328 WRIGHT! CRYSTALS AND CRYSTAL FORCES 



that the presence, in the solution, of minute quantities of cer- 

 tain colloids or semicolloids may influence profoundly the rate 

 and character of crystal growth, these colloids probably enter- 

 ing into the surficial adsorbed layer and changing greatly the rate 

 at which the molecules diffuse through the adsorbed film to the 

 growing crystal. Gibbs concluded from thermodynamical rela- 

 tions that, because of the vectorial character of crystals, the 

 surface tension on different crystal faces is different and that, 

 therefore, a difference in the solubility of these faces must exist ; 

 Curie inferred that in a growing crystal the tendency exists, by 

 virtue of surface tensional forces, to develop in such form that 

 the total surface energy is a minimum. Ritzel and Marc 4 con- 

 cluded further that because of the differences in solubility of the 

 different faces the tendency also exists for the less soluble forms 

 to develop at the expense of the more soluble and that, therefore, 

 the final crystal form represents the equilibrium adjustment 

 between these two tendencies, namely, toward total minimum 

 surface energy and toward faces of minimum solubility. The 

 same relations should, of course, obtain for vapor pressures 

 over different faces, since vapor pressure may be looked upon as 

 solubility in a vacuum. Experimentally these relations are very 

 difficult to test satisfactorily, partly because of the formation of 

 etch figures. It is probable that accurate vapor pressure meas- 

 urements will furnish results least open to criticism. 



Field of atomic forces. The distances through which atomic 

 and molecular forces act effectively have been shown by differ- 

 ent methods to be of the order of magnitude of 5 mm- Lehmann 

 observed that small acicular liquid crystals of ammonium oleate 

 on precipitation from solution exert, if sufficiently close to- 

 gether, a mutual orienting influence; and that, finally, after having 

 attained strict parallelism, they coalesce. Certain liquid crys- 

 tals are susceptible, moreover, to the orienting influences of a 

 magnetic field. The analogy between a magnetic field and the 

 field of the atoms and molecules in a crystal is more significant 

 than may appear at first sight. In modern theories on the 



4 Zcitschrift fur Physikalische Chemie, 76: 584. 1911. 



