1910] Tim Chemical Significance of Crystal Structure. 828 



WEEKLY EVENING MEETING, . 



Friday, April 15, 1910. 



Sir Francis Laking, Bart., G.O.WO. M.D. LL.I)., 

 Vice-President, in the Chair. 



Professor William J. Pope, M.A. F.R.S. 



The Chemical Significance of Crystal Structure. 



Large numbers of chemical substances occur on the earth's surface 

 as definite geometrical forms bounded by plane faces : these poly- 

 hedral shapes are called crystals. Inspection of the crystal forms 

 assumed by mineral substances shows that, roughly speaking, each 

 crystalline substance affects some specific geometrical shape which is 

 characteristic for the material : further that, whilst crystals of any 

 particular mineral attain vastly different dimensions and are bounded 

 by planes which vary greatly in relative area, one geometrical feature 

 remains constant. The angles between corresponding pairs of faces 

 on any two crystals of the same substance are the same, notwithstand- 

 ing the existence of difference in size, or in relative face magnitude 

 between the two crystals. The constancy of interfacial angle amongst 

 crystals of the same substance is a law of nature, and has been amply 

 demonstrated by the very careful crystallographic measurements made 

 by Tutton during the last 20 years. 



It is, however, not essential to study mineral substances alone in 

 order to obtain a knowledge of the laws governing crystal growth. 

 Great numbers of laboratory products can be caused to crystallise by 

 condensation from some fluid condition ; thus, the crystals of various 

 alums exhibited were obtained by slow evaporation of aqueous solu- 

 tions of these salts. 



The examination of a crystal shows that many of its physical 

 properties differ according to the direction in the crystal in which 

 the property is determined : the hardness of crystals, the speed at 

 which light travels through them, and many other properties, are 

 commonly dependent on the direction in which the material is 

 examined. 



The dependence of crystal properties on direction indicates the 

 most essential feature of the crystal to be a definite and orderly 

 arrangement of its ultimate particles ; this arrangement is referred 

 to as the crystal structure. Further evidence that crystals possess an 

 aiTanged structure is furnished by the observation that crystallisation 

 is not necessarily a spontaneous process. Thus, on melting benzo- 

 phenone and rapidly cooHng the clear molten mass, the liquid state is 



