APPLICATIONS OF CRYSTALLOGRAPHY, ETC. 583 



Camphoric acid crystallises from acetone in large trans- 

 parent tablets containing acetone of crystallisation, and 

 having the composition Ci.jHioO^, |(CH3).,CO, which can 

 be considered as built up on three rectangular axes, the 

 ratios of the lengths of which are a:b\c= i •2386 : 1 : i 7 i 72. 

 Camphoric anhydride, which is closely related chemically to 

 camphoric acid, crystallises without acetone in tablets of 

 very similar appearance ; these have the composition 

 C10H14O3, and may be referred to the rectangular axes 

 a : b : c = i 'ooi i : i : i 7270. The ratio of the axes ^ : (5 is 

 almost the same in the two cases, so that many of the angles 

 measured are nearly identical on both kinds of crystals ; the 

 crystals of both substances cleave readily along a plane 

 perpendicular to the f-axis, and less readily perpendicular 

 to the ^-axis. In each case the optic axes lie in the plane 

 perpendicular to the a- axis, the smaller optic axial angle is 

 bisected by the axis b and the double refraction is negative 

 in sign ; finally both kinds of crystals are striated in similar 

 ways and almost the same planes are developed on each. 

 The resemblances between the crystals, notwithstanding 

 the great difference in composition, are extraordinarily close 

 and cannot be regarded as due to mere coincidence ; they 

 indicate clearly that one plane section of each structure is of 

 the same pattern in the two cases but that there is no such 

 similarity in other sections. Morphotropic relationships of 

 similar natures to the foregoing are frequently discovered ; 

 they indicate not only that the kind of grouping of the 

 molecules in the crystalline substances upon which they are 

 observed is of a somewhat similar character but also support 

 the view that the physical molecules in crystalline substances 

 are in general identical with the chemical molecules. The 

 information to be gained by the systematic study of morpho- 

 tropic relationships is undoubtedly great and cannot fail to 

 well repay investigation. 



In the preceding pages an attempt is made to demon- 

 strate that many of those problems of physical chemistry 

 which are now so quickly coming to the fore are so closely 

 interlinked with crystallographic phenomena that to study 



