorphic molecules unite, the enantio- 

 morphism of crystalline structure is also destroyed, as 

 a rule. In the cases hitherto studied the following axial 

 ratios have been found : 



Potassium bitartrate, KC 4 H-O 6 , 

 Rhombic a : b : c = 0-7168 : i : 0*7373 

 Potassium biracemate, (KC 4 H 5 O 6 ) 2 , 

 Triclinic a : b : c = 0-7053 : i : 0*7252 

 a = 88 36' /3=:io2 22' y=87i6', 

 so that at any rate two axial angles are not far from 90. 



If we have here a striking relation between the configura- 

 tion of the molecule and crystalline structure, it becomes 

 much more so on considering the phenomena of isomorphism, 

 which Mitscherlich first put in the right light. That two 

 bodies like potassium-magnesium sulphate and rubidium- 

 cadmium sulphate, despite their entirely different com- 

 position : 



19.4 / Potassium 29-2 / Rubidium 



6 ,, Magnesium 19*3 ,, Cadmium 



I 5*9 > Sulphur n Sulphur 



55-7 ,, Oxygen 38-5 Oxygen 

 3 Hydrogen 2 Hydrogen 



should possess the same monoclinic crystalline form, which, 

 moreover, indicates so close a similarity in internal structure 

 that they form completely homogeneous mixed crystals in 

 which one compound has replaced the other, is a most con- 

 clusive proof that the molecular-mechanical conception of the 

 compounds, attributing to them the analogous formulae, 



K 2 Mg (S0 4 ) 2 6 H 2 0, Rb 2 Cd (SO 4 ) 2 6 H 2 O, 



comes very near to the facts if only schematically. 



It is on this ground of isomorphism that considerations 

 may be developed allowing of a deeper insight into the 

 structural relations in crystals. 



In the first place we must consider the investigation 

 started by Groth on so-called morphotropy, i. e. small 

 changes of form which are produced by change of composi- 



