CHEMICAL CRYSTALLOGRAPHY. Chemical Crystallography is concerned with the relations between the chemical constitution of crystallized substances and the structure of their crystals as revealed by their form and physical characters.
In 1800 Abbe Rene Just Hairy announced that to every specific substance of definite chemical composition capable of existing in the solid condition there appertains a crystalline form peculiar to and characteristic of that sub stance. Despite the many controversies involv ing polymorphism, and isomorphism, this law needs little modifying and it is still the belief that each chemically definite body crystallizes only in one symmetry class and has for any definite temperature and pressure constant angles and constant volume just as it has a definite molecular weight and a definite chemical structure. Furthermore, all the in vestigations prove that substances in any way chemically different possess different shape and volume.
The observation by Mitscherlich in 1819 that the phosphates of ammonium and potassium and the arsenates of ammonium and potassium occur in very similar crystalline shapes led to the announcement in 1821 of the principle of isomorphism which was in effect that analogous elements can replace each other in their crys tallized compounds without any apparent change in crystalline form.
For many years criteria were Solight by which substances "truly isomorphous* could be recognized; for instance, similarity in their chemical molecules and reactions, close rela tions in symmetry and angles and above all capacity to crystallize together in homogeneous mixed crystals tin which the physical characters are continued functions of their chemical com position."
It is now established that except in the iso metric substances, the replacement of one ele ment by another inevitably produces some i change in the angles and sometimes even in the symmetry, and while the change increases in general with increasing chemical dissimilarity there are no sharp lines which can be drawn and series exist with striking resemblances in angles and physical characters and little or no chemical resemblance.
Von broth in 1870 broadened and systema tized the field of chemical mineralogy by direct ing attention to the definiteness of the changes produced by the replacement of one atom or group by another. For instance, he showed that the derivatives of benzene, GI., underwent definite changes of structure by the substitution of new atoms or groups, and that even while the crystal system might change there remained a striking similarity in angle in certain zones, and the alteration consisted chiefly in an elonga tion or shortening of this zone axis, usually a crystal axis. The axial ratios given below show that the change is principally with refer ence to the f axis.
d : b : 0.891:1:0.799 C4114(0 H)2 0.910:1:0.540 C41is(OH)(NOsh 0.933:1:0.7S3