204 Professor W. Lawrence Bragg [May 28, 



being linked up by electron sharing, a view which is supported by 

 the density and hardness of the diamond. A crystal such as quartz 

 probably has the same type of structure. The structure of carborun- 

 dum resembles very much that of diamond, and here again every 

 atom shares electrons with all its neighbours. 



The crystal of an electropositive element, such as lithium or 

 potassium/ has a different structure. Each atomic nucleus has a 

 stable arrangement of electrons surrounding it, corresponding to one 

 of the inert gases, and in addition one or more electrons must be 

 associated with each atom according to its valency, as the whole mass 

 of the metal is electrically neutral. The crystal may be regarded as 

 composed of ions and electrons, the electrons having no definite 

 place in the crystalline structure. If an electromotive force is applied 

 to the metal they are driven through, it ; in other words, the metal is 

 a conductor of electricity. 



Still another type of association is that represented by water of 

 crystallization, where some residual forces of attraction must be 

 supposed to hold the electrically neutral water molecules in the 

 crystalline structure. 



15. Crystals where all the atoms hold electrons in common are 

 characterized by great hardness ; large forces are necessary to break 

 the atoms apart. The typical salt, on the other hand, is soft, as in 

 parting the atoms the only forces which have to be overcome are the 

 electrostatic attractions between the atoms. An exception occurs 

 when the ions have a double or treble charge and are close together, 

 as in magnetite, Fe 3 4 , and ruby, A1 2 3 , for then the forces must be 

 supposed to be so great as to give the crystalline structure consider- 

 able hardness. 



In the crystal of a metal the ions, held together by electrons 

 which probably have no fixed positions in the structure, are still freer 

 to move past each other and give the metal its characteristic malleable 

 and ductile properties. 



Since the first two classes of crystals have no free electrons, they 

 are non-conductors of electricity. This is the case for crystals of 

 the electronegative elements where the atoms hold electrons in 

 common. It is the last class of crystals, those of the electropositive 

 elements, which are conductors. 



16. In studying crystal structure we are studying the arrange- 

 ment of the atoms in the solid state, for all true solids are crystalline 

 in nature. It is the extremely short wave-length of the X-rays 

 which makes this feasible. The limit to the minuteness of detail 

 which it is possible to see under the microscope is fixed by the wave- 

 length of the light used to illuminate the object, for it is impossible 

 to distinguish two objects which are closer together than that wave- 

 length. In the case of X-rays the wave-length is ten or twenty 

 times smaller than the distances between the atoms, so that by illu- 

 minating the crystal with X-rays the atomic arrangement may be 



